International Journal of Cosmetic Science, 2013, 35, 613–621
doi: 10.1111/ics.12085
Effect of emulsifier type and concentration, aqueous phase volume and wax ratio on physical, material and mechanical properties of water in oil lipsticks A. Beri, J. E. Norton and I. T. Norton Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
Received 12 April 2013, Accepted 27 July 2013
Synopsis OBJECTIVES: Water-in-oil emulsions in lipsticks could have the potential to improve moisturizing properties and deliver hydrophilic molecules to the lips. The aims of this work were (i) to investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size and (ii) to investigate the effect of wax ratio (carnauba wax, microcrystalline wax, paraffin wax and performalene) and aqueous phase volume on material properties (Young’s modulus, point of fracture, elastic modulus and viscous modulus). METHODS: Emulsion formation was achieved using a high shear mixer. RESULTS: Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (droplet size ~18–25 lm). Polyglycerol polyricinoleate (PGPR) resulted in emulsions with the smallest droplets (~3–5 lm), as expected from previous studies that show that it produces a thick elastic interface. The results also showed that both Young’s modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax, suggesting that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young’s modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. CONCLUSIONS: This work suggests the potential use for emulsions in lipstick applications, particularly when PGPR is used as an emulsifier, and with the addition of solid wax, as it increases network strength. sume Re OBJECTIFS: Les emulsions eau dans huile dans les rouges a levres pourraient avoir le potentiel pour ameliorer les proprietes hydratantes et delivrer des molecules hydrophiles aux levres. Les objectifs de ce travail etaient: (i) d’etudier l’effet du type d’emulsifiant (polymere vs monomere, et de la cha^ıne saturee vs. insatures) et de la Correspondence: Akash Beri, Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK. Tel.: +44(0)1214145082; fax: +44(0) 1214145377; e-mail: [email protected]
concentration sur la taille des gouttelettes, et (ii) d’etudier l’effet du taux de cire (cire de carnauba, la cire microcristalline, de la cire de paraffine et PERFORMALENE) et du volume de la phase aqueuse sur les proprietes du materiau (module de Young, le point de rupture, le module elastique et module visqueux). METHODES: la formation d’emulsion a ete realisee en utilisant un cisaillement eleve. melangeur a RESULTATS: Les resultats ont montre que la nature saturee de l’emulsifiant a eu tres peu d’effet sur la taille des gouttelettes, pas plus que l’utilisation d’un emulsifiant avec un groupe de t^ete plus grosse (taille des gouttelettes ~ 18–25 lm). Le polyricinoleate de des emulsions des plus petites polyglycerol (PGPR) a conduit a partir d’etudes anterieures gouttelettes (~ 3–5 lm), comme prevu a qui montrent que cela produit une interface elastique epaisse. Les resultats ont egalement montre que le module de Young et le point de fracture augmentent avec un pourcentage croissant de la cire de carnauba (selon a une dependance en loi de puissance de 3), mais diminuent avec un pourcentage croissant de la cire microcristalline, suggerant que la cire de carnauba est inclus dans l’ensemble du reseau de cire forme par les composants satures, tandis que la cire microcristalline forme des cristaux irreguliers qui perturbent l’ensemble du reseau. Le module de Young, le module elastique et le module visqueux diminuent avec l’augmentation du volume de la phase aqueuse dans l’emulsion, bien que la pente de la diminution de modules elastiques et visqueux soit fonction de l’addition de cire solide, en consequence du renforcement du reseau. CONCLUSIONS: Ce travail suggere l’utilisation potentielle des emulsions dans des applications de rouge a levres, surtout quand PGPR est utilisee comme emulsifiant, et avec l’ajout de cire solide, car elle augmente la force du reseau. Introduction Lipsticks form an intrinsic part of most cosmetic companies product range and typically consist mainly of hydrophobic ingredients (e.g. waxes, pigments and oils). The high solid wax and pigment content provide the lipstick with material properties, for example strength, rigidity and elasticity; however, the excessive use of hydrophobic ingredients can result in dryness of lips owing to layering the lip with a hydrophobic layer preventing natural lubrication (from saliva or water vapour in the atmosphere). Emulsions could be used to deliver moisture to the lips, whilst maintaining material properties. Water-in-oil (W/O) emulsions are used in many industries, including food, cosmetics, pharmaceuticals and insecticides [1].
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie
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In the food industry, various fats/oils, including milk fats [2], cocoa butter [3] and vegetable oils [4], have been used as the continuous phase to produce W/O emulsions. When producing an emulsionbased lipsticks, there are two critical parameters to be considered: (i) physical properties (e.g. droplet size; small droplets can increase the release rate of any functional ingredients within the aqueous phase as the surface area is increased [5] and the processes of destabilization can be slowed) and (ii) material properties (e.g. Young’s modulus and elastic modulus). These parameters can be controlled by many factors, including the emulsifier used, the aqueous phase volume and the properties of the continuous lipid phase. Emulsifiers are required to reduce the interfacial tension between the two phases, which enhances droplet break-up and delays coalescence during emulsification. Emulsifiers can be polymeric (e.g. polyglycerol polyrinceoleate (PGPR)) or monomeric (e.g. monoglycerides and sorbitan olivate). Polymeric emulsifiers stretch across the interface creating a physical barrier against coalescence [6]. Some monomeric emulsifiers (sorbitan olivate) sit at the interface, whereas monoglycerides (monoolein) can crystallize at the interface causing Pickering stabilization, which has been well documented in literature [1, 7–9]. The mechanical properties, such as strength (maximum stress before failure), hardness (ability to withstand surface indentation), rigidity/stiffness (resistance to deformation in response to applied force), plasticity (ability to undergo irreversible deformations) and elasticity (ability to return to original shape after deformation), are governed by the structure of the network, including the number of cross-links and defects. Typically, mechanical properties can be measured using texture analysis or rheology. Following texture analysis, a stress–strain curve is plotted, indicating the amount of strain (deformation) at increasing stresses (loading). A number of different measures of mechanical properties can be taken, including the Young’s modulus/tensile modulus (measure of the stiffness; the slope of the initial linear region of the stress–strain curve), bulk modulus/compressibility (resistance to uniform compression; the slope of the second linear region of the stress–strain curve), point of failure/fracture and total work of failure (area under the curve) [10]. Similarly, oscillatory rheology (study of flow) can be used to characterize samples, providing values of elastic/shear storage modulus (G’), viscous/shear loss modulus (G”) and the loss factor (tan d). G’ is a measure of the deformation energy which is stored within a sample during shear, thus showing the elastic (solid-like) behaviour. G” is the deformation energy that is dissipated as heat during shear and indicates viscous (liquid-like) behaviour. The loss factor is a ratio of loss to storage moduli (tan d = G”/G’) and gives an indication of viscoelastic properties. A phase angle (d) of 0° is indicative of an elasticity, whereas a phase angle of 90° represents a purely viscous material [11]. Mechanical properties can be manipulated or controlled by the ratio of different waxes used in the formulation, and in this case the amount and size of the water droplets within the emulsion. The introduction of water into the structure in the form of water droplets has varying effects, as described in the literature. Although Wang and Lee (1997) reported that increasing water content (5– 15%) increased hardness [12], Le Reverend et al. [5] showed that the hardness (measured using penetration) decreases with increasing water content (10–40%, droplet size ~ 5 lm). The latter agreed with literature in the food industry [5, 13]. Le Reverend et al. (2011) suggested that wax crystals move to the W/O interface from the continuous phase, which weakened the structure. In addition, the solid nature of the continuous phase should result in
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long-term stability, slowing down the processes of coalescence and phase separation by trapping water droplets within the wax network [14]. In this work, we investigated (i) the effect of concentration for four emulsifiers (Sorbitan olivate, monolein, monostearate and PGPR) on the emulsion droplet size for a variety of aqueous phase volumes (10–40 wt%) (ii) the effect of emulsifier concentration on long-term stability (iii) the effect of different wax ratios (carnauba wax and microcrystalline wax) on elasticity and point of fracture and (iv) the effect of introducing water droplets on the elastic and rheological properties. Monoolein and monostearate were compared with investigate the effect that the nature of saturation of monoolein has on the steric hindrance at the interface. Sorbitan olivate was then compared with the monoglycerides to investigate the effect head group size. The monomeric emulsifiers were then compared with a polymeric (PGPR) emulsifier. It is important to note that for this paper a model system (without pigments) was investigated to understand the role of waxes on material properties. Materials and methods Materials All materials (castor oil, carnauba wax, microcrystalline wax, hard paraffin and performalene) used to produce the continuous phase of emulsions were supplied by Alliance Boots PLC (U.K.). These were used in combination with double distilled water and various emulsifiers: PGPR (HLB 1.5) (Palsgaard, Denmark), sorbitan olivate (HLB 4.3) (Aston Chemicals, U.K.), monoolein (HLB 3.6) (Cargill, Netherlands) or monostearate (HLB 3.2) (Cargill, Netherlands) to produce emulsions. Emulsion preparation A continuous phase formulation containing castor oil (40–95 wt %), carnauba wax (0–20 wt%), microcrystalline wax (0–20 wt%) and paraffin (5 wt%) or performalene (5 wt%) was melted with either PGPR, sorbitan olivate, monoolein or monostearate (0.2–5 wt%) and aqueous phase (distilled water, 10–40 wt%) in varying quantities. The pre-mixture was emulsified for 5 minutes using a Silverson L4RT (Silverson Machines Ltd, U.K.) high shear mixer at ~75°C, fitted with a fine emulsifier screen (pores ~1 mm) at ~10 000 g. Following emulsification, the emulsion was placed in a freezer for 20 min (cooling rate: 2.4 0.2°C min1) to allow for the continuous phase to crystallize. Determination of droplet size Droplet size measurements were performed using a pulsed field gradient NMR equipped with a water droplet size application (restricted diffusion) (minispec mq series, Bruker Optics, U.K.) at ~5°C to measure the volume-weighted mean droplet diameter (d3,3). The use of NMR for droplet size determination in W/O emulsions has been well documented by van Duynhoven et al. [15], and is beneficial as its non-invasive, and can measure opaque samples. It should be noted, however, that the technique assumes a log-normal droplet size distribution and spherical droplets. The samples were poured into 10 mm NMR tubes and filled to a height of 10 mm. Samples were then placed into a freezer for 20 min. All NMR analysis was conducted either on day 0, 1, 7, 10 and 180 after manufacture (where day 0 is day of manufacture).
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Physical and material properties of emulsion lipsticks
The mean droplet size (d3,2) was then calculated using equation 1 [15]:
(a)
2
ð1Þ
where d3,2 is the surface-weighted mean droplet diameter, d3,3 is the volume-weighted mean droplet diameter, and r is the standard deviation of the logarithm of the droplet diameter. Microscopy Cyro-scanning electron microscopy was used to visualize the emulsions. A XL-30ESEM (Philips, Eindhoven, the Netherlands) was used for the analysis. The structure of the sample was preserved using cryogenic temperatures (below 150°C). The sample was attached onto a sample holder and rapidly frozen by immersion into a liquid nitrogen bath for approximately 2 min. The sample was then transferred to the high vacuum cryo-unit chamber and freezefractured. A thin layer of platinum was scattered onto the surface. The sample was the moved into the observation chamber, and observations were carried out at 3–5 kV at temperature between 100 and 175°C. Texture profile analysis The mechanical properties of both continuous phase formulations and emulsions were determined by compression. This was conducted using a TaXT+ (Stable Microsystems, Surrey, U.K.) texture analyser. Measurements involved using a standard compression plate (SMS P/40) with a 40 mm diameter cylindrical aluminium probe. All samples had a diameter of 20 mm, and their length was kept at ~20 mm. All measurements were carried out in quadruplet with a compression speed of 1 mm s1. The data (force/distance) obtained from the texture analyser were converted into true stress and true strain using equations 2–5 (obtained from Moresi & Bruno, 2007 [16]); eE ¼
Ho h Ho
eH ¼ lnð1 eE Þ rE ¼
F Ao
rH ¼ rE ð1 þ eH Þ
ð2Þ ð3Þ ð4Þ ð5Þ
where eE and eH are the engineering and true strain, respectively, Ho and h are initial height and height of each sample as recorded during the compression test, rE and rH are the engineering and true stress, respectively, and F and Ao are compression force applied and initial cross sectional area of sample. From the true stress/true strain curves (typical curves shown in Fig. 1), the Young’s modulus (as shown by Norton et al.,[10]) was taken (the slope of the first linear region of the stress–strain curve) and plotted against either percentage of wax or percentage of water to give an insight into the material properties of the formulations. Determination of melting properties The melting properties of continuous phase formulations were investigated using a Differential Scanning Calorimeter (Perkin Elmer DSC Series 7, Cambridge, U.K.), equipped with thermal analysis software (Pyris). The instrument was calibrated for
10% CW and 90 % CO 10% CW and 90 % CO
0.002
0.001
0.000 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.08
0.10
0.12
True strain (b) 0.003
True stress (M Pa)
d3;2 ¼ d3;3 e0:5r
True stress (M Pa)
0.003
5% CW, 5% MW and 90 % CO 5% CW, 10% MW and 85 % CO
0.002
0.001
0.000 0.00
0.02
0.04
0.06
True strain Figure 1 Typical True Stress (MPa) vs. True strain curves for bulk oil formulations, where (a) is 10% carnauba wax (CW) in castor oil (CO), for measurement 1 (●) and measurement 2 (○), and (b) is 5% microcrystalline wax (MW) and 5% CW was in CO (●) and 10% MW and 5% CW was in CO (○). All samples were melted and stirred using a magnetic stirrer until molten (~ 30–40 min) and then cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
temperature using indium and tin, with an empty aluminium pan as a reference. Samples were loaded into 50 lL capacity aluminium pans and sealed with aluminium covers. Pans were heated at a rate of 10°C min1, from a range of 10 to 120°C. Rheology The elastic modulus (G’) and viscous modulus (G”) were determined to investigate the effect of waxes on the viscoelastic properties of an emulsion. G’ and G” were measured using a Bohlin Gemini Nano Rheometer (Malvern, U.K.), and a 20 mm parallel plate (1 mm gap width) geometry. To allow analysis to be carried out in the linear viscoelastic region (LVR), an initial amplitude sweep experiment was conducted to calculate the appropriate stress/strain to use for a frequency sweep experiment. All experiments were run at 32°C (lip temperature). All samples were prepared immediately after production by pouring molten solutions into cylindrical moulds and cooled in a freezer for 20 min. Thin slices (~1 mm in thickness and 20 mm in diameter) were placed in a parallel plate geometry for testing.
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Results and discussion Physical properties A series of experiments were conducted to investigate the effect of both emulsifier type and concentration on droplet size. Emulsification were conducted at 75°C to ensure that the waxes were molten, thus preventing Pickering particles (i.e. wax crystals) stabilizing the emulsion. Different monomeric (monoolein and monostearate) emulsifiers were compared with investigate the difference between saturated and unsaturated chains [7, 17]. These were then compared with sorbitan olivate to explore the effect of head group size. These were compared with a polymeric emulsifier (PGPR). Figure 2 shows the droplet size (d3,2, lm) as a function of emulsifier type and concentration. Results show that for emulsions formed using monoglycerides the droplet size was approximately 20–28 lm, regardless of the emulsifier concentration. Results also indicate that there is no difference in droplet size when using a saturated or unsaturated monoglyceride. Sorbitan olivate is a combination of both sorbitan monoolein and sorbitan monostearate (head group ~ 13.9 A, obtained from ChemDraw 12.0). Even though the head group is twice the size of the head group for monoglycerides (5 A, obtained from ChemDraw 12.0), no difference in droplet size was observed for all emulsifier concentrations. However, a greater concentration of sorbitan olivate is required (≥1%) to produce an emulsion with droplets in the same size range (~25 lm). At lower concentrations (0.2–1%), the NMR was unable to measure droplet size, which suggests that the droplets were greater than 100 lm (upper size limit for NMR restricted diffusion). The difference in concentration of emulsifier required could either be owing to (i) insufficient emulsifier for surface coverage, (ii) the ability of the emulsifier to get to the interface during the process, (iii) affinity for the interface. The amount of emulsifier required for surface coverage (assuming a monolayer of emulsifier and spherical droplets) was calculated by dividing the total area of interface by the surface coverage of each. This was compared with the amount Sorbitan Olivate Monoolein
30
Monostearate PGPR
25
d3,2 (µm)
20
added in reality (Table I). It is clear that there is an excess of emulsifier, and therefore, the difference in concentration required is either owing to the emulsifier’s ability to get to the interface or it’s affinity for the interface. Sorbitan olivate has a greater molecular weight (~430 g mol1) than monoglycerides (356 g mol1) and will therefore take longer to diffuse to the interface. If the governing factor was the mass of the emulsifier, one would expect PGPR (~500 g mol1) to produce the largest droplets. However, Fig. 2 illustrates that even at low concentrations of PGPR, droplets are formed in the range of 4–7 lm. This is owing to PGPR’s ability to produce a thick elastic interface preventing coalescence of droplets. The horizontal nature of all the points displayed in Fig. 2 is a result of the system being saturated by emulsifier, as suggested by the calculations displayed in Table I. The effect of different PGPR concentrations and aqueous phase volume on droplet size was investigated by NMR restricted diffusion. Table II shows that for all aqueous phase volumes, droplet size decreases as PGPR concentration increases. At higher aqueous phase volumes (30% and 40%), the lower concentrations of PGPR (0.2 and 0.5 wt%) are unable to stabilize droplets, resulting in larger droplets (>100 lm). This is a result of there being a greater surface area, and not being sufficient time for the emulsifier to diffuse to the interface. To test this hypothesis, the processing time for an emulsion (40% aqueous phase and 0.5% PGPR) was doubled from 5 to 10 min. This resulted in a reduction in droplet size from >100 to 7.4 lm. Table II also shows that the droplet size does not reduce after 2% PGPR, owing to the limitation of the process to produce smaller droplets. The SEM micrographs (Fig. 3) show droplets embedded within a continuous network. The continuous network provides network stabilization which has been previously described in the literature [14]. The network stabilization explains the stability against coalescence as the wax crystals provide a physical barrier, preventing movement. From the image it is unclear whether the wax particles are in the interface (displacing emulsifier during crystallization) or at the interface (moving towards the interface during crystallization). These images are similar to those previously published by Norton et al. [3] for chocolate. The droplet size observed in the micrographs are comparable with those obtained by the NMR. The effect of PGPR on long-term coalescence stability was investigated over a period of 180 days. Samples were produced with varying PGPR concentrations and various aqueous phase volumes, and the droplet size was measured. As can be seen in Table III, droplet size does not vary over a period of 180 days for any of the Table I Theoretical surface coverage values (m2 g1) for emulsifiers used, calculated using mean droplet size measured by NMR restricted diffusion (values of 23, 25 and 5 lm were used for monoglycerides, sorbitan olivate and polyglycerol polyricinoleate (PGPR), respectively), and head group size of the emulsifiers taken from (a) Choi, Lee, Kim and Kim [18], (b) calculated using ChemDraw (CambridgeSoft, U.K.) and (c) calculated from hydronamic radii of PGPR taken from [19]
15 10 5 0 0
1
2
3
4
5
6
Emulsifier
Surface coverage (m2 g−1)
Amount required (mg)
Amount added (mg)
Monoglyceridesa Sorbitan Olivateb PGPRc
500 8207 37800
1.57 0.09 0.9
600 600 600
Emulsifier concentration (%) Figure 2 Mean droplet diameter (d3,2, lm) of 10% water W/O emulsions, as measured by NMR restricted diffusion on day of emulsification, as a function of emulsifier concentration for sorbitan olivate (●), monoolein (■), monostearate (♦) and polyglycerol polyricinoleate (▲).
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© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Table II Mean droplet diameter (d3,2, lm) measured by NMR restricted diffusion on day of formation as a function of aqueous phase volume and polyglycerol polyricinoleate (PGPR) concentration. Standard deviation is of triplicate measurements
Phase (%)
10
30
PGPR concentration (%)
d3,2 (lm)
Standard deviation
0.2 0.5 1 2 5 0.2 0.5 1 2 5
5.3 4.6 4.4 2.4 2.5 >100 14.2 6.4 2.9 2.6
1.5 1.1 0.8 0.2 0.1 – 3.3 0.6 0.3 0.1
Aq Phase (%)
20
40
PGPR concentration (%)
d3,2 (lm)
Standard deviation
0.2 0.5 1 2 5 0.2 0.5 1 2 5
>100 5.8 3.4 2.8 2.3 >100 >100 3.4 2.7 2.8
– 1.8 0.2 0.1 0.1 – – 0.0 0.4 0.1
(b)
(a)
Acc.V Spot Magn Det WD 5.00 kV 3.0 28991x SE 6.3
2 µm
Acc.V Spot Magn Det WD 5.00 kV 3.0 13678x SE 5.0
5 µm
Figure 3 Cyro-SEM micrographs of water droplets surrounded by a hydrophobic continuous phase where (a) is 10% aqueous phase with 2% polyglycerol polyricinoleate (PGPR) and (b) 40% aqueous phase with 2% polyglycerol polyricinoleate. Table III Mean droplet diameter (d3,2, lm) measured by NMR restricted diffusion from day 0 to day 180 for emulsions produced with varying polyglycerol polyricinoleate (PGPR) concentrations (0.5, 1, 2 and 5 wt%). All emulsions contain 30% water
Day 0
Day 1
Day 7
Day 10
Day 180
PGPR concentration (%)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
0.5 1 2 5
14.2 6.4 2.9 2.6
3.3 0.6 0.3 0.1
14.0 6.1 2.9 2.5
3.2 0.6 0.3 0.3
14.2 6.1 2.8 2.5
3.2 0.6 0.3 0.0
14.1 6.1 2.8 2.3
3.4 0.6 0.3 0.1
13.8 5.8 2.8 2.2
3.2 0.6 0.4 0.2
formulations, indicating long-term stability. The stability is caused by a combination of the emulsifier and network stabilization. Pawlik et al. [20] in 2010 showed that emulsions made with only PGPR are not stable after 1 month. This would suggest that in this case, the stability of the emulsions can be attributed to the network stabilization, although the systems studied are different. Future work should eliminate the effects by storing samples above the melting point of the continuous wax phase. Material properties The introduction of water droplets will result in defects throughout the microstructure. These defects will affect the mechanical properties
of the emulsion. Therefore, the material properties (Young’s modulus) of both the continuous phase (wax blends containing carnauba wax and microcrystalline wax.) and the emulsion (containing different aqueous phase volumes and different continuous phases (various wax to oil ratios)) were investigated. It is important to note that either paraffin or performalene were added to the continuous phase of the emulsion in an attempt to restore the mechanical properties that were lost on the addition of an aqueous phase (discussed later). Four samples for each continuous phase blend (containing castor oil (40–95 wt%), carnauba wax (0–20 wt%) and microcrystalline wax (0–20 wt%).) were compressed using a texture analyser. The data (force/distance) were then converted into true stress and true
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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0.025 Youngs Modulus Point of Fracture
0.04
0.020
0.03
0.015
0.02
0.010
0.01
0.005
0.00 5
0
10
15
20
Pint of fracture (M Pa)
Youngs modulus (M Pa)
(a) 0.05
0.000 25
% of Carnauba wax 0.025 Youngs Modulus Point of Fracture
0.04
0.020
0.03
0.015
0.02
0.010
0.01 R2 = 0.97
0.005
R2 = 0.97
0.00 0
5
10
15
20
Point of fracture (M Pa)
Youngs modulus (M Pa)
(b) 0.05
0.000 25
0.1
log youngs modulus (M Pa)
strain (using equations 2–5), and the Young’s modulus was calculated using a method described by Norton et al.[10]. Figure 4(a) shows the relationship between carnauba wax concentration and both Young’s modulus and point of fracture: as the percentage of carnauba wax increases from 5% to 20%, Young’s modulus increases from 0.001 to 0.04 MPa, respectively. This is similar to the relationships observed in polymer gels. Li (2002) showed power law dependencies (103.03) of methylcellulose in water as concentration of methylcellulose increased [21]. Figure 5 shows a power law dependency of 3 for Young’s modulus as a function of wax concentration. This is indicative of a stronger wax network being formed, which is similar to work published for polymer gels. Nakayama et al. (2004) showed that by increasing the amount of cross-linking, the modulus increases, indicating a critically cross-linked network [22]. This behaviour has also been observed in fat crystal networks. Where various studies have shown that the hardness of a fat crystal network follows a power law relationship in regard to solid fat content [23–25]. Carnauba wax consists of a complex mixture of high molecular weight esters of acids and hydroxy acids [26]. Thus, by increasing the carnauba wax concentration, the intrinsic bonding between crystals can increase, providing greater connections, which strengthens the network. During the moulding process, samples with just microcrystalline wax were too soft to form the mould required to conduct the
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Slope ~ 3 0.001
0.0001 1
10
100
log CW Wax percentatge (%) Figure 5 Dependence of Young’s modulus on carnauba wax (CW) concentration (wt%). The solid line is a fit to represent the power law dependence.
experiment. Therefore, 5 wt% carnauba wax was added to 5–20 wt% microcrystalline wax to investigate the effect of microcrystalline wax on material properties. Figure 4(b) shows a negative correlation (R2 = 0.97) for increasing the amount of microcrystalline wax from 5 wt% to 20 wt%, resulting in the Young’s modulus decreasing from 0.005 to 0.001 MPa. Microcrystalline wax consists of iso-alkanes and naphthene containing alkanes. Owing to the large amount of branched and naphthenic hydrocarbons in microcrystalline waxes, they mainly form small irregular crystals during crystallization [27]. These irregular crystals can disrupt the strength of the crystal network resulting in a weaker structure. The disruption in crystal network can also be observed by a shift in the onset temperature for melting. As microcrystalline wax concentration increases from 5 wt% to 20 wt%, the onset temperature shifts from 33 to 23°C (Table IV, Fig. 6), suggesting the existence of mixed crystals. This agrees with work conducted in the dental field on dental waxes, which shows that microcrystalline wax weakens the structure [28]. Figure 7 shows that the introduction of water droplets (in the range of 2–4 lm) into the formulation lowers the Young’s modulus from 0.03 MPa to 0.0015 MPa and decreases the point of fracture from 0.003 MPa to 0.001 MPa, which is a result of water (soft filler particles) being introduced into the microstructure, wax crystals moving from the continuous network to the W/O interface and an overall reduction in the solid wax content [29]. The introduction of water droplets reduces Young’s modulus from 0.0026 to 0.0014 MPa which could be owing to crystals
Table IV Onset (Tonset), peak (Tpeak 1 and Tpeak 2) and end (Tend) temperatures as a function of microcrystalline wax (MW) concentration (left)
Micro crystalline wax (%)
Tonset (°C)
5 10 15 20
33.8 30.5 26.5 23.5
% of Microcrystalline wax Figure 4 Young’s modulus (MPa) and point of fracture (MPa) of wax combinations containing either (a) carnauba wax or (b) microcrystalline wax (with 5% carnauba wax). All samples were melted and stirred using a magnetic stirrer until molten (~ 30–40 min) and cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
0.01
0.8 0.7 0.7 2.1
Tpeak
60.2 61.7 62.8 62.5
1
(°C)
Tpeak
2
(°C)
Tend (°C)
0.7 0.4 0.2 0.3
75.8 72.6 73.2 72.3
0.3 0.5 0.3 0.5
80 78.7 78.9 78.5
0.4 0.6 0.4 1
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Temperature (°C) Figure 6 Differential Scanning Calorimeter (DSC) curves for same samples (right); all samples contain 5% carnauba wax (CW). All samples were measured using a DSC from 10 to 120°C at 10°C/min.
moving to the interface. However, increasing aqueous phase volume does not seem to have an impact on the Young’s modulus. Two different waxes (paraffin and performalene) were added into the continuous phase formulation to investigate their effect on the overall mechanical properties of the emulsion. Performalene is a polyethylene molecule that forms small crystals, which allow an increase in number of connections in the wax network. Hard paraffin is a straight-chain hydrocarbon [30] and was chosen as previous literature has shown that paraffin increases strength of emulsions [5], by producing large crystals which will not travel to W/O interface, resulting in a stronger continuous wax network. When Fig. 7 (emulsions containing carnauba and microcrystalline waxes) are compared with Fig. 8 (the addition of solid wax), the data show an increase in Young’s modulus and point of fracture with the addition of solid wax (particularly performalene). For example, for emulsions containing 10 wt% aqueous phase, the
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Aqueous phase (%) Figure 7 Young’s modulus (MPa) and point of fracture (MPa) of emulsions containing 2 wt% polyglycerol polyricinoleate as a function of aqueous phase volume (10–40wt%), where the continuous phase contains 5% carnauba wax and 10% microcrystalline wax. All emulsions were produced using a Silverson high shear mixer (for 5 min and at ~10 000 g) and cooled quiescently in the freezer till solid and measured with a compression rate of 1 mm s1 at 32°C.
Figure 8 Young’s modulus (YM) (MPa) and point of fracture (PF) (MPa) of emulsions containing 2 wt% polyglycerol polyricinoleate as a function of aqueous phase volume (10–40 wt%) where the continuous phase contains 5% carnauba wax and 10% microcrystalline wax, and either 5% paraffin (■) or 5% performalene (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
addition of performalene increases the Young’s modulus from ~ 0.003 MPa to ~ 0.005 MPa, and the point of fracture from ~ 0.001 to ~ 0.005 MPa, indicating a stiffer emulsion. Interestingly, the emulsions containing 10 wt% aqueous phase and performalene are stronger than the control without performalene (i.e. the nonemulsified bulk wax system): the Young’s moduli are ~0.005 MPa and ~0.0038 MPa, respectively, and the point of fractures are ~0.005 MPa and ~0.003 MPa, respectively. The samples containing 20 and 30 wt% aqueous phase and performalene are more similar to the control formulation in terms of both Young’s modulus and point of fracture, but there is some weakening of the structure for samples containing 40 wt% water. The droplet size for all emulsions produced were similar (in the range of 2–4 lm); therefore, it is thought that performalene produces a greater number of connections in the wax network resulting in an emulsion that is more resistant to compression, up to a point where the number of droplets (i.e. ‘defects’) increases and reduced the mechanical strength of the structure. However, it should be noted that the error bars are large. Overall, the results show that the introduction of water lowers both Young’s modulus and point of fracture, resulting in a weaker structure. This observation concurs with previously reported data on lipsticks and foods, which showed that the introduction of water droplets decreases strength [12, 13]. Wang and Lee reported that when the water content was increased from 5 to 15 wt%, the strength increased [12]. The results obtained in this research show that the Young’s modulus does not increase as water content increases from 10 to 40 wt%. Therefore, the differences between samples must be owing to a difference in the wax crystal network; hence, the rheological properties of the emulsion were investigated. Both the elastic modulus (G’) and the viscous modulus (G”) were investigated directly after emulsion formation. Previous literature has shown that by increasing the number of crystals, a more rigid network is produced [5]. Therefore, G’ (i.e. solid-like behaviour) should increase with the addition of crystalline material (paraffin and performalene). Figure 9 shows that there is a negative correlation between aqueous phase volume and G’ for all the waxes investigated. This is owing to the solid wax content decreasing as aqueous phase
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Water phase (%) Figure 9 G’ (MPa) (solid symbols) and G’’ (open symbols) as a function of aqueous phase volume for emulsions containing 2 wt% polyglycerol polyricinoleate, where the continuous phase contains 5% carnauba wax (CW) and 10% microcrystalline wax (MW) (●), and either 5% paraffin (P) (■) or 5% performalene (PF) (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid and measured via oscillation rheology. G’ value taken at a strain of 1.4 9 10^-5 and a frequency of 5 Hz.
volume increases, resulting in fewer connections between wax moieties. The addition of either paraffin or performalene increases G’ (from 0.4 to 0.8 and 1.3 MPa, respectively, for 10 wt% aqueous phase) with performalene increasing G’ by a greater amount for all aqueous phase volumes. This could be attributed to its ability to create connections in the crystal network, thereby increasing the strength of the crystal network. The reduction in G’ at greater aqueous phase volumes is caused by the surface area of the w/o interface increasing with increasing aqueous phase volume, resulting in crystals from the continuous network moving to the interface during cooling. As a result, a rigid crystal network cannot be formed. Figure 9 also shows that for all emulsions tested, G’ is greater than G’’ indicating that the emulsion behaves more like a solid. On further analysis of both G’ and G’’, the phase angle can be calculated (d). Figure 10 shows that as the aqueous phase volume increases (from 10% to 40%), the d increases from 5 to 15°. It is known that if the phase angel is between 0 and 90°, then the
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Figure 10 Phase angle As a function of aqueous phase volume for emulsions containing 2 wt% polyglycerol polyricinoleate, where the continuous phase contains 5% carnauba wax (CW) and 10% microcrystalline wax (MW) (●), and either 5% paraffin (P) (■) or 5% performalene (PF) (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid and measured via oscillation rheology.
5% CW, 10% MW and 85% CO 5% CW, 10% MW, 5% P and 80% CO 5% CW, 10% MW, 5% PF and 80% CO
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material can be classified as viscoelastic [11]. From Fig. 10, we can conclude that for all aqueous phases, the emulsion behaves viscoelastically; however, at higher aqueous phase volumes, the emulsion behaves slightly more viscously. This can be attributed to the fact that the existence of a crystal network decreases as the water content increases. This is likely to be caused by a combination of (i) an increase in the number of droplets (and thus the size of the w/o interface) with increasing aqueous phase volume, creating an increasing number defects in the structure and (ii) movement of crystals from the bulk to the interface, weakening the network. Conclusion The aims of this work were to (i) investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size, and (ii) investigate the effect of wax ratio (carnauba wax and microcrystalline wax) and aqueous phase volume on material properties (Young’s modulus, point of fracture, elastic modulus and viscous modulus). Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (~18– 25 lm). Polyglycerol polyricinoleate resulted in emulsions with the smallest droplets (~3–5 lm), as a result of a thick elastic interface. The results also showed that both Young’s modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax. This suggests that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young’s modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. Although the addition of water droplets (typically in the size range of 2–4 lm) weakens the structure to compressibility, the addition of solid wax (particularly performalene) increases strength,
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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particularly for the lower aqueous phase volume emulsions produced. Specifically, the emulsions containing up to 30 wt% aqueous phase had very similar mechanical properties to the nonemulsified control. Rheological data showed that all emulsions behaved as viscoelastic solids regardless of the aqueous phase volume, with the samples becoming slightly more viscous as the aqueous phase volume increased. Thus, the work suggests the potential for the use of emulsions in the formulation of lipstick products. Future work should consider the moisturizing or lubricating properties of emulsions within wax continuous systems and possibly
consider the release of hydrophilic compounds from the aqueous phase under simulated shear conditions similar to those experienced during lipstick application. Acknowledgement The authors thank Alliance Boots PLC for providing materials as well as the EPSRC for funding. The authors would also like to thank Dr Antonio Sullo (University of Birmingham) for helpful scientific discussion.
References 1. Rousseau, D. Fat crystals and emulsion stability - a review. Food Res. Int. 33, 3–14 (2000). 2. Aguilera, J.M., Stanley, D.W., eds. Microstructural Principles of Food Processing and Engineering. 2nd ed. Springer, London (1999). 3. Norton, J.E., Fryer, P.J., Parkinson, J. and Cox, P.W. Development and characterisation of tempered cocoa butter emulsions containing up to 60% water. J. Food Eng. 95, 172–178 (2009). 4. Rousseau, D. and Hodge, S.M. Stabilization of water-in-oil emulsions with continuous phase crystals. Colloids Surf. A Physicochem. Eng. Asp. 260, 229–237 (2005). 5. Le Reverend, B.J.D., Taylor, M.S. and Norton, I.T. Design and application of water-in-oil emulsions for use in lipstick formulations. Int. J. Cosmet. Sci. 33, 263–268 (2011). 6. Ghosh, S. and Rousseau, D. Freeze-thaw stability of water-in-oil emulsions. J. Colloid Interface Sci. 339, 91–102 (2009). 7. Kulkarni, C.V., Wachter, W., Iglesias-Salto, G., Engelskirchen, S. and Ahualli, S. Monoolein: a magic lipid? Phys. Chem. Chem. Phys. 13, 3004–3021 (2011). 8. Dickinson, E. Food emulsions and foams: stabilization by particles. Curr. Opin. Colloid Interface Sci. 15, 40–49 (2010). 9. Melle, S., Lask, M. and Fuller, G.G. Pickering Emulsions with Controllable Stability. Langmuir 21, 2158–2162 (2005). 10. Norton, A.B., Cox, P.W. and Spyropoulos, F. Acid gelation of low acyl gellan gum relevant to self-structuring in the human stomach. Food Hydrocolloids 25, 1105–1111 (2011). 11. Tabilo-Munizaga, G. and Barbosa-C anovas, G.V. Rheology for the food industry. J. Food Eng. 67, 147–156 (2005).
12. Wang, T.S. and Lee, G. The effect of formulation on the hardness and crystallization of emulsion lipsticks. J. Soc. Cosmet. Chem. 48, 41–50 (1997). 13. Vereecken, J., Meeusssen, W., Lesaffer, A. and Dewettinck, K. Effect of water and monoglyceride concentration on the behaviour of monoglyceride containing fat systems. Food Res. Int. 43, 872–881 (2010). 14. Ghosh, S. and Rousseau, D. Fat crystals and water-in-oil emulsion stability. Curr. Opin. Colloid Interface Sci. 16, 421–431 (2011). 15. van Duynhoven, J.P.M., Goudappel, G.J.W., van Dalen, G., van Bruggen, P.C., Blonk, J.C.G. and Eijkelenboom, A. Scope of droplet size measurements in food emulsions by pulsed field gradient NMR at low field. Magn. Reson. Chem. 40, S51–S59 (2002). 16. Moresi, M. and Bruno, M. Characterisation of alginate gels using quasi-static and dynamic methods. J. Food Eng. 82, 298–309 (2007). 17. Mackles, L., inventor Product Resources International, assignee. Aerosol Foam. USA (1987). 18. Choi, J.H., Lee, H.Y., Kim, J.-C. and Kim, Y.C. Monoolein Cubic Phases Containing Hydrophobically Modified Poly(N-isopropylacrylamide). J. Ind. Eng. Chem. 13, 380–386 (2007). 19. Gunes, D.Z., Clain, X., Breton, O., Mayor, G. and Burbidge, A.S. Avalanches of coalescence events and local extensional flows – Stabilisation or destabilisation due to surfactant. J. Colloid Interface Sci. 343, 79–86 (2010). 20. Pawlik, A., Cox, P.W. and Norton, I.T. Food grade duplex emulsions designed and stabilised with different osmotic pressures. J. Colloid Interface Sci. 352, 59–67 (2010).
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21. Li, L. Thermal Gelation of Methylcellulose in Water: scaling and Thermoreversibility. Macromolecules 35, 5990–5998 (2002). 22. Nakayama, A., Kakugo, A., Gong, J.P., Osada, Y., Takai, M., Erata, T., and Kawano, S. High mechanical strength double-network hydrogel with bacterial cellulose. Adv. Funct. Mater. 14, 1124–1128 (2004). 23. Narine, S.S. and Marangoni, A.G. Relating structure of fat crystal networks to mechanical properties: a review. Food Res. Int. 32, 227–248 (1999). 24. Haighton, A.J. Blending, chilling, and tempering of margarines and shortenings. J. Am. Oil Chem. Soc. 53, 397–399 (1976). 25. Narine, S.S. and Marangoni, A.G. Fractal nature of fat crystal networks. Phys. Rev. E 59, 1908–1920 (1999). 26. Lacerda, S.P., Cerize, N.N.P. and Re, M.I. Preparation and characterization of carnauba wax nanostructured lipid carriers containing benzophenone-3. Int. J. Cosmet. Sci. 33, 312–321 (2011). 27. Petersson, M., Gustafson, I. and Stading, M. Comparison of microstructural and physical properties of two petroleum waxes. J. Mater. Sci. 43, 1869–1879 (2008). 28. McCabe, J. and Wallis, A. Applied Dental Materials. 9th ed. Wiley-Blackwell, Oxford (2008). 29. Shrinivas, P., Kasapis, S. and Tongdang, T. Morphology and Mechanical Properties of Bicontinuous Gels of Agarose and Gelatin and the Effect of Added Lipid Phase. Langmuir 25, 8763–8773 (2009). 30. Wang, J.F., Calhoun, M.D. and Severtson, S.J. Dynamic rheological study of paraffin wax and its organoclay nanocomposites. J. Appl. Polym. Sci. 108, 2564–2570 (2008).
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doi: 10.1111/ics.12085
Effect of emulsifier type and concentration, aqueous phase volume and wax ratio on physical, material and mechanical properties of water in oil lipsticks A. Beri, J. E. Norton and I. T. Norton Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
Received 12 April 2013, Accepted 27 July 2013
Synopsis OBJECTIVES: Water-in-oil emulsions in lipsticks could have the potential to improve moisturizing properties and deliver hydrophilic molecules to the lips. The aims of this work were (i) to investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size and (ii) to investigate the effect of wax ratio (carnauba wax, microcrystalline wax, paraffin wax and performalene) and aqueous phase volume on material properties (Young’s modulus, point of fracture, elastic modulus and viscous modulus). METHODS: Emulsion formation was achieved using a high shear mixer. RESULTS: Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (droplet size ~18–25 lm). Polyglycerol polyricinoleate (PGPR) resulted in emulsions with the smallest droplets (~3–5 lm), as expected from previous studies that show that it produces a thick elastic interface. The results also showed that both Young’s modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax, suggesting that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young’s modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. CONCLUSIONS: This work suggests the potential use for emulsions in lipstick applications, particularly when PGPR is used as an emulsifier, and with the addition of solid wax, as it increases network strength. sume Re OBJECTIFS: Les emulsions eau dans huile dans les rouges a levres pourraient avoir le potentiel pour ameliorer les proprietes hydratantes et delivrer des molecules hydrophiles aux levres. Les objectifs de ce travail etaient: (i) d’etudier l’effet du type d’emulsifiant (polymere vs monomere, et de la cha^ıne saturee vs. insatures) et de la Correspondence: Akash Beri, Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK. Tel.: +44(0)1214145082; fax: +44(0) 1214145377; e-mail: [email protected]
concentration sur la taille des gouttelettes, et (ii) d’etudier l’effet du taux de cire (cire de carnauba, la cire microcristalline, de la cire de paraffine et PERFORMALENE) et du volume de la phase aqueuse sur les proprietes du materiau (module de Young, le point de rupture, le module elastique et module visqueux). METHODES: la formation d’emulsion a ete realisee en utilisant un cisaillement eleve. melangeur a RESULTATS: Les resultats ont montre que la nature saturee de l’emulsifiant a eu tres peu d’effet sur la taille des gouttelettes, pas plus que l’utilisation d’un emulsifiant avec un groupe de t^ete plus grosse (taille des gouttelettes ~ 18–25 lm). Le polyricinoleate de des emulsions des plus petites polyglycerol (PGPR) a conduit a partir d’etudes anterieures gouttelettes (~ 3–5 lm), comme prevu a qui montrent que cela produit une interface elastique epaisse. Les resultats ont egalement montre que le module de Young et le point de fracture augmentent avec un pourcentage croissant de la cire de carnauba (selon a une dependance en loi de puissance de 3), mais diminuent avec un pourcentage croissant de la cire microcristalline, suggerant que la cire de carnauba est inclus dans l’ensemble du reseau de cire forme par les composants satures, tandis que la cire microcristalline forme des cristaux irreguliers qui perturbent l’ensemble du reseau. Le module de Young, le module elastique et le module visqueux diminuent avec l’augmentation du volume de la phase aqueuse dans l’emulsion, bien que la pente de la diminution de modules elastiques et visqueux soit fonction de l’addition de cire solide, en consequence du renforcement du reseau. CONCLUSIONS: Ce travail suggere l’utilisation potentielle des emulsions dans des applications de rouge a levres, surtout quand PGPR est utilisee comme emulsifiant, et avec l’ajout de cire solide, car elle augmente la force du reseau. Introduction Lipsticks form an intrinsic part of most cosmetic companies product range and typically consist mainly of hydrophobic ingredients (e.g. waxes, pigments and oils). The high solid wax and pigment content provide the lipstick with material properties, for example strength, rigidity and elasticity; however, the excessive use of hydrophobic ingredients can result in dryness of lips owing to layering the lip with a hydrophobic layer preventing natural lubrication (from saliva or water vapour in the atmosphere). Emulsions could be used to deliver moisture to the lips, whilst maintaining material properties. Water-in-oil (W/O) emulsions are used in many industries, including food, cosmetics, pharmaceuticals and insecticides [1].
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In the food industry, various fats/oils, including milk fats [2], cocoa butter [3] and vegetable oils [4], have been used as the continuous phase to produce W/O emulsions. When producing an emulsionbased lipsticks, there are two critical parameters to be considered: (i) physical properties (e.g. droplet size; small droplets can increase the release rate of any functional ingredients within the aqueous phase as the surface area is increased [5] and the processes of destabilization can be slowed) and (ii) material properties (e.g. Young’s modulus and elastic modulus). These parameters can be controlled by many factors, including the emulsifier used, the aqueous phase volume and the properties of the continuous lipid phase. Emulsifiers are required to reduce the interfacial tension between the two phases, which enhances droplet break-up and delays coalescence during emulsification. Emulsifiers can be polymeric (e.g. polyglycerol polyrinceoleate (PGPR)) or monomeric (e.g. monoglycerides and sorbitan olivate). Polymeric emulsifiers stretch across the interface creating a physical barrier against coalescence [6]. Some monomeric emulsifiers (sorbitan olivate) sit at the interface, whereas monoglycerides (monoolein) can crystallize at the interface causing Pickering stabilization, which has been well documented in literature [1, 7–9]. The mechanical properties, such as strength (maximum stress before failure), hardness (ability to withstand surface indentation), rigidity/stiffness (resistance to deformation in response to applied force), plasticity (ability to undergo irreversible deformations) and elasticity (ability to return to original shape after deformation), are governed by the structure of the network, including the number of cross-links and defects. Typically, mechanical properties can be measured using texture analysis or rheology. Following texture analysis, a stress–strain curve is plotted, indicating the amount of strain (deformation) at increasing stresses (loading). A number of different measures of mechanical properties can be taken, including the Young’s modulus/tensile modulus (measure of the stiffness; the slope of the initial linear region of the stress–strain curve), bulk modulus/compressibility (resistance to uniform compression; the slope of the second linear region of the stress–strain curve), point of failure/fracture and total work of failure (area under the curve) [10]. Similarly, oscillatory rheology (study of flow) can be used to characterize samples, providing values of elastic/shear storage modulus (G’), viscous/shear loss modulus (G”) and the loss factor (tan d). G’ is a measure of the deformation energy which is stored within a sample during shear, thus showing the elastic (solid-like) behaviour. G” is the deformation energy that is dissipated as heat during shear and indicates viscous (liquid-like) behaviour. The loss factor is a ratio of loss to storage moduli (tan d = G”/G’) and gives an indication of viscoelastic properties. A phase angle (d) of 0° is indicative of an elasticity, whereas a phase angle of 90° represents a purely viscous material [11]. Mechanical properties can be manipulated or controlled by the ratio of different waxes used in the formulation, and in this case the amount and size of the water droplets within the emulsion. The introduction of water into the structure in the form of water droplets has varying effects, as described in the literature. Although Wang and Lee (1997) reported that increasing water content (5– 15%) increased hardness [12], Le Reverend et al. [5] showed that the hardness (measured using penetration) decreases with increasing water content (10–40%, droplet size ~ 5 lm). The latter agreed with literature in the food industry [5, 13]. Le Reverend et al. (2011) suggested that wax crystals move to the W/O interface from the continuous phase, which weakened the structure. In addition, the solid nature of the continuous phase should result in
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long-term stability, slowing down the processes of coalescence and phase separation by trapping water droplets within the wax network [14]. In this work, we investigated (i) the effect of concentration for four emulsifiers (Sorbitan olivate, monolein, monostearate and PGPR) on the emulsion droplet size for a variety of aqueous phase volumes (10–40 wt%) (ii) the effect of emulsifier concentration on long-term stability (iii) the effect of different wax ratios (carnauba wax and microcrystalline wax) on elasticity and point of fracture and (iv) the effect of introducing water droplets on the elastic and rheological properties. Monoolein and monostearate were compared with investigate the effect that the nature of saturation of monoolein has on the steric hindrance at the interface. Sorbitan olivate was then compared with the monoglycerides to investigate the effect head group size. The monomeric emulsifiers were then compared with a polymeric (PGPR) emulsifier. It is important to note that for this paper a model system (without pigments) was investigated to understand the role of waxes on material properties. Materials and methods Materials All materials (castor oil, carnauba wax, microcrystalline wax, hard paraffin and performalene) used to produce the continuous phase of emulsions were supplied by Alliance Boots PLC (U.K.). These were used in combination with double distilled water and various emulsifiers: PGPR (HLB 1.5) (Palsgaard, Denmark), sorbitan olivate (HLB 4.3) (Aston Chemicals, U.K.), monoolein (HLB 3.6) (Cargill, Netherlands) or monostearate (HLB 3.2) (Cargill, Netherlands) to produce emulsions. Emulsion preparation A continuous phase formulation containing castor oil (40–95 wt %), carnauba wax (0–20 wt%), microcrystalline wax (0–20 wt%) and paraffin (5 wt%) or performalene (5 wt%) was melted with either PGPR, sorbitan olivate, monoolein or monostearate (0.2–5 wt%) and aqueous phase (distilled water, 10–40 wt%) in varying quantities. The pre-mixture was emulsified for 5 minutes using a Silverson L4RT (Silverson Machines Ltd, U.K.) high shear mixer at ~75°C, fitted with a fine emulsifier screen (pores ~1 mm) at ~10 000 g. Following emulsification, the emulsion was placed in a freezer for 20 min (cooling rate: 2.4 0.2°C min1) to allow for the continuous phase to crystallize. Determination of droplet size Droplet size measurements were performed using a pulsed field gradient NMR equipped with a water droplet size application (restricted diffusion) (minispec mq series, Bruker Optics, U.K.) at ~5°C to measure the volume-weighted mean droplet diameter (d3,3). The use of NMR for droplet size determination in W/O emulsions has been well documented by van Duynhoven et al. [15], and is beneficial as its non-invasive, and can measure opaque samples. It should be noted, however, that the technique assumes a log-normal droplet size distribution and spherical droplets. The samples were poured into 10 mm NMR tubes and filled to a height of 10 mm. Samples were then placed into a freezer for 20 min. All NMR analysis was conducted either on day 0, 1, 7, 10 and 180 after manufacture (where day 0 is day of manufacture).
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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The mean droplet size (d3,2) was then calculated using equation 1 [15]:
(a)
2
ð1Þ
where d3,2 is the surface-weighted mean droplet diameter, d3,3 is the volume-weighted mean droplet diameter, and r is the standard deviation of the logarithm of the droplet diameter. Microscopy Cyro-scanning electron microscopy was used to visualize the emulsions. A XL-30ESEM (Philips, Eindhoven, the Netherlands) was used for the analysis. The structure of the sample was preserved using cryogenic temperatures (below 150°C). The sample was attached onto a sample holder and rapidly frozen by immersion into a liquid nitrogen bath for approximately 2 min. The sample was then transferred to the high vacuum cryo-unit chamber and freezefractured. A thin layer of platinum was scattered onto the surface. The sample was the moved into the observation chamber, and observations were carried out at 3–5 kV at temperature between 100 and 175°C. Texture profile analysis The mechanical properties of both continuous phase formulations and emulsions were determined by compression. This was conducted using a TaXT+ (Stable Microsystems, Surrey, U.K.) texture analyser. Measurements involved using a standard compression plate (SMS P/40) with a 40 mm diameter cylindrical aluminium probe. All samples had a diameter of 20 mm, and their length was kept at ~20 mm. All measurements were carried out in quadruplet with a compression speed of 1 mm s1. The data (force/distance) obtained from the texture analyser were converted into true stress and true strain using equations 2–5 (obtained from Moresi & Bruno, 2007 [16]); eE ¼
Ho h Ho
eH ¼ lnð1 eE Þ rE ¼
F Ao
rH ¼ rE ð1 þ eH Þ
ð2Þ ð3Þ ð4Þ ð5Þ
where eE and eH are the engineering and true strain, respectively, Ho and h are initial height and height of each sample as recorded during the compression test, rE and rH are the engineering and true stress, respectively, and F and Ao are compression force applied and initial cross sectional area of sample. From the true stress/true strain curves (typical curves shown in Fig. 1), the Young’s modulus (as shown by Norton et al.,[10]) was taken (the slope of the first linear region of the stress–strain curve) and plotted against either percentage of wax or percentage of water to give an insight into the material properties of the formulations. Determination of melting properties The melting properties of continuous phase formulations were investigated using a Differential Scanning Calorimeter (Perkin Elmer DSC Series 7, Cambridge, U.K.), equipped with thermal analysis software (Pyris). The instrument was calibrated for
10% CW and 90 % CO 10% CW and 90 % CO
0.002
0.001
0.000 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.08
0.10
0.12
True strain (b) 0.003
True stress (M Pa)
d3;2 ¼ d3;3 e0:5r
True stress (M Pa)
0.003
5% CW, 5% MW and 90 % CO 5% CW, 10% MW and 85 % CO
0.002
0.001
0.000 0.00
0.02
0.04
0.06
True strain Figure 1 Typical True Stress (MPa) vs. True strain curves for bulk oil formulations, where (a) is 10% carnauba wax (CW) in castor oil (CO), for measurement 1 (●) and measurement 2 (○), and (b) is 5% microcrystalline wax (MW) and 5% CW was in CO (●) and 10% MW and 5% CW was in CO (○). All samples were melted and stirred using a magnetic stirrer until molten (~ 30–40 min) and then cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
temperature using indium and tin, with an empty aluminium pan as a reference. Samples were loaded into 50 lL capacity aluminium pans and sealed with aluminium covers. Pans were heated at a rate of 10°C min1, from a range of 10 to 120°C. Rheology The elastic modulus (G’) and viscous modulus (G”) were determined to investigate the effect of waxes on the viscoelastic properties of an emulsion. G’ and G” were measured using a Bohlin Gemini Nano Rheometer (Malvern, U.K.), and a 20 mm parallel plate (1 mm gap width) geometry. To allow analysis to be carried out in the linear viscoelastic region (LVR), an initial amplitude sweep experiment was conducted to calculate the appropriate stress/strain to use for a frequency sweep experiment. All experiments were run at 32°C (lip temperature). All samples were prepared immediately after production by pouring molten solutions into cylindrical moulds and cooled in a freezer for 20 min. Thin slices (~1 mm in thickness and 20 mm in diameter) were placed in a parallel plate geometry for testing.
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Results and discussion Physical properties A series of experiments were conducted to investigate the effect of both emulsifier type and concentration on droplet size. Emulsification were conducted at 75°C to ensure that the waxes were molten, thus preventing Pickering particles (i.e. wax crystals) stabilizing the emulsion. Different monomeric (monoolein and monostearate) emulsifiers were compared with investigate the difference between saturated and unsaturated chains [7, 17]. These were then compared with sorbitan olivate to explore the effect of head group size. These were compared with a polymeric emulsifier (PGPR). Figure 2 shows the droplet size (d3,2, lm) as a function of emulsifier type and concentration. Results show that for emulsions formed using monoglycerides the droplet size was approximately 20–28 lm, regardless of the emulsifier concentration. Results also indicate that there is no difference in droplet size when using a saturated or unsaturated monoglyceride. Sorbitan olivate is a combination of both sorbitan monoolein and sorbitan monostearate (head group ~ 13.9 A, obtained from ChemDraw 12.0). Even though the head group is twice the size of the head group for monoglycerides (5 A, obtained from ChemDraw 12.0), no difference in droplet size was observed for all emulsifier concentrations. However, a greater concentration of sorbitan olivate is required (≥1%) to produce an emulsion with droplets in the same size range (~25 lm). At lower concentrations (0.2–1%), the NMR was unable to measure droplet size, which suggests that the droplets were greater than 100 lm (upper size limit for NMR restricted diffusion). The difference in concentration of emulsifier required could either be owing to (i) insufficient emulsifier for surface coverage, (ii) the ability of the emulsifier to get to the interface during the process, (iii) affinity for the interface. The amount of emulsifier required for surface coverage (assuming a monolayer of emulsifier and spherical droplets) was calculated by dividing the total area of interface by the surface coverage of each. This was compared with the amount Sorbitan Olivate Monoolein
30
Monostearate PGPR
25
d3,2 (µm)
20
added in reality (Table I). It is clear that there is an excess of emulsifier, and therefore, the difference in concentration required is either owing to the emulsifier’s ability to get to the interface or it’s affinity for the interface. Sorbitan olivate has a greater molecular weight (~430 g mol1) than monoglycerides (356 g mol1) and will therefore take longer to diffuse to the interface. If the governing factor was the mass of the emulsifier, one would expect PGPR (~500 g mol1) to produce the largest droplets. However, Fig. 2 illustrates that even at low concentrations of PGPR, droplets are formed in the range of 4–7 lm. This is owing to PGPR’s ability to produce a thick elastic interface preventing coalescence of droplets. The horizontal nature of all the points displayed in Fig. 2 is a result of the system being saturated by emulsifier, as suggested by the calculations displayed in Table I. The effect of different PGPR concentrations and aqueous phase volume on droplet size was investigated by NMR restricted diffusion. Table II shows that for all aqueous phase volumes, droplet size decreases as PGPR concentration increases. At higher aqueous phase volumes (30% and 40%), the lower concentrations of PGPR (0.2 and 0.5 wt%) are unable to stabilize droplets, resulting in larger droplets (>100 lm). This is a result of there being a greater surface area, and not being sufficient time for the emulsifier to diffuse to the interface. To test this hypothesis, the processing time for an emulsion (40% aqueous phase and 0.5% PGPR) was doubled from 5 to 10 min. This resulted in a reduction in droplet size from >100 to 7.4 lm. Table II also shows that the droplet size does not reduce after 2% PGPR, owing to the limitation of the process to produce smaller droplets. The SEM micrographs (Fig. 3) show droplets embedded within a continuous network. The continuous network provides network stabilization which has been previously described in the literature [14]. The network stabilization explains the stability against coalescence as the wax crystals provide a physical barrier, preventing movement. From the image it is unclear whether the wax particles are in the interface (displacing emulsifier during crystallization) or at the interface (moving towards the interface during crystallization). These images are similar to those previously published by Norton et al. [3] for chocolate. The droplet size observed in the micrographs are comparable with those obtained by the NMR. The effect of PGPR on long-term coalescence stability was investigated over a period of 180 days. Samples were produced with varying PGPR concentrations and various aqueous phase volumes, and the droplet size was measured. As can be seen in Table III, droplet size does not vary over a period of 180 days for any of the Table I Theoretical surface coverage values (m2 g1) for emulsifiers used, calculated using mean droplet size measured by NMR restricted diffusion (values of 23, 25 and 5 lm were used for monoglycerides, sorbitan olivate and polyglycerol polyricinoleate (PGPR), respectively), and head group size of the emulsifiers taken from (a) Choi, Lee, Kim and Kim [18], (b) calculated using ChemDraw (CambridgeSoft, U.K.) and (c) calculated from hydronamic radii of PGPR taken from [19]
15 10 5 0 0
1
2
3
4
5
6
Emulsifier
Surface coverage (m2 g−1)
Amount required (mg)
Amount added (mg)
Monoglyceridesa Sorbitan Olivateb PGPRc
500 8207 37800
1.57 0.09 0.9
600 600 600
Emulsifier concentration (%) Figure 2 Mean droplet diameter (d3,2, lm) of 10% water W/O emulsions, as measured by NMR restricted diffusion on day of emulsification, as a function of emulsifier concentration for sorbitan olivate (●), monoolein (■), monostearate (♦) and polyglycerol polyricinoleate (▲).
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Table II Mean droplet diameter (d3,2, lm) measured by NMR restricted diffusion on day of formation as a function of aqueous phase volume and polyglycerol polyricinoleate (PGPR) concentration. Standard deviation is of triplicate measurements
Phase (%)
10
30
PGPR concentration (%)
d3,2 (lm)
Standard deviation
0.2 0.5 1 2 5 0.2 0.5 1 2 5
5.3 4.6 4.4 2.4 2.5 >100 14.2 6.4 2.9 2.6
1.5 1.1 0.8 0.2 0.1 – 3.3 0.6 0.3 0.1
Aq Phase (%)
20
40
PGPR concentration (%)
d3,2 (lm)
Standard deviation
0.2 0.5 1 2 5 0.2 0.5 1 2 5
>100 5.8 3.4 2.8 2.3 >100 >100 3.4 2.7 2.8
– 1.8 0.2 0.1 0.1 – – 0.0 0.4 0.1
(b)
(a)
Acc.V Spot Magn Det WD 5.00 kV 3.0 28991x SE 6.3
2 µm
Acc.V Spot Magn Det WD 5.00 kV 3.0 13678x SE 5.0
5 µm
Figure 3 Cyro-SEM micrographs of water droplets surrounded by a hydrophobic continuous phase where (a) is 10% aqueous phase with 2% polyglycerol polyricinoleate (PGPR) and (b) 40% aqueous phase with 2% polyglycerol polyricinoleate. Table III Mean droplet diameter (d3,2, lm) measured by NMR restricted diffusion from day 0 to day 180 for emulsions produced with varying polyglycerol polyricinoleate (PGPR) concentrations (0.5, 1, 2 and 5 wt%). All emulsions contain 30% water
Day 0
Day 1
Day 7
Day 10
Day 180
PGPR concentration (%)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
d3,2 (lm)
SD (r)
0.5 1 2 5
14.2 6.4 2.9 2.6
3.3 0.6 0.3 0.1
14.0 6.1 2.9 2.5
3.2 0.6 0.3 0.3
14.2 6.1 2.8 2.5
3.2 0.6 0.3 0.0
14.1 6.1 2.8 2.3
3.4 0.6 0.3 0.1
13.8 5.8 2.8 2.2
3.2 0.6 0.4 0.2
formulations, indicating long-term stability. The stability is caused by a combination of the emulsifier and network stabilization. Pawlik et al. [20] in 2010 showed that emulsions made with only PGPR are not stable after 1 month. This would suggest that in this case, the stability of the emulsions can be attributed to the network stabilization, although the systems studied are different. Future work should eliminate the effects by storing samples above the melting point of the continuous wax phase. Material properties The introduction of water droplets will result in defects throughout the microstructure. These defects will affect the mechanical properties
of the emulsion. Therefore, the material properties (Young’s modulus) of both the continuous phase (wax blends containing carnauba wax and microcrystalline wax.) and the emulsion (containing different aqueous phase volumes and different continuous phases (various wax to oil ratios)) were investigated. It is important to note that either paraffin or performalene were added to the continuous phase of the emulsion in an attempt to restore the mechanical properties that were lost on the addition of an aqueous phase (discussed later). Four samples for each continuous phase blend (containing castor oil (40–95 wt%), carnauba wax (0–20 wt%) and microcrystalline wax (0–20 wt%).) were compressed using a texture analyser. The data (force/distance) were then converted into true stress and true
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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0.025 Youngs Modulus Point of Fracture
0.04
0.020
0.03
0.015
0.02
0.010
0.01
0.005
0.00 5
0
10
15
20
Pint of fracture (M Pa)
Youngs modulus (M Pa)
(a) 0.05
0.000 25
% of Carnauba wax 0.025 Youngs Modulus Point of Fracture
0.04
0.020
0.03
0.015
0.02
0.010
0.01 R2 = 0.97
0.005
R2 = 0.97
0.00 0
5
10
15
20
Point of fracture (M Pa)
Youngs modulus (M Pa)
(b) 0.05
0.000 25
0.1
log youngs modulus (M Pa)
strain (using equations 2–5), and the Young’s modulus was calculated using a method described by Norton et al.[10]. Figure 4(a) shows the relationship between carnauba wax concentration and both Young’s modulus and point of fracture: as the percentage of carnauba wax increases from 5% to 20%, Young’s modulus increases from 0.001 to 0.04 MPa, respectively. This is similar to the relationships observed in polymer gels. Li (2002) showed power law dependencies (103.03) of methylcellulose in water as concentration of methylcellulose increased [21]. Figure 5 shows a power law dependency of 3 for Young’s modulus as a function of wax concentration. This is indicative of a stronger wax network being formed, which is similar to work published for polymer gels. Nakayama et al. (2004) showed that by increasing the amount of cross-linking, the modulus increases, indicating a critically cross-linked network [22]. This behaviour has also been observed in fat crystal networks. Where various studies have shown that the hardness of a fat crystal network follows a power law relationship in regard to solid fat content [23–25]. Carnauba wax consists of a complex mixture of high molecular weight esters of acids and hydroxy acids [26]. Thus, by increasing the carnauba wax concentration, the intrinsic bonding between crystals can increase, providing greater connections, which strengthens the network. During the moulding process, samples with just microcrystalline wax were too soft to form the mould required to conduct the
618
Slope ~ 3 0.001
0.0001 1
10
100
log CW Wax percentatge (%) Figure 5 Dependence of Young’s modulus on carnauba wax (CW) concentration (wt%). The solid line is a fit to represent the power law dependence.
experiment. Therefore, 5 wt% carnauba wax was added to 5–20 wt% microcrystalline wax to investigate the effect of microcrystalline wax on material properties. Figure 4(b) shows a negative correlation (R2 = 0.97) for increasing the amount of microcrystalline wax from 5 wt% to 20 wt%, resulting in the Young’s modulus decreasing from 0.005 to 0.001 MPa. Microcrystalline wax consists of iso-alkanes and naphthene containing alkanes. Owing to the large amount of branched and naphthenic hydrocarbons in microcrystalline waxes, they mainly form small irregular crystals during crystallization [27]. These irregular crystals can disrupt the strength of the crystal network resulting in a weaker structure. The disruption in crystal network can also be observed by a shift in the onset temperature for melting. As microcrystalline wax concentration increases from 5 wt% to 20 wt%, the onset temperature shifts from 33 to 23°C (Table IV, Fig. 6), suggesting the existence of mixed crystals. This agrees with work conducted in the dental field on dental waxes, which shows that microcrystalline wax weakens the structure [28]. Figure 7 shows that the introduction of water droplets (in the range of 2–4 lm) into the formulation lowers the Young’s modulus from 0.03 MPa to 0.0015 MPa and decreases the point of fracture from 0.003 MPa to 0.001 MPa, which is a result of water (soft filler particles) being introduced into the microstructure, wax crystals moving from the continuous network to the W/O interface and an overall reduction in the solid wax content [29]. The introduction of water droplets reduces Young’s modulus from 0.0026 to 0.0014 MPa which could be owing to crystals
Table IV Onset (Tonset), peak (Tpeak 1 and Tpeak 2) and end (Tend) temperatures as a function of microcrystalline wax (MW) concentration (left)
Micro crystalline wax (%)
Tonset (°C)
5 10 15 20
33.8 30.5 26.5 23.5
% of Microcrystalline wax Figure 4 Young’s modulus (MPa) and point of fracture (MPa) of wax combinations containing either (a) carnauba wax or (b) microcrystalline wax (with 5% carnauba wax). All samples were melted and stirred using a magnetic stirrer until molten (~ 30–40 min) and cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
0.01
0.8 0.7 0.7 2.1
Tpeak
60.2 61.7 62.8 62.5
1
(°C)
Tpeak
2
(°C)
Tend (°C)
0.7 0.4 0.2 0.3
75.8 72.6 73.2 72.3
0.3 0.5 0.3 0.5
80 78.7 78.9 78.5
0.4 0.6 0.4 1
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1.0 0.8 0.6 0.4
0.005
0.005
0.004
0.004
0.003
0.003
0.002
0.002
0.001
0.001
0.000
0.2
0.000 0
10
20
30
40
Aqueous phase (%)
0.0 20
30
40
50
60
70
80
Temperature (°C) Figure 6 Differential Scanning Calorimeter (DSC) curves for same samples (right); all samples contain 5% carnauba wax (CW). All samples were measured using a DSC from 10 to 120°C at 10°C/min.
moving to the interface. However, increasing aqueous phase volume does not seem to have an impact on the Young’s modulus. Two different waxes (paraffin and performalene) were added into the continuous phase formulation to investigate their effect on the overall mechanical properties of the emulsion. Performalene is a polyethylene molecule that forms small crystals, which allow an increase in number of connections in the wax network. Hard paraffin is a straight-chain hydrocarbon [30] and was chosen as previous literature has shown that paraffin increases strength of emulsions [5], by producing large crystals which will not travel to W/O interface, resulting in a stronger continuous wax network. When Fig. 7 (emulsions containing carnauba and microcrystalline waxes) are compared with Fig. 8 (the addition of solid wax), the data show an increase in Young’s modulus and point of fracture with the addition of solid wax (particularly performalene). For example, for emulsions containing 10 wt% aqueous phase, the
0.006
0.006 Youngs Modulus Point of Fracture
0.005
0.005
0.004
0.004
0.003
0.003
0.002
0.002
0.001
0.001
Point of fracture (M Pa)
Youngs modulus (M Pa)
0.006 YM for Emulsions containing Paraffin YM for Emulsions containing Performalene PF for Emulsions containing Paraffin PF for Emulsions containing Performalene
Point of fracture (M Pa)
1.2
Youngs modulus (M Pa)
Specific heat (J/g* °C)
0.006
5 % MW and 5 % CW 10 % MW and 5 % CW 15 % MW and 5 % CW 20 % MW and 5 % CW
1.4
0.000
0.000 0
10
20
30
40
Aqueous phase (%) Figure 7 Young’s modulus (MPa) and point of fracture (MPa) of emulsions containing 2 wt% polyglycerol polyricinoleate as a function of aqueous phase volume (10–40wt%), where the continuous phase contains 5% carnauba wax and 10% microcrystalline wax. All emulsions were produced using a Silverson high shear mixer (for 5 min and at ~10 000 g) and cooled quiescently in the freezer till solid and measured with a compression rate of 1 mm s1 at 32°C.
Figure 8 Young’s modulus (YM) (MPa) and point of fracture (PF) (MPa) of emulsions containing 2 wt% polyglycerol polyricinoleate as a function of aqueous phase volume (10–40 wt%) where the continuous phase contains 5% carnauba wax and 10% microcrystalline wax, and either 5% paraffin (■) or 5% performalene (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid then measured at a compression rate of 1 mm s1 at 32°C.
addition of performalene increases the Young’s modulus from ~ 0.003 MPa to ~ 0.005 MPa, and the point of fracture from ~ 0.001 to ~ 0.005 MPa, indicating a stiffer emulsion. Interestingly, the emulsions containing 10 wt% aqueous phase and performalene are stronger than the control without performalene (i.e. the nonemulsified bulk wax system): the Young’s moduli are ~0.005 MPa and ~0.0038 MPa, respectively, and the point of fractures are ~0.005 MPa and ~0.003 MPa, respectively. The samples containing 20 and 30 wt% aqueous phase and performalene are more similar to the control formulation in terms of both Young’s modulus and point of fracture, but there is some weakening of the structure for samples containing 40 wt% water. The droplet size for all emulsions produced were similar (in the range of 2–4 lm); therefore, it is thought that performalene produces a greater number of connections in the wax network resulting in an emulsion that is more resistant to compression, up to a point where the number of droplets (i.e. ‘defects’) increases and reduced the mechanical strength of the structure. However, it should be noted that the error bars are large. Overall, the results show that the introduction of water lowers both Young’s modulus and point of fracture, resulting in a weaker structure. This observation concurs with previously reported data on lipsticks and foods, which showed that the introduction of water droplets decreases strength [12, 13]. Wang and Lee reported that when the water content was increased from 5 to 15 wt%, the strength increased [12]. The results obtained in this research show that the Young’s modulus does not increase as water content increases from 10 to 40 wt%. Therefore, the differences between samples must be owing to a difference in the wax crystal network; hence, the rheological properties of the emulsion were investigated. Both the elastic modulus (G’) and the viscous modulus (G”) were investigated directly after emulsion formation. Previous literature has shown that by increasing the number of crystals, a more rigid network is produced [5]. Therefore, G’ (i.e. solid-like behaviour) should increase with the addition of crystalline material (paraffin and performalene). Figure 9 shows that there is a negative correlation between aqueous phase volume and G’ for all the waxes investigated. This is owing to the solid wax content decreasing as aqueous phase
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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1.6
25
R2 = 0.98
G' (M Pa)
1.2 1.0
R2 = 0.95
0.8 0.6
R2 = 0.96
0.4
5% CW, 10% MW and 85% CO 5% CW, 10% MW, 5% P and 80% CO 5% CW, 10% MW, 5% PF and 80% CO
20
Phase angle (°)
1.4
5% CW, 10% MW and 85% CO 5% CW, 10% MW, 5% P and 80% CO 5% CW, 10% MW, 5% PF and 80% CO
15
10
5 0.2 0.0 0
10
20
30
40
50
0 0
Water phase (%)
G'' (M Pa)
0.3
0.2
0.1
0.0 10
20
30
40
50
Water phase (%) Figure 9 G’ (MPa) (solid symbols) and G’’ (open symbols) as a function of aqueous phase volume for emulsions containing 2 wt% polyglycerol polyricinoleate, where the continuous phase contains 5% carnauba wax (CW) and 10% microcrystalline wax (MW) (●), and either 5% paraffin (P) (■) or 5% performalene (PF) (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid and measured via oscillation rheology. G’ value taken at a strain of 1.4 9 10^-5 and a frequency of 5 Hz.
volume increases, resulting in fewer connections between wax moieties. The addition of either paraffin or performalene increases G’ (from 0.4 to 0.8 and 1.3 MPa, respectively, for 10 wt% aqueous phase) with performalene increasing G’ by a greater amount for all aqueous phase volumes. This could be attributed to its ability to create connections in the crystal network, thereby increasing the strength of the crystal network. The reduction in G’ at greater aqueous phase volumes is caused by the surface area of the w/o interface increasing with increasing aqueous phase volume, resulting in crystals from the continuous network moving to the interface during cooling. As a result, a rigid crystal network cannot be formed. Figure 9 also shows that for all emulsions tested, G’ is greater than G’’ indicating that the emulsion behaves more like a solid. On further analysis of both G’ and G’’, the phase angle can be calculated (d). Figure 10 shows that as the aqueous phase volume increases (from 10% to 40%), the d increases from 5 to 15°. It is known that if the phase angel is between 0 and 90°, then the
620
30
40
50
Figure 10 Phase angle As a function of aqueous phase volume for emulsions containing 2 wt% polyglycerol polyricinoleate, where the continuous phase contains 5% carnauba wax (CW) and 10% microcrystalline wax (MW) (●), and either 5% paraffin (P) (■) or 5% performalene (PF) (♦). All emulsions were produced using a Silverson high shear mixer and cooled quiescently in the freezer till solid and measured via oscillation rheology.
5% CW, 10% MW and 85% CO 5% CW, 10% MW, 5% P and 80% CO 5% CW, 10% MW, 5% PF and 80% CO
0
20
Water phase (%)
0.5
0.4
10
material can be classified as viscoelastic [11]. From Fig. 10, we can conclude that for all aqueous phases, the emulsion behaves viscoelastically; however, at higher aqueous phase volumes, the emulsion behaves slightly more viscously. This can be attributed to the fact that the existence of a crystal network decreases as the water content increases. This is likely to be caused by a combination of (i) an increase in the number of droplets (and thus the size of the w/o interface) with increasing aqueous phase volume, creating an increasing number defects in the structure and (ii) movement of crystals from the bulk to the interface, weakening the network. Conclusion The aims of this work were to (i) investigate the effect of emulsifier type (polymer vs. monomer, and saturated vs. unsaturated chain) and concentration on droplet size, and (ii) investigate the effect of wax ratio (carnauba wax and microcrystalline wax) and aqueous phase volume on material properties (Young’s modulus, point of fracture, elastic modulus and viscous modulus). Results showed that the saturated nature of the emulsifier had very little effect on droplet size, neither did the use of an emulsifier with a larger head group (~18– 25 lm). Polyglycerol polyricinoleate resulted in emulsions with the smallest droplets (~3–5 lm), as a result of a thick elastic interface. The results also showed that both Young’s modulus and point of fracture increase with increasing percentage of carnauba wax (following a power law dependency of 3), but decrease with increasing percentage of microcrystalline wax. This suggests that the carnauba wax is included in the overall wax network formed by the saturated components, whereas the microcrystalline wax forms irregular crystals that disrupt the overall wax crystal network. Young’s modulus, elastic modulus and viscous modulus all decrease with increasing aqueous phase volume in the emulsions, although the slope of the decrease in elastic and viscous moduli is dependent on the addition of solid wax, as a result of strengthening the network. Although the addition of water droplets (typically in the size range of 2–4 lm) weakens the structure to compressibility, the addition of solid wax (particularly performalene) increases strength,
© 2013 Society of Cosmetic Scientists and the Société Française de Cosmétologie International Journal of Cosmetic Science, 35, 613–621
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Physical and material properties of emulsion lipsticks
particularly for the lower aqueous phase volume emulsions produced. Specifically, the emulsions containing up to 30 wt% aqueous phase had very similar mechanical properties to the nonemulsified control. Rheological data showed that all emulsions behaved as viscoelastic solids regardless of the aqueous phase volume, with the samples becoming slightly more viscous as the aqueous phase volume increased. Thus, the work suggests the potential for the use of emulsions in the formulation of lipstick products. Future work should consider the moisturizing or lubricating properties of emulsions within wax continuous systems and possibly
consider the release of hydrophilic compounds from the aqueous phase under simulated shear conditions similar to those experienced during lipstick application. Acknowledgement The authors thank Alliance Boots PLC for providing materials as well as the EPSRC for funding. The authors would also like to thank Dr Antonio Sullo (University of Birmingham) for helpful scientific discussion.
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