Article pubs.acs.org/Langmuir

Preparation of Double Pickering Emulsions Stabilized by Chemically Tailored Nanocelluloses Ana G. Cunha,*,†,‡,§ Jean-Bruno Mougel,† Bernard Cathala,† Lars A. Berglund,‡,§ and Isabelle Capron*,† †

UR1268 Biopolymères, Interactions et Assemblages, INRA, F-44316 Nantes, France Wallenberg Wood Science Center and §Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden



S Supporting Information *

ABSTRACT: Nanocelluloses are bio-based nanoparticles of interest as stabilizers for oil-in-water (o/w) Pickering emulsions. In this work, the surface chemistry of nanocelluloses of different length, nanofibrillated cellulose (NFC, long) and cellulose nanocrystals (CNC, short), was successfully tailored by chemical modification with lauroyl chloride (C12). The resulting nanofibers were less hydrophilic than the original and able to stabilize water-in-oil (w/o) emulsions. The combination of the two types of nanocelluloses (C12-modified and native) led to new surfactant-free oilin-water-in-oil (o/w/o) double emulsions stabilized by nanocellulose at both interfaces. Characterization was performed with respect to droplet size distribution, droplet stability over time, and stability after centrifugation. Nanocellulose-based Pickering emulsions can be designed with a substantial degree of control, as demonstrated by the stability of the chemically tailored NFC double emulsions. Furthermore, it was demonstrated that increased nanofiber length leads to increased stability.

1. INTRODUCTION Emulsions are systems consisting of dispersed droplets of one immiscible liquid into another. Simple emulsions are either oilin-water (o/w) or water-in-oil (w/o) type. Surface-active agents, i.e. emulsifiers, are used to stabilize these systems by lowering the interfacial tension at the liquid/liquid interface. Conventionally, surfactants are utilized for this purpose. Multiple emulsions are complex systems, in which both w/o and o/w emulsion types exist simultaneously.1,2 Such emulsions are important carrier systems that find applications in numerous fields, such as food, pharmaceuticals, and cosmetics.3 The two major types of multiple emulsions are water-in-oil-inwater (w/o/w) and oil-in-water-in-oil (o/w/o) double emulsions, which typically require two or more emulsifiers. One of the emulsifiers is predominantly hydrophobic stabilizing the w/o emulsion, and the other is predominantly hydrophilic stabilizing the o/w emulsion. Both surfactants may interact with both interfaces but also interfere with each other’s stabilizing performance.4 Moreover, lifetime of the films at the interfaces and their permeation properties are governed by the composition of the binary surfactant mixture.5 Synthetic polymeric amphiphiles and naturally occurring biopolymers (proteins or hydrocolloids) were recognized as promising emulsifiers for making multiple emulsions. Polymers are known to be multianchoring amphiphiles, increasing adsorption and stabilization capabilities. The gain in free energy by such adsorption was already greater than that of the adsorbed monomeric surfactants. Pioneering studies by Ramsden6 and Pickering7 then showed that solid colloidal © 2014 American Chemical Society

particles could be used as a third class of surface-active agents. They can irreversibly adsorb at liquid interfaces leading to the so-called Pickering emulsions.8,9 Another advantage is the wetting specificity of the particle, which depends on its surface chemistry. An amphiphilic character is required. Particles with a more hydrophilic character lead to o/w emulsions, whereas more hydrophobic particles lead to inverse w/o emulsions.8,9 The use of colloidal particles is of great interest for double emulsion preparation since they are not sensitive to osmotic variations. This inhibits compositional ripening and allows formation of stable emulsions of large-sized drops without coalescence. Barthel et al.10 used fumed silica particles with different SiOH content for the stabilization of surfactant-free o/ w/o and w/o/w double Pickering emulsions. In a similar fashion, Zou et al.11 used silica, iron oxide, and clay nanoparticles differing in their wettability. In both studies, stable double emulsions were obtained. Previous works on Pickering emulsions mostly concern the use of inorganic particles as emulsifiers, including silica,12−15 calcium carbonate,16,17 montmorillonite,18 among others. Biobased particles from renewable resources19 allow preparing environmentally friendly materials. Cellulosic nanofibers are good candidates in this context.20 They can provide a range of aspect ratios and different lengths, while the chemical structure may be preserved. A few studies have used cellulose to stabilize Received: May 7, 2014 Revised: July 15, 2014 Published: July 21, 2014 9327

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simple emulsions.21,22 In a recent study, our group showed that it is possible to obtain highly stable o/w Pickering emulsions stabilized by unmodified cellulose nanocrystals from different sources.23−25 It was also shown that the use of nanorods of different lengths might lead to different types of emulsions assembling, from individual to entangled droplets. Although most studies are on the o/w emulsion type, a few inverse w/o emulsions have been investigated. Notably, cellulose derivatives such as ethylcellulose,26 and chemically modified micro/ nanocelluloses, from plant27,28 or bacterial29 origin, have been used for the inverse emulsions. However, the use of solid particles from renewable resources to stabilize double emulsions appears to be practically unexplored. The few examples of w/o/w emulsions in the literature concern the combination of natural particles, such as microcrystalline cellulose,30 α-form fat microcrystals,31 and modified quinoa starch granules,32 and surfactants. To the best of our knowledge, double emulsions only stabilized by naturally occurring organic particles have not been prepared before. In this work we report the combination of unmodified and chemically modified cellulose nanoparticles as bio-based emulsifiers for o/w/o double emulsions. The chemical modification adopted in this approach involved the partial esterification of the hydroxyl groups from cellulose with a fatty acid chloride, namely lauroyl chloride (C12). The emulsifying capacities of long nanofibrillated cellulose (NFC) and shorter cellulose nanocrystals (CNC) were compared, and the mechanical stability of the resulting emulsions was investigated.

Table 1. Dimensions and Surface Charge Density of the Nanocelluloses Used in This Work nanocelluloses NFC CNC cotton CNC wood pulp

widtha (nm)

lengtha (nm)

thicknessb (nm)

surface charge densityc (e/nm2)

8±3 13 ± 4 10 ± 4

>1000 189 ± 51 134 ± 55

7±4 18 ± 5 12 ± 4

∼0 0.13 0.30

a c

Estimated from TEM analysis. bEstimated from AFM analysis. Obtained from conductometric titration with NaOH.24

AKL 250-14, Wilmington, DE) and redispersion steps (Q700 sonicator, QSonica, Newtown, CT), using acetone as intermediary solvent. For both NFC and CNC, a desired amount of nanocellulose dispersion in toluene (2 and 4 g/L, respectively) was poured into a round-bottom flask, followed by the addition, under stirring, of 1 equiv (relative to the total OH groups of cellulose) of lauroyl chloride (C12) and of pyridine. The reaction was conducted at 80 °C for 1 h. At the end of the reaction, the esterified nanocelluloses were sequentially washed with toluene, acetone, ethanol, and again with acetone and toluene, by consecutive centrifugation and redispersion steps, before being solvent-exchanged to hexadecane using the same method. This washing procedure ensured the removal of residual free C12 in the system, and no trace of C12 was identified by ATR-FTIR or 13C NMR. Modified nanocellulose dispersions at 2 and 4 g/L in hexadecane were obtained for NFC and CNC, respectively. 2.2.3. Preparation of the Simple Emulsions. Oil-in-water (o/w) Pickering emulsions were prepared using hexadecane and 2 or 4 g/L aqueous NFC or cotton-derived CNC suspensions, respectively. In all emulsions the oil:water ratio was 20:80. NaCl (50 mM) was added to the sulfated CNC aqueous suspension in order to prevent electrostatic repulsions that may lead to unstable emulsions. The mixtures were ultrasonicated (Q700 sonicator, QSonica, Newtown, CT) for 20 s. Water-in-oil (w/o) Pickering emulsions were prepared using Milli-Q water and 2 or 4 g/L modified NFC (NFCC12) or CNC (CNCC12) dispersions in hexadecane, respectively. A water:oil ratio of 20:80 was used in all cases. The emulsification process was conducted by handpremixing followed by homogenization using a rotor-stator (Heidolph Silent Crusher M homogenizer, Germany) at 5000 rpm for 30 s. 2.2.4. Preparation of the Double Emulsions. Oil-in-water-in-oil (o/w/o) double emulsions were prepared by adding a suitable volume of the modified nanocelluloses dispersions in hexadecane to the primary o/w emulsions diluted twice with water, stabilized by either NFC or CNC, at a ratio 20:80 (o/w:o). The final mixture was premixed by hand and subsequently homogenized using a rotor-stator (Heidolph Silent Crusher M homogenizer, Germany) at 5000 rpm for 30 s. Four different types of o/w/o double emulsions were prepared as summarized in Table 2.

2. MATERIALS AND METHODS 2.1. Materials. All the reagents used were of analytical grade (Sigma-Aldrich), and water was purified with Milli-Q reagent system (Millipore). Hexadecane was purified by extensive extraction with water. For CLSM, BODIPY 665/676 [(E,E)-3,5-bis(4-phenyl-1,3butadienyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)] was purchased from Molecular Probes Invitrogen (Eugene, OR) and used without purification. 2.2. Methods. 2.2.1. Nanocelluloses. Nanofibrillated cellulose (NFC) was prepared from softwood sulfite pulp fibers (DP of 1200, lignin and hemicelluloses content of 0.7% and 13.8%, respectively, Nordic Pulp and Paper, Sweden) according to a method previously described.33 The pulp was first dispersed in water and subjected to a pretreatment step involving enzymatic degradation and mechanical beating. Subsequently, the pretreated pulp was disintegrated by homogenization process with a Microfluidizer M-110EH (Microfluidics Ind.), and a 2 wt % NFC dispersion in water was obtained. Sulfated cellulose nanocrystals (CNC) were prepared according to a method described by Revol et al.34 with minor modifications. Briefly, Whatman filters (grade 20 Chr) were hydrolyzed in 58% (v/v) sulfuric acid at 70 °C until the dispersion coloration changed from milky white to pale yellow-orange. After hydrolysis, the suspension was washed by centrifugation, dialyzed to neutrality against Milli-Q water, and deionized using mixed bed resin (TMD-8). The final dispersion (ca. 8 g/L) was sonicated for 15 min (Q700 sonicator, QSonica, Newtown, CT), filtered, and stored at 4 °C. CNC from wood pulp fibers at a high concentration, ca. 60 g/L, were also supplied by University of Maine. ATR-FTIR and XRD data revealed that this CNC is a mixture of cellulose polymorphs type I and type II. The purpose of using this latter type of CNC was simply to facilitate the solvent-exchange process before chemical modification. The dimensions and surface charge density of the nanocelluloses used in this work are listed in Table 1. 2.2.2. Chemical Modification of the Nanocelluloses. Prior to chemical modification the aqueous nanocellulose suspensions (NFC and CNC from wood pulp) were solvent exchanged to toluene, by successive centrifugation (Thermo Scientific KR25i centrifuge, Rotor

Table 2. Identification of the o/w/o Double Emulsions Prepared nanocellulose type double emulsion

inner interface

outer interface

CNC/CNCC12 CNC/NFCC12 NFC/CNCC12 NFC/NFCC12

CNC CNC NFC NFC

CNCC12 NFCC12 CNCC12 NFCC12

2.2.5. Fourier-Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded using a Nicolet Magna 550 series II FTIR spectrometer equipped with an attenuated total reflectance (ATR) system. Dried samples were analyzed directly after pressing them against the crystal. The acquisition conditions were 100 scans and 2 cm−1 resolution. 2.2.6. Atomic Force Microscopy (AFM). AFM measurements were performed with a Innova Microscope (Bruker). The unmodified 9328

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nanocellulose dispersions were diluted to 0.001 g/L (water). A 20 μL drop of nanocellulose dispersion was placed on the surface of freshly cleaved mica for 1 min, then rinsed with water, and dried under a gaseous N2 stream. For the charged CNC, the dispersion was dropped on a mica surface previously coated with 20 μL drop of 0.1% (w/v) poly(allylamine hydrochloride) solution under similar procedure (1 min standing time, rinsing with water and drying under gaseous N2 stream). All measurements were carried out in the tapping mode with a scan rate of 0.5−0.7 Hz using noncontact antimony-doped silicon cantilevers (TESPA, Bruker) with a tip radius of 8 nm and a spring constant of 42 N/m. The micrographs were taken from the height images over a 5 × 5 or 10 × 10 μm2 area. 2.2.7. Transmission Electron Microscopy (TEM). TEM was performed using a JEOL JEM-1230 and a Hitachi HT-7700 (Japan) operating at 80 kV, for CNC and NFC, respectively. A 0.1% (w/v) nanocellulose suspension in water was deposited on freshly glowdischarged carbon-coated electron microscope grids (200 mesh copper, Delta Microscopies, France) for 2 min, and the excess water was removed by blotting. Negative staining was carried out using uranyl acetate solution (2% w/v) for 2 min. The excess solution was removed by blotting, and the grids were dried in an oven at 40 °C just before observation. 2.2.8. Optical Microscopy (OM). The emulsions were observed using an Olympus BX-51 optical microscope. Single drops of diluted emulsions (ca. 100× dilution) were poured onto glass slides and observed under the optical microscope. Fluorescein was used to stain the aqueous phase, and the dark field microscopy (DFM) observations were performed under an excitation lamp X-cite 120Q. The droplet size and size distribution of the different emulsions were measured from the corresponding micrographs using the free software ImageJ. 2.2.9. Confocal Laser Scanning Microscopy (CLSM). CLSM images of the emulsions were acquired using a Zeiss LSM 410 confocal microscope (Zeiss, Gottingen, Germany) equipped with a 40× water-immersion lens with an optical section thickness of approximately 1 μm. Water was stained with fluorescein and hexadecane with BODIPY 665/676 at 0.04 g/L. 2.2.10. Emulsion Stability Studies. The stability of the emulsions was assessed over 46 days of storage time. Photographs of the emulsion containers were captured along the time using a P1 digital camera (Olympus), and the emulsions were observed under OM and DFM. The mechanical stability of the emulsions was also investigated by submitting them to centrifugation-induced shear forces at 100g, 500g, 1000g, 2500g, 5000g, and 10000g for 1 min. Photographs were captured before and after centrifugation, and the emulsions were observed under OM and DFM.

Figure 1. TEM micrographs of the nanocellulosic substrates used for chemical modification: (a) nanofibrillated cellulose (NFC) and (b) cellulose nanocrystals (CNC). AFM height images of (c) NFC and (d) CNC.

samples presented new resonances attributed to aliphatic and carboxylic carbons at 10−40 and ∼175 ppm, respectively. The relative intensity of these new signals, particularly those corresponding to the aliphatic carbons, are related to the extent of esterification in terms of bulk degree of substitution (bulk-DS) (see Supporting Information for details). Values of 0.27 and 0.15 were obtained for the C12-modified NFC (NFCC12) and CNC (CNCC12), respectively. However, only the OH groups exposed at the nanoparticle surface are accessible to the esterifying reagent. Thus, the extent of modification is better represented by the surface degree of substitution (surface-DS; see Supporting Information for details). Surface-DS of 0.93 and 0.60 were obtained for NFCC12 and CNCC12, respectively. For CNCC12, the estimated value is probably representative since the nanoparticle size is quite well-defined. For the NFCC12, the estimated surface-DS may be overestimated. NFC has some disordered regions, which are known to be promptly reactive to chemical modifications (more than the ordered domains).39 Thus, the real surface-DS in NFCC12 might be lower than the 0.93 estimate. Nevertheless, in both cases surface-DS is lower than the “theoretical” maximum value 1.5.40,41 This is in support of topochemical surface modification. XRD measurements revealed that the crystallinity of the nanocelluloses was not affected by the esterification reaction, since the diffraction patterns of unmodified and C12-modified nanocelluloses are similar (see Figure S2 in Supporting Information). This is also in favor of surface esterification only. Contact angle measurements with water were performed to assess changes in the wettability of the nanocelluloses after chemical modification. Water contact angles (θw) of 22 ± 1° and 41 ± 2° were measured for unmodified CNC and NFC cast films (see Figure S3 in Supporting Information), respectively. Similar values can be found in the literature for unmodified CNC smooth cast films42 and NFC films.43 The difference in θw for the two types of unmodified nanocelluloses

3. RESULTS AND DISCUSSION 3.1. Chemical Modification of the Nanocelluloses. In Pickering emulsions the stabilization of the solid particles at the oil−water interface is governed by their large size and their wettability. Here, long and slender nanofibrillated cellulose (NFC) fibrils and rodlike cellulose nanocrystals (CNC) were used as nanocellulosic particles20 (Figure 1 and Table 1) and submitted to an esterification reaction with a fatty acid chloride, namely lauroyl chloride (C12). The success of the chemical modification was assessed by FTIR-ATR spectroscopy, through the monitoring of a new band at ∼1735 cm−1, attributed to the CO stretching frequency of the new ester function (Figure 2).35 Additionally, new peaks at ∼2850 and 2905 cm−1 were observed. These frequencies, previously assigned to C−H stretching in CH2 and CH3 groups,36,37 respectively, confirmed the presence of long carbon-chain moieties (C12) attached to the nanocellulose surfaces (Figure 2).35 Solid-state 13C NMR measurements were also carried out for both unmodified and modified nanocelluloses (see Figure S1 in Supporting Information). Apart from the resonances typical from the cellulose backbone,38 the spectra of the modified 9329

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Figure 2. FTIR-ATR spectra of unmodified and C12-modified nanocelluloses for (a) nanofibrillated cellulose and (b) cellulose nanocrystals.

must be related with the different nature of their surface. The cast film of charged CNC is likely to be wetted more easily by water and thus possesses the lowest water contact angle. After esterification, the water contact angle increased to 76 ± 1° and 115 ± 3° for CNCC12 and NFCC12 (see Figure S3 in Supporting Information), respectively. This confirms the less hydrophilic character of the chemically modified nanocelluloses. The roughness (Rz) increased somewhat for the cast films made of C12-modified nanocelluloses (from Rz = 35−50 nm for unmodified to Rz = 70−140 nm for modified; see Figure S3 in Supporting Information). Roughness is known to influence wettability and water contact angle.9,44 Either way, none of the two types of modified nanocelluloses were dispersible in water anymore, but dispersible in hexadecane after a solvent-exchange process (although they were not colloidally stable and tended to sediment with time). Also, when mixed with hexadecane and water, they were able to stabilize w/o emulsions, although the water−air contact angle θw obtained for CNCC12 was still below 90°. This fact is not contradictory, since it is the three phase contact angle, particle−water−oil, that dictates the stabilization of the emulsion, which is difficult to measure by conventional techniques. 3.2. Preparation of the o/w/o Double Emulsions. Prior to the buildup of the o/w/o emulsions, simple o/w and w/o emulsions were prepared with unmodified and C12-modified nanocelluloses, respectively, in order to evaluate their individual capacity to stabilize the oil−water interface. In all emulsions, hexadecane was chosen as “oil” phase. For the direct o/w emulsions, unmodified nanocelluloses were used. Sodium chloride (50 mM) was added to the CNC dispersion in order to screen sulfate ester charges on CNC. Since NFC is neutral, no salt was added to NFC compositions. In previous work on CNC-stabilized o/w emulsions, very stable emulsions were obtained at a concentration of 4 g/L in the aqueous phase. All the oil could be emulsified without release of unadsorbed CNC in the aqueous phase.25 A lower concentration of 2 g/L was selected for NFC- and NFCC12-stabilized emulsions in order to avoid entangled emulsion particles due to interlinking of the long NFC fibrils. This is in analogy with the long Cladophora cellulose nanocrystals tested in the same study.25 As shown in Figures 3a and 3b, o/w emulsions were effectively stabilized by the unmodified NFC and CNC, respectively. The droplet size, as assessed by optical microscopy (OM), was slightly polydisperse in both cases (see Figure S6 in Supporting Information), with an average diameter of 3.3 ± 1.2 and 2.6 ± 0.8 μm for NFC- and CNC-stabilized emulsions (Table 3), respectively. For the CNC-stabilized o/w emulsion

Figure 3. DFM micrographs of o/w emulsions stabilized by (a) NFC and (b) CNC; w/o emulsions stabilized by (c) NFCC12 and (d) CNCC12; o/w/o double emulsions stabilized by (e) NFC/NFCC12, (f) CNC/NFCC12, (g) NFC/CNCC12, and (h) CNC/CNCC12. Water was stained with fluorescein. Scale bar is 50 μm.

the droplets were fairly isolated in the continuous aqueous phase (Figure 3b) under dilute conditions, whereas for the NFC-analogue cluster-like aggregates of interconnected droplets were observed (Figure 3a). This is mainly due to the high aspect ratio (∼100−150) and long length of NFC (>1 μm). Furthermore, owing to the formation of this threedimensional nanofibrils-droplets network, the o/w emulsions stabilized by the long NFC do not tend to cream over the 9330

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The double emulsions were prepared according to a two-step emulsification process, in which the unmodified nanocelluloses were used to stabilize the inner interface (o/w) and the C12modified nanocelluloses the outer interface (w/o). This is the most common method for double emulsion preparation.1 While the primary o/w emulsion is prepared under high shear conditions (here ultrasonication), the secondary emulsification step is usually carried out under milder shear conditions (here hand-premixing followed by rotor stator), in order to avoid the disruption of the primary o/w emulsion. After homogenization using a rotor stator, the droplets were well dispersed in the oily continuous phase and did not immediately sediment due to density differences (see Figure S5 in Supporting Information). Four different types of o/w/o double emulsions were prepared (Figure 3e−h) combining the two unmodified and the two C12-modified nanocelluloses (Table 2). Hereafter, the inner droplets of the double emulsions will be referred as droplets and the outer droplets as globules. At a first glance, the droplets seem efficiently encapsulated in all the double emulsions prepared and their size, as visualized through the optical microscope, seems to be preserved. Some differences appeared in the globule size distribution. A higher polydispersity can be perceived from the graphs of globule size distribution in Figure 4. The globules were larger when NFCC12 was used to stabilize the outer interface (Figure 4a,b), as previously observed for the simple w/o emulsions (Table 3). Moreover, the NFCC12-stabilized double emulsions presented this time a “bimodal” globule size distribution with two populations at diameters around 40 and 70 μm (Figure 4). It is noteworthy that the size of globules from CNCC12-stabilized double emulsions (43 and 51 μm) are similar to the droplets from w/o simple-analogues (40 μm). The same holds true in the case of the NFCC12-stabilized double emulsions (Table 3), showing that the emulsification process is not strongly influenced by the presence of the small droplets in the continuous oily phase. Confocal laser scanning microscopy (CLSM) was used to study the double emulsions. Fluorescein and BODIPY 665/676 were employed in the staining of the intermediary aqueous

Table 3. Average Droplet and Globule Diameter in the Simple and Double Emulsions, Respectively o/w simple emulsion

NFC

av droplet diam (μm)

3.3 ± 1.2

double emulsion av globule diam (μm)

w/o CNC 2.6 ± 0.8 o/w/o

NFCC12

CNCC12

66 ± 26

40 ± 14

NFC/ NFCC12

CNC/ NFCC12

NFC/ CNCC12

CNC/ CNCC12

65 ± 27

76 ± 32

43 ± 18

51 ± 21

storage time,45 conversely to the CNC-stabilized counterparts.23,25 As depicted in Figures 3c and 3d for NFCC12 and CNCC12, respectively, the C12-modified nanocelluloses dispersed in hexadecane successfully formed very stable w/o emulsions without coalescence for months or formation of clusters. The absence of such droplet aggregates is related to the much larger size of the droplets compared to the nanofibril length as well as the lower shear emulsification (rotor stator at 5000 rpm). The droplets size presented a higher polydispersity (see Figure S6 in Supporting Information) with an average of 66 ± 26 μm for the NFCC12- and 40 ± 14 μm for the CNCC12-stabilized w/o emulsions (Table 3). AFM analysis of the modified nanocelluloses revealed that after chemical modification the nanocelluloses formed large aggregates (see Figure S4 in Supporting Information). This is possibly due to a preaggregation phenomenon upon solvent exchange, since the unmodified nanocelluloses were not colloidally stable in toluene. Moreover, postflocculation of the modified nanocelluloses induced by hydrophobic interactions may also have contributed to the formation of the aggregates. These large aggregates may have caused lower coverage of the interface, so that the droplet diameters became larger. Also, NFCC12 was used at 2 g/L, whereas CNCC12 was used at 4 g/L. This can explain the significant difference in droplet size for the two w/o emulsions.

Figure 4. Globule size distribution in the four o/w/o double emulsions prepared: (a) NFC/NFCC12, (b) CNC/NFCC12, (c) NFC/CNCC12, and (d) CNC/CNCC12. The black curves serve to guide the eyes of the reader. 9331

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droplets were still individual, confirming efficient encapsulation and the absence of coalescence. 3.3. Stability of the o/w/o Double Emulsions. Instability is a major problem in design of double emulsions. This is mainly attributed to the presence of two thermodynamically unstable interfaces. The characteristic instability of these ternary systems may be overcome by stabilization of the inner and/or outer interfaces using polymeric emulsifiers/ macromolecular amphiphiles or solid colloidal particles.3 It is expected that such emulsifiers can provide stabilization by forming strong and rigid films at the interfaces.3 In solid particle-stabilized double emulsions, the surface specificity inhibits the migration of particles from inner to outer interfaces or vice versa after emulsion formation.8 No coalescence should then take place during storage as long as no stress is involved, and such emulsions are stable for several months. In the case of the o/w/o double emulsions, sedimentation was always observed, even within a short period of time (

Preparation of double Pickering emulsions stabilized by chemically tailored nanocelluloses.

Nanocelluloses are bio-based nanoparticles of interest as stabilizers for oil-in-water (o/w) Pickering emulsions. In this work, the surface chemistry ...
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