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Adsorption at the air-water and oil-water interfaces, and the self-assembly in aqueous solution of ethoxylated polysorbate nonionic surfactants Jeffrey Penfold, Robert K. Thomas, Peixun X. Li, Jordan T. Petkov, Ian M. Tucker, John Robert Peter Webster, and Ann E. Terry Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00151 • Publication Date (Web): 20 Feb 2015 Downloaded from http://pubs.acs.org on February 22, 2015

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Adsorption at the air-water and oil-water interfaces, and the self-assembly in aqueous solution of ethoxylated polysorbate nonionic surfactants Jeffrey Penfold1,2, Robert K Thomas1, Peixun X Li1, Jordan T Petkov3, Ian Tucker3, John R P Webster2, Ann E Terry2

1.Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK; 2. STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, UK 3. Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, UK

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Corresponding Author: Jeff Penfold, [email protected]

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ABSTRACT

The Tween nonionic surfactants are ethoxylated sorbitan esters, which have twenty ethylene oxide groups attached to the sorbitan headgroup and a single alkyl chain, Lauric, Palmitic, Stearic or Oleic. They are an important class of surfactants which are extensively used in emulsion and foam stabilization; in applications associated with foods, cosmetics and pharmaceuticals. A range of ethoxylated polysorbate surfactants, with differing degrees of ethoxylation from three to fifty ethylene oxide groups, have been synthesized and characterized by neutron reflection, small angle neutron scattering and surface tension. In conjunction with the different alkyl chain groups, this provides the opportunity to modify their surface properties, their self-assembly in solution, and their interaction with macromolecules, such as proteins. The adsorption at the air-water and oil-water interfaces and the solution self-assembly of the range of ethoxylated polysorbate surfactants synthesised are presented and discussed.

KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide significant keywords to aid the reader in literature retrieval.

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INTRODUCTION The Tween surfactants are an important class of biocompatible nonionic surfactants, which are extensively used in foods (1-4), cosmetics and pharmaceuticals (5, 6) and other bio-medical applications (7-9). The Tween surfactants are ethoxylated sorbitan esters of different fatty acids. The commercially available series, Tween20, Tween40, Tween60 and Tween80, all have twenty ethylene oxide groups attached to the sorbitan headgroup, and attached to the headgroup is a single long chain carboxylic acid, Lauric, Palmitic, Stearic, and Oleic, respectively (see figure 1).

Figure 1. Structure of Tween surfactants (shown here for Tween60), reproduced from reference 10.

The Tween surfactants have relatively low critical micelle concentrations, CMC, in the range 10 to 50 µM, and hydrophilie-lipophile balance, HLB, values ~ 15-17. This makes them effective as oil-in-water emulsifiers, detergents and solubilisers (11-14).

These properties and their inherent biocompatibility make the Tween surfactants an important class of food grade nonionic surfactants; and they are widely exploited in emulsion and foam stabilization (2, 15). The properties of the Tween surfactants as emulsion and foam stabilisers have been extensively studied in a wide range of oil-in-water systems (16-20). In many applications, especially bio-medical, non-specific protein interactions are undesirable. Surface ethylene oxide group provide an effective 3 Environment ACS Paragon Plus

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route for the development of protein resistant surfaces (21); and Shen et al (22) explored the potential of Tween surfactants to provide protein-resistant surface coatings. Their adsorption properties have attracted a diverse range of potential applications. Graca et al (23) demonstrated that Tween adsorption onto hydrophobic surfaces provides an effective surface for soft lubrication. Li et al (24) showed how polysorbate surfactants can be used to promote and manipulate the formation of porous low dielectric surfactant films. Specific interactions between proteins and the different Tween surfactants give rise to competitive and synergistic interactions at interfaces (1-3, 25) and in self-assembly (4, 26). The specific interactions between ethoxylated nonionic surfactants and biomolecules gives rise to chaperoning properties. This has been exploited, for example, to suppress aggregation and promote refolding in recombinant human growth hormone and interferon (7-9). Nonionic surfactants are potentially attractive in the extraction and solubilising of membrane proteins (27). Although not necessarily optimal in extraction efficiency, the nonionic based surfactants are effective at inhibiting protein unfolding; and Tween surfactants have been exploited specifically for that purpose by Arachea et al (28).

Although the changes in surfactant properties of the ethoxylated polysorbate surfactants with different alkyl chain lengths and geometries are well established, there is relatively little information on the impact of changing the degree of ethoxylation. The differences in vesicle formation in Tween20 (Tween21) mixtures with cholesterol (29) have provide an important insight into the impact of the degree of ethoxylation on self-assembly. The Tween21 / Cholesterol vesicles and bulk phases (Tween21 has a lower degree of ethoxylation of four ethylene oxide groups compared to twenty for Tween20) show greater pH sensitivity, and a different evolution in the solution phases. The behaviour of Span / Tween mixtures at interfaces (30, 31) (where Span is the sorbitan fatty acid ester without any ethoxylation) further suggest that changing the degree of ethoxylation could have important consequences for the properties of the ethoxylated polysorbate surfactants. Tucker et al (32) have demonstrated that the ethoxylated polysorbate surfactants with a lower degree of ethoxylation (eight ethylene oxide groups, EO8) interact with the protein hydrophobin at the air-water interface to form 4 Environment ACS Paragon Plus

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well-defined multilayer structures at the interface. Furthermore it was shown that, although the surface self-assembly existed for ethoxylated polysorbate surfactants with the lower degrees of ethoxylation and with alkyl chain lengths from Lauric to Stearic/Oleic, it was not present for the Tween surfactants (with twenty ethylene oxide groups, EO20). These results strongly imply that adjusting the degree of ethoxylation of the ethoxylated polysorbate surfactants will result in some interesting changes in their surface and self-assembly properties and provide the ability to adjust their interaction with a range of proteins and other biomolecules.

The focus of this paper is to explore the fundamental properties, adsorption and self-assembly, of a range of ethoxylated polysorbate surfactants, with Lauric, Palmitic, Stearic and Oleic alkyl chains; with degrees of ethoxylation of the headgroup ranging from EO3 to EO50. This range of ethoxylated polysorbate surfactants has been custom synthesized. Their adsorption behaviour at the air-water interface has been characterized using neutron reflectivity, NR, and their adsorption at the oil-water interface and their self-assembly in aqueous solution have been characterized using small angle neutron scattering, SANS.

EXPERIMENTAL DETAILS

The neutron reflectivity measurements were made at the air-water interface on the INTER reflectometer at the ISIS neutron source (33). The reflectivity R(Q) was measured over a Q range of 0.03 to 0.5 Å-1 was covered using an angle of incidence of 2.3° and neutron wavelengths from 0.5 to 15 Å. The reflectivity, R(Q), was calibrated with respect to the direct beam intensity and the reflection from a D2O surface. The measurements were made in sealed Teflon troughs at 25 °C with sample volumes ~ 25 mL. Each neutron reflectivity profile took ~ 20-30 minutes. In the kinematic approximation (34) the reflectivity is related to the square of the Fourier transform of the scattering length density profile, ρ(z), normal to the surface (ρ(z)=∑ini(z) bi, ni(z) and bi are the number density and neutron scattering length 5 Environment ACS Paragon Plus

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of the ith component, and ρ(z) is related to the neutron refractive index, n(z), and n(z)=1-λ2ρ(z)/2π). By manipulation of ρ(z) through deuterium labeling (H, D have different scattering lengths, -3.7 × 10-6 Å for H, and 6.67 × 10-5 Å for D) the neutron reflectivity profile can directly provide information about the amount adsorbed at the air-water interface, and the structure of the adsorbed layer. This has been extensively demonstrated and exploited for a range of surfactant systems (34); and this approach is used here to characterize the ethoxylated polysorbate surfactant adsorption at the air-water interface.

The SANS measurements were made on the SANS2D diffractometer (35) at the ISIS pulsed neutron source. The measurements were made using the white beam time of flight method, with a neutron wavelength range from 2 to 16.5 Å and a sample to detector distance of 4.0 meters, to cover a Q range of 0.006 to 0.8 Å-1. The solutions were contained in 1 mm path length quartz spectrophotometer cells, and measured at 25°C. All the measurements were made in D2O at 25 °. The scattering from the empty cell and solvent were subtracted from the data. The data were normalized for the detector response, spectral distribution of the incident beam and solid angle, to establish the scattering intensity I(Q) on an absolute scattering cross-section (in cm-1), using standard procedures (36). Each individual measurement took ~ 10 to 30 minutes.

A range different ethoxylated polysorbate surfactants were synthesized. The ethoxylated sorbitan alkanoate (polysorbate) surfactants were prepared by reacting ethylene oxide with the appropriate sorbitan alkanoate. Sorbitan alkanoates are available as Span20 (dodecanoate), Span40 (hexadecanoate), Span60 (octadecanoate), and Span80 (oleate) following a procedure similar to that devised for synthesizing ethylene oxide-propylene oxide triblock copolymers (37). In this procedure a known amount of ethylene oxide is distilled into a calibrated side arm where it is held and measured volumetrically at room temperature. The side arm is connected to a vessel containing the Span at 110 oC. The whole system is initially evacuated and the reaction proceeds by transfer of the ethylene oxide through the vapour phase and normally takes about 12 hr for completion. A chosen weight of the 6 Environment ACS Paragon Plus

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appropriate Span, typically 3-5 g was first dried under vacuum at 90 oC with stirring. A catalytic amount of potassium t-butoxide (0.05 g) was added and the majority of the butanol produced by reaction with the Span was removed under vacuum at 90 oC. The temperature was raised to 110 oC and reaction with the ethylene oxide in the side arm was then started by connecting the two. The reaction vessel was continuously stirred and the reaction was easily monitored by the amount of ethylene oxide left in the side arm. Reaction was always quantitative and complete in 12 hr or less. The reaction was terminated by addition of water and enough HCl to neutralize the butoxide. Water and residual butanol were removed by freeze drying. The dried sample was dissolved in ethyl acetate (sometimes with the addition of a small amount of ethanol to ensure solution). Anhydrous sodium sulphate was added and the solution filtered to remove KCl left from the reaction. The final ethoxylated product was obtained by removing the solvent on a rotary evaporator and finally by warming under vacuum. The stoichiometry could be controlled to an accuracy of ±10% in ethylene oxide and we use the nomenclature PES720, where 20 refers to the original Span and the 7 is the average number of ethylene oxide units per molecule. The commercially available Tweens normally have 20 ethylene oxide units per molecule. In every case, matched samples containing normal ethylene oxide (BOC) and ethylene oxide-d4 (98% D, supplied by Dr D. A. Styrkas) were prepared.

The polyethylene sorbitan monolaurate, monostearate, and monooleate surfactants were prepared with seven, eight, thirteen and twenty ethylene oxide groups. Polyethylene sorbitan monostearate surfactants were also synthesised with three, seven, eight, nine, thirteen, seventeen, twenty, thirty and fifty ethylene oxide groups. For consistency with the conventional Tween nomenclature we abbreviate the surfactants synthesised here as PESn20, PESn60 and PESn80. A prefix, d-, or h- is used to indicate hydrogeneous or deuterated ethylene oxide groups. High purity water (Elga ultrapure) was used and the D2O was obtained from Sigma-Aldrich. All the glassware, the Teflon troughs (used for the NR measurements) and the quartz spectrophotometer cells (used for the SANS measurements) were cleaned in alkali detergent (Decon 90) and rinsed thoroughly in high purity water. Deuterium labeled and hydrogeneous 7 Environment ACS Paragon Plus

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hexadecane (d-, h-hexadecane) (for the oil-water interface measurements) were obtained from Sigma Aldrich.

The NR measurements were made at the air-water interface at surfactant concentrations from 3.0 to 1000.0 µM, for the ethoxylated polysorbate surfactants, with in ethylene oxide groups deuterium labeled,

in null reflecting water, nrw, (92/8 mole ratio mixture of H2O and D2O). The SANS

measurements were made for surfactant concentrations of 5, 10 and 20 mM for the ethoxylated polysorbate surfactants in D2O, for surfactants with the ethylene oxides groups hydrogeneous or deuterated. The surfactant adsorption at the oil-water interface was measured using SANS at a hexadecane- water interface of well-defined sub-micron emulsion droplets. Stable hexadecane in water emulsions (≤ 10% volume fraction) were prepared at low surfactant (SDS) concentrations (at approximately 10% coverage of the available emulsion surface) using ultrasonic homogenization, as described in detail elsewhere (38). The emulsions were prepared with the minimum amount of SDS required to stabilize the emulsion and to provide a sufficient electrostatic barrier to coalescence. The emulsion droplets were characterized by dynamic light scattering, DLS, using a Malvern Zetasizer Nano. The emulsion droplets had a mean diameter ~ 0.2 microns and a relatively narrow polydispersity, ~ 0.15; and the emulsion volume fraction was 0.067. The particle size was sufficiently small to prevent creaming and sufficient to provide adequate scattering from the adsorbed layer of surfactant. The measurements were made with a mixture of d- and h-hexadecane, which was neutron refractive index matched to D2O, in D2O, with the addition of increasing concentrations of the ethoxylated polysorbate surfactant with hydrogeneous ethylene oxide groups. In these circumstances and in the absence of micellar aggregates in solution only the adsorbed layer of surfactant contributes to the scattering. The incoherent scattering contribution (background) contribution is evident at high Q (Q> 0.2 Å-1), and is subtracted as described in detail elsewhere (38).

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RESULTS and DISCUSSION

(a) Adsorption at the air-water interface The adsorption of the ethoxylated polysorbate surfactants at the air-water interface was measured by neutron reflectivity in null reflecting water, nrw, using surfactants with deuterium labeled ethylene oxide groups. For all the surfactants measurements were made at a concentration >> cmc to establish the equilibrium saturation adsorption. The reflectivity was consistent with a thin monolayer of uniform composition adsorbed at the interface. The data were analysed using the exact expression for a thin film of uniform composition (34), to obtain a thickness, d, and a scattering length density, ρ. This provides a direct determination of the adsorbed amount, Γ, or area / molecule, A, at the interface (34), where Γ=1/NaA and A=∑b/dρ, Na is Avogadros number and ∑b is the sum of scattering length for the ethylene oxide deuterated polysorbate surfactant.

The ∑b values for the different surfactants

investigated are listed in table S1 in the Supporting Information. The thickness of the adsorbed layer depends upon the alkyl chain length and the degree of ethoxylation: and varies from 23 to 34 Å. For PES760, PES1360, and PES2060 the adsorption isotherms from ~ 3.0 to 1000 µM were measured and are shown in figure 2.

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Adsorbed amount (x10-10 mol cm-2)

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3

2

1

0 1

10

100

1000

Surfactant concentration (uM)

Figure 2. Adsorption isotherms for PES760 (●), PES1360 (▲), and PES2060 (■). The solid lines are fits to a Langmuir isotherm as described in the main text.

Similar measurements were made for PES720, PES1320, PES2020, PES780, PES1380, and PES2080; and are plotted in figure S1 and S2 in the Supporting Information. For the adsorption data in figure 2 the adsorbed layer thickness at saturation is 23 ± 1Å for PES760, 27 ± 1Å for PES1360, and 32 ± 1Å for PES2060 (see table S2 in the Supporting Information for a summary of the adsorbed layer thicknesses for PESn60, PESn20 and PESn80). The solid lines in figure 2 are least squares fits to a Langmuir isotherm (where Γ=Γsat C/(kd+C), Γsat is the saturation adsorption, C the surfactant concentration, and kd the adsorption coefficient). This gives Γsat and kd values for PES760, PES1360, and PES2060 of 3.2 x10-10 mol cm-2, 18.6 µM; 1.9 x10-10 mol cm-2, 12.5 µM and 1.4 x10-10 mol cm-2, 2.5 µM respectively. The corresponding values for PESn20 and PESn80 are listed in table S3 in the Supporting Information. The mean values of the adsorption, taken from figure 2 for surfactant concentrations between 50 and 1000 µM give values of 2.8x10-10, 1.78x10-10, and 1.34x10-10 mol cm-2 for PESn60, n=7, 13, 20; and the corresponding area/molecule are 59, 93, and 124 Å2. Comparison with the equivalent values for PESn80 (see figure S2 and tables S2 and S3 in the Supporting Information) indicate that changing from the saturated stearic to the unsaturated oleic alkyl chain slightly disrupts the packing. This results in the 10 Environment ACS Paragon Plus

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PESn80 adsorption being marginally less than the PESn60 adsorption. The difference decreases as the degree of ethoxylation increases. The PES780 adsorption is ~ 10% less than for PES760, but for the higher degrees of ethoxylation, 13 and 20 ethylene oxide groups, the differences are only 4 and 1% respectively. This implies that the packing at the interface is increasingly dominated by the ethoxylated headgroup, as the degree of ethoxylation increases. This is also evident from the comparison between the PESn60 and PESn20 isotherms. The PESn20 adsorption is 14, 10 and 4% lower than the equivalent PESn60 adsorption , for n=7, 13, and 20, respectively. There are few directly reported values for the adsorption in the literature, but Yang et al (15) quote a value of 70 Å2 for the area/molecule of Tween80. This compares with a value ~ 125 Å2 measured here by NR (see table S2 in the Supporting Information). The dependence of the area/molecule on the degree of ethoxylation is further illustrated in figure 3 for PESn60, for n varying from 3 to 50.

350

300

250

2

area/molecule (A )

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200

150

100 polysorbate surfactants linear nonionics

50

0 0

10

20

30

40

50

60

Degree of ethoxylation

Figure 3. Variation in area/molecule (from NR data) for nonionic surfactants with increasing degree of ethoxylation, (●) PESn60 ethoxylated polysorbate for n=3 to 50, (▲) dodecyl oligopolyethylene glycol (34) and the data point for C12EO23 is from surface tension data (39).The solid lines are guides to the eye only.

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Figure 3 shows that the variation in the area / molecule with the degree of ethoxylation for PESn60 is linear, for degrees of ethoxylation varying from 3 to 50. The variation in the area / molecule for the linear dodecyl oligopolyethylene glycol surfactants is also plotted in figure 3. There is agreement between both surfactants series up to a degree of ethoxylation of order 8. The variation in the area / molecule with the degree of ethoxylation for PESn60 remains constant up to a degree of ethoxylation of 30.0. It is only between the degrees of ethoxylation of 30 to 50 that the variation in the area / molecule starts to deviate from the linear dependence. Whereas for the linear dodecyl oligopolyethylene glycol surfactants the area/molecule starts to level off markedly for degrees of ethoxylation ≥ 8.

The variation in the area / molecule is explained by changes in the structure of the adsorbed layer, and has been discussed in detail for the linear dodecyl oligopolyethylene glycol nonionic surfactants (34, 40). The general increase in the area/molecule with the increase in the degree of ethoxylation for both series of nonionic surfactants is associated with an increased steric repulsion due to the larger ethoxylated headgroup. Detailed structural measurements were made for the linear dodecyl oligopolyethylene glycol surfactants (34, 40). From those measurements it was shown that the extent of the alkyl chain at the interface varies little with the size of the ethoxylated headgroup. For ethylene oxide chain lengths ≤ 6 the ethylene oxide chains are fully extended. For the longer ethylene oxide chains, their distribution at the interface is shorter than the fully extended length and so are more disordered. At this point they exhibit the polymer-like behaviour predicted by Samoria and Blankschtein (41) using the rotational isomeric state model. This would, however, indicate that the area / molecule should continue to increase linearly with the degree of ethoxylation, but as shown in figure 3 it does not. There are other factors which contribute, the attractive forces between the alkyl chains, and the increasing inter-mixing between the alkyl and ethylene oxide chains, which partially suppress the increase in the area / molecule. This is not observed for the more distributed ethylene oxide groups of the ethoxylated polysorbate surfactants, where the variation in the area / molecule remains linear up to much higher levels of ethoxylation. 12 Environment ACS Paragon Plus

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The variation in the surface layer thickness with the degree of ethoxylation is complex, and also depends upon the type and length of the alkyl chain. Comparing the results for PESn60 and PESn80, changing the degree of ethoxylation from 7 to 20 results in the adsorbed layer thickness increasing by ~ 9Å for PESn60 and by ~ 7Å for PESn80. Whereas the corresponding change for PESn20 is only ~ 5 Å. the comparison between PESn60 and PESn80 shows that changing from saturated to unsaturated alkyl chains not only has an impact on the packing at the surface, but also on the structure of the adsorbed layer. Reducing the alkyl chain length from stearyl to lauryl has an even greater effect. Assuming that the ethylene oxide groups are evenly distributed amongst the three sites on the sorbitan group (see figure 1) then the increase in thickness is substantially less than that associated with a fully extended ethylene oxide chain. Hence the ethylene oxide chains must be increasingly disordered as the degree of ethoxylation, as they are for the linear nonionic surfactants (34, 39). However, unlike the linear nonionic surfactants this does not reach a point where variation in the area/molecule no longer linear with the degree of ethoxylation until the degree of ethoxylation is between 30 and 50.

(b) Evaluation of the cmc

Cmc values for Tween20, Tween60 and Tween80 are quoted in the literature, with average values of 0.047, 0.021 and 0.016 mM for Tween20, Tween60 and Tween80 respectively (11-13). However, there is some spread in the quoted values; for Tween20 it varies from 0.042 to 0.056 mM and for Tween80 from 0.010 to 0.028 mM. Patist et al (11) discussed how impurities can affect the cmc determination, and its dependence upon the method used; but there are other factors that contribute. As discussed recently, these relatively low cmc values are in the region where depletion effects can start to dominate measurements such as surface tension at low concentrations (42). It was demonstrated (42) that by integration of the Gibbs equation,

σ

∫σ dσ = − RT ∫ 0

c

c0

Γ(c ) dc , the surface tension for the nonionic c

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surfactants can be reliably reconstructed from NR adsorption data; and this was demonstrated for the nonionic surfactant C18EO12. Hence from the isotherms measured and reported here and the surface tension in the region about the cmc, before depletion effects occur, the surface tension can be evaluated more reliably.

Figure 4. (a) Surface tension for Tween60. The solid line is a calculated curve as described in the main text, (b) Adsorption isotherm, from NR data (see also figure 2) for Tween60, and the solid line is a fit to a Langmuir Isotherm

The coverage of Tween60 (PES2060) determined by NR fit very well to a Langmuir isotherm, as shown in figure 2 and repeated here in figure 4b. The fitted Langmuir isotherm was used to calculated the 14 Environment ACS Paragon Plus

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surface tension (line shown in figure 4b), using the integrated Gibbs equation and the plateau value of the surface tension measured, as described above. The surface tension data and the surface tension recreated from the NR derived adsorption data are shown in figure 4a. The choice of the cmc value is in principle arbitrary and the value used in figure 4a was 50 µmol. However, if a higher value was used the calculated curve will lie above the first two of the last three data points. If a lower value is used the calculated line will lie below all of the last three data points. Thus the combination of the NR data and surface tension makes it possible to locate the cmc with much greater accuracy than possible using either technique on its own. The error in the value of the cmc is at most ±15 µM. The value of 50 µM obtained for Tween60 is higher than the values quoted in the literature (11-13). In figure S3 in the Supporting Information the surface tension data from Patist et al (11) for Tween20 is reproduced (figure 4 in reference 10). The measurements look reasonable, but the data at low concentrations, where depletion effects are evident, were used to determine the cmc. This results in a value which is too low. The NR data measured in the study can be used (as discussed above for Tween60) to calculate the slope just below the cmc (using the integrated Gibbs equation), and gives the red line in figure S3. The cmc is then the intercept of this curve with the plateau part of the surface tension, and this gives a cmc of 20 µm. This is lower than that determined by Patist et al (11). At first sight it is a surprise that the cmc of Tween20 is lower than for Tween60, and contrary to that reported in the literature. However, the limiting surface tension value for Tween20 is much lower at about 34 compared to 39 mNm-1 for Tween60, and so the trend in the cmc should not be entirely unexpected. (c) Adsorption at the oil-water interface The adsorption of PESn20, for n=7, 13, 20, was measured by SANS at the hexadecane-water interface, as described in the Experimental Details. The data in figure 5 are the scattering for PES2020 adsorption in the surfactant concentration range 0.4 to 1.8 mM.

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Scattered Intensity, I(Q) (cm-1)

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1

0.1 0.4 mM 0.6 mM 0.8 mM 1.0 mM 1.2 mM 1.5 mM 1.8 mM

0.01

0.001 0.01

0.1 -1

Wave vector transfer, Q (A )

Figure 5. Scattered Intensity for 6.7 vol% Hexadecane in D2O emulsion with PES2020 adsorption, for PES2020 concentrations from 0.4 to 1.8mM. The solid lines are model fits for a thin adsorbed layer of surfactant, using equation 1 as described in the text.

Similar data were obtained for PES720 and PES1320, but are not shown. The data shown in figure 5 arises only from the thin layer of hydrogeneous surfactant at the oil-water interface. At higher surfactant concentrations the additional contribution to the scattering arising from the formation of free surfactants in solution is evident, as described in detail elsewhere (38). For the dilute emulsions of the size used, the scattering in the Q range 0.004 to 0.25 Å-1 is in the Porod regime, and can be approximated as,

[

I (Q ) ≈ NV 2 Q −4 (ρ1 − ρ 0 ) + (ρ 2 − ρ1 ) + 2(ρ1 − ρ 0 )(ρ 2 − ρ1 ) cos Qd 2

2

]

(1)

where N and V are the emulsion droplet number density and volume, ρ0, ρ1, and ρ2 are the scattering length densities of the hexadecane, adsorbed layer (shell), and D2O, and d is the adsorbed layer thickness. The absolutely scaled SANS data are analysed using a standard core-shell sphere model (34), taking into account polydispersity, the known emulsion concentration and the instrumental resolution, to give thickness of the adsorbed layer, d, and a scattering length density, ρf.; as shown by the solid lines in figure 5. From this the adsorbed amount, Г, can be obtained,

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Γ=

d (ρ s − ρ1 ) N aVm (ρ s − ρ a )

(2)

where Vm is the surfactant molecular volume, ρs=ρ0=ρ2, ρa, and ρ1 are the scattering lengths of the solvent, surfactant, and the value for the adsorbed layer obtained from the model fits. The adsorbed layer is ~ 12 Å, and the adsorbed amounts, obtained using equation 2, are shown in figure 6.

-10

-2

mol cm )

2.5

Adsorbed amount (x10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0

1.5

1.0

0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Surfactant concentration (mM)

Figure 6. Adsorption isotherm for PESn20 at the hexadecane-water interface for, (●) PES2020, (▲) PES1320, and (■) PES720.

The adsorbed amounts for PESn20 at the lower surfactant concentrations are broadly independent of the degree of ethoxylation, and is similar to that observed at the air-water interface for PES2020 (see figure S1 in the Supporting Information). This is different to what is observed for PESn60 and PESn80, (see figure 2 and figure S2 in the Supporting Information) where the onset of adsorption is shifted to lower concentrations for EO7 compared to EO13 and EO20. Apart from the data for PES2020 in figure 6 there is no adsorption plateau visible before the onset of micellisation. However the adsorption immediately before micellisation is broadly similar to the saturation adsorption values at the air-water interface. The values at the oil-water interface are 1.25, 1.44 and 2.0 x 10-10 mol cm-2 for PESn20, for n=7, 13, 20 respectively; and this compares with values of 1.34, 1.78, and 2.8 x10-10 mol cm-2 at the airwater interface. The oil-water interface values were obtained by assuming that the SDS initially at the 17 Environment ACS Paragon Plus

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interface to stabilize the emulsion contributes little to the scattering. As the more surface active polysorbate surfactant is added it will rapidly replace the SDS, and from previous studies (38) it is estimated that at the lowest polysorbate surfactant concentration the SDS contribution will be less than 10%. The difference systematically increases as the degree of ethoxylation decreases. There are few values in the current literature for a direct comparison, but Apenten and Zhu (43) obtained a value of 1.7x10-10 mol cm-2 for the saturation adsorption of Tween20 at the hexane-water interface, which contrasts with the value of 1.25x10-10 mol cm-2 reported here for the hexadecane-water interface. This difference is consistent with the expected increase in adsorption as the chain length of the alkane decreases.

(d) Self-assembly

SANS measurements were made for PESn20 and PESn60, for n=7, 13 and 20, in D2O for hydrogeneous and deuterated ethylene oxide groups, at surfactant concentrations of 5, 10 and 20 mM. For the lower degree of ethoxylation of 7.0 the scattering from both types of surfactant has a Q-2 dependence, as shown in figure 7a for PES760.

100

Scattered Intensity, I(Q) (in cm-1)

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5 m|M 10 mM 20 mM

10

1

0.1

0.01

0.001 0.01

0.1

Wave vector transfer, Q (A-1)

(a)

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10

Scattered Intensity, I(Q) (cm-1)

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1

0.1

0.01

h, 20 mM h, 10 mM h, 5 mM d, 20 mM d, 10 mM d, 5 mM

0.001 0.01

0.1 -1)

Wave vector transfer, Q (A

(b) Figure 7. Scattered Intensity for (a) 5, 10, 20 mM PES760 in D2O, (b) 5, 10 , 20 mM PES2060 in D2O, with deuterated and hydrogeneous ethylene oxide group, see legend for details. The solid lines in 7b are model fits as described in the main text and for the model parameters summarized in table 2.

The Q-2 dependent scattering is consistent with the formation of planar structures. The lack of further structure in the Q-2 scattering is consistent with the formation of unilamellar or relatively flexible multilamellar vesicles. At the higher degree of ethoxylation of 20 the scattering data for both of the surfactant types are consistent with small globular micellar structures, as shown in figure 7b. The transition between micellar and planar structures depends upon the alkyl chain length and degree of ethoxylation, as summarized in table 1. Surfactant type

Degree of ethoxylation, n

Solution Phase

PESn20

7

Lamellar

13

Micellar

20

Micellar

7

Lamellar

13

Lamellar/micellar

20

Micellar

PESn60

Table 1. Solution phase behaviour of the ethoxylated polysorbate surfactants PESn20 and PESn60. 19 Environment ACS Paragon Plus

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For the lower degree of ethoxylation of 7.0 the solution structures are planar, and for the highest degree of ethoxylation of 20.0 the structures are micellar, independent of the alkyl chain length. At the intermediate degree of ethoxylation of 13.0 the transition from planar to micellar structures depends upon the alkyl chain length; and there is a greater tendency towards planar structures for the PESn60 with the longer stearyl alkyl chain length.

The data measured with hydrogeneous and deuterated ethylene oxide groups, in both the planar and micellar regions, show no systematic changes other than in the form factor and the absolute scattering; which are both due just to changes in the ‘contrast’. The variations with surfactant concentration show no systematic changes with concentration apart from the change in the absolute scattering and the scattering scales directly with solution concentration.

The scattering data in the micellar regime were analysed using a well established core-shell model for globular interacting micelles (44). In that case the scattering can be described within the decoupling approximation as, I (Q ) = N p  S (Q ) F (Q ) 

2 Q

+ F (Q )

− F (Q )

2 Q

2 Q

 

(3)

S(Q) is the inter-micelle structure factor, which in this case is essentially a hard sphere structure factor,; and is characterized by the micelle number density, Np and the micelle diameter, σ. F(Q) is the micelle form factor, which describes the micelle shape and size. Here it is modeled as a core-shell particle with an inner core of alkyl chains with a radius R1, and an outer shell of headgroups and associated hydration with a radius R2 (σ=2R2), as described in detail elsewhere (44). Here R2 is taken as an adjustable parameter to accommodate the packing of the headgroups in the outer shell. Molecular constraints are included and the micelle inner radius is limited to the fully extend alkyl; chain length, lc, such that R1=lc for spherical micelles. For aggregation numbers, ν, larger than can be packed into a spherical geometry,

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it is assumed that the micelles are elliptical (prolate ellipses) with core dimensions of R1 and R1.ee (ee is the elliptical ratio). From the known molecular volumes, neutron scattering lengths and solution concentration, the scattering can be calculated using equation 3 on an absolute scale, and compared with the data. The key refinable model parameters are then ν and R2. An acceptable model fit is when the functional form of the data is represented and the absolute intensity is predicted to within ±20%; and the comparison is evaluated using a least squares criterion. There is an additional constraint, in that the data for the hydrogeneous and deuterated ethylene oxide surfactants fit the same model. The solid lines in figure 7b are model fits using the model described above, and the model parameters for the measurements with deuterated and hydrogeneous ethylene oxide groups are within error the same. Furthermore there is no significant variation with surfactant concentration, in the range 5 to 20 mM. The key model parameters from the micelle analysis summarized in table 2 are an average over the three different concentrations and the two different ethylene oxide contrasts.

Surfactant

Aggregation number, ν, (±10)

R1(±1Å)

R2 (±2Å)

Ellipticity, ee (±0.05)

PES2020

90

17

30

1.6

PES1320

150

17

27

2.8

PES2060

125

24

39

1.1

Table 2. Key model parameters from analysis of micelle data for PES2020, PES1320 and PES2060 For both the dodecyl and stearyl alkyl chains the relatively large headgroup ensures that the micelles are globular, and are best described as ellipsoids with a modest axial ratio or ellipticity, ee. For PES2020 an aggregation number ~ 90 is comparable to a value ~ 100 for the linear nonionic surfactant C12EO12 (45). For PES1320 the reduced level of ethoxylation results in a smaller R2 value and micellar growth, such that the aggregation number is now ~150. For PES2060 the mean aggregation number is now larger than for PES2020, and this reflects the larger stearyl alkyl chain of PES2060. However the larger micelle core 21 Environment ACS Paragon Plus

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results in a small elliptical distortion, and the increased aggregation number can be accommodated in a more globular structure. The trends summarized in table 2 are qualitatively consistent with the simple packing criteria of Israelachvili, Mitchell and Ninham (46). There are some limited estimates of micelle aggregation numbers for the Tween Surfactants in the literature to compare with the results presented here. From a pair distribution distance function analysis of scattering data from Tween80 aggregates, Varade et al (47) report a core radius ~20 Å and an outer radius ~ 40 Å. Using a similar approach Bester-Rogac (48) reported a core radius ~21 to 24 Å and an outer radius ~ 45 Å for Tween40; and translated this into an aggregation number ~95. Behera et al (49) quote values of 86, 90, 111, and 123 for the aggregation number of Tween20, Tween40, Tween60 and Tween80 respectively from fluorescence studies. Bhattacharjee et al (5) quote an aggregation number for 0.1 M Tween80 of 179, with a core radius of 25.6 Å and a hard sphere radius of 46.9 Å. The results of Bhattacharjee et al (5) for Tween80 are in good agreement with the PES2060 (Tween60) data presented here, but the values for the aggregation number from Behera et al (49) are significantly smaller than those reported here or by Bhattacharjee et al (5). The values quoted by Behera et al (49) are from dye solubilisation studies by Bhattacharya et al (50). These are a more indirect evaluation and assume a 1:1 complex formation between the surfactant and safranine dye molecule. Although the quoted value for Tween20 is in good agreement with the data presented here, the values obtained for Tween60 and SDS are significantly underestimated. Bhattacharya et al (50) quote a value of 52 for SDS, whereas Hayter and Penfold (45) quote a value of 75. Hence it seems likely that the complex formation will substantially affect the aggregation process. The results for Tween20 and Tween40 from Bester-Rogac (48) and Behera et al (49) for Tween20 are consistent with those reported here for PES2020 (Tween20). The micellar growth and ultimate transition to planar structures for the ethoxylated polysorbate surfactants with decreasing degree of ethoxylation is consistent with the observations of Varade et al (47). Varade et al (47) reported micellar growth in Tween80 micelles with the addition of the linear nonionic surfactants, C12EO3 and C14EO3; due to a decrease in the effective headgroup area with the addition of 22 Environment ACS Paragon Plus

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the CnEO3 cosurfactant. Bester-Rogac (48) also reported micellar growth with increasing surfactant for Tween40 micelles, but at solution concentrations much higher than studied here, >10 wt %.

SUMMARY

We have showed how the adsorption at the air-water and oil-water interface varies with the alkyl chain length and the degree of ethoxylation of the ethoxylated polysorbate surfactants. Reducing the degree of ethoxylation and increasing the alkyl chain length result in an increase in the adsorption and both interfaces. Reducing the degree of ethoxylation results in a transition from the formation of globular micelles to planar structures. This transition is sensitive to the alkyl chain length and occurs at a higher degree of ethoxylation for longer alkyl chain lengths. The range of ethoxylated polysorbate surfactants synthesized and characterized provide a range of surface and self-assembly properties. This provides the opportunity to substantially extend the range of applications of Tween surfactants into areas requiring new or modified functionalities.

ACKNOWLEDGEMENTS

The provision of beam time on the INTER and SANS2D instruments at ISIS is acknowledged. The invaluable scientific and technical assistance of the Instrument Scientists, and support staff is gratefully recognized.

SUPPORTING INFORMATION AVAILABLE

Tables and figures of supporting data and key model parameters are included in the Supporting Information. This material is available free of charge via the internet at http://pubs.acs.org. 23 Environment ACS Paragon Plus

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AUTHOR INFORMATION

Corresponding Author: Jeff Penfold, [email protected] Author Contributions All the authors have given their approval of the final version of the manuscript

Funding Sources Neutron beam time at the ISIS Facility, UK (STFC).

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TABLE OF CONTENT GRAPHIC

Adsorption at the air-water and oil-water interfaces, and the selfassembly in aqueous solution of ethoxylated polysorbate nonionic surfactants Jeffrey Penfold, Robert K Thomas, Peixun X Li, Jordan T Petkov, Ian Tucker, John R P Webste2, Ann E Terry

140

Tween60EO7 Tween60EO13 Tween60EO20

-2

mol cm )

4

120

2

area/molecule (A )

3

-10

Adsorbed amount (x10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

2

100

80

60

1

polysorbate surfactants linear nonionics

40

0

20 1

10

100

1000

0

5

Surfactant concentration (micromoles)

31 Environment ACS Paragon Plus

10

15

Degree of ethoxylation

20

25

Adsorption at air-water and oil-water interfaces and self-assembly in aqueous solution of ethoxylated polysorbate nonionic surfactants.

The Tween nonionic surfactants are ethoxylated sorbitan esters, which have 20 ethylene oxide groups attached to the sorbitan headgroup and a single al...
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