Structure of High Internal Phase Aqueous-in-Oil ... - ACS Publications

J. Phys. Chem. B 2009, 113, 12243–12256

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Structure of High Internal Phase Aqueous-in-Oil Emulsions and Related Inverse Micelle Solutions. 4. Surfactant Mixtures Philip A. Reynolds, Elliot P. Gilbert,† Mark J. Henderson, and John W. White* Research School of Chemistry, The Australian National UniVersity, Canberra ACT 0200, Australia ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: July 1, 2009

The effects of combinations of surfactants on the structure and stability of high internal phase water-inhexadecane and saturated ammonium nitrate-in-hexadecane oil-based emulsions and oil-based inverse micellar solutions are reported. The combinations were 750, 1200, and 1700 molecular weight monodisperse and 450 and 1000 molecular weight polydisperse polyisobutylene acid amides, and sorbitan monooleate. The samples made from mixtures have qualitatively similar nanostructures to emulsions made from single surfactants. Again, for the emulsions, micrometer-scale aqueous droplets are dispersed in a continuous oil phase, which contains inverse spherical micelles composed of surfactant, hexadecane, and water. In quantitative terms, lower average surfactant molecular weight, lower ammonium nitrate content, and lower surfactant content increased the swelling of micelles, their water content, and the tendency of the emulsion to be unstable and form a sponge phase. This instability also allows micelle plasticity such that their geometry and content in mixed surfactant systems are not simply predictable by interpolation from single surfactant systems. An example was found of a mixed micelle 3 times larger than either single component micelle. The observed behavior suggests that mixing surfactant molecules of very different molecular weights destabilizes the emulsions, while mixing surfactants close in molecular weight has the opposite effect. The synergistic effects of surfactant molecular weight polydispersity and binary mixing are most marked for 1:1 molecular mixtures of surfactant. Introduction Mixtures of surfactants can produce synergistic effects, in that a property cannot be linearly interpolated from the properties of the single surfactant components. In this paper, as part of a continuing study,1-3 we report such effects in the structural properties in high internal phase emulsions caused by mixing monodisperse polyisobutylene (PIBSA) surfactants of different molecular weight and related effects when polydisperse surfactants are used. These synergies are of potential use in emulsions of importance in the food and explosives industries, where stability may be improved in an economic manner. In previous papers,1,2 isotopic (D/H) contrast variation and the small angle neutron scattering (SANS) from aqueous/ hydrocarbon high internal phase emulsions, and related inverse micelle solutions, have elucidated the emulsion nanostructure for the case of single PIBSA surfactants of molecular weight (MW) about 1200. The emulsions have about 90% aqueous phaseseither water or almost saturated ammonium nitrate solutionssdispersed as micrometer-scale droplets in a continuous hexadecane oil phase. The SANS results can be modeled consistently with a single model invoking the sum of the scattering from the oil phase, containing nanometer-scale surfactant/water inverse micellar structures, and scattering from the aqueous droplet-oil interface. The amount of surfactant adsorbed at the aqueous droplet interface, the roughness of that interface and the inverse micellar amounts, dimensions and compositions in the oil phase were all measured. The dependence of the scattering on surfactant concentration2 proved that the micellar structures were spherical and that the surfactant * To whom correspondence should be addressed. E-mail: jww@ rsc.anu.edu.au. Fax: (61) 2 6125 4903. Phone: (61) 2 6125 3578. † Current address: ANSTO, Private Mailbag 1, Menai, NSW 2234, Australia.

loading at the oil-water droplet interface was almost independent of dilution. At the highest concentrations, only 5% of the surfactant was at the interface and the rest is in the oil phase. The headgroup area per molecule at the interface was 140 Å2, which corresponded well with that expected for almost a monolayer of surfactant. In a further paper,3 both emulsions and inverse micellar solutions were studied by SANS to determine the effect of varying monodisperse surfactant molecular weight (MW): MW 750, MW 1200, and MW 1700 PIBSAs were used. A variety of structures of increasing curvatureslamellar, sponge, spherical micellar, and emulsion phasesswere detected with various surfactant concentrations and aqueous phases. We have been able to generalize our results to state that phases of higher curvature, and in emulsions higher dispersed phase surface area, are favored by higher surfactant molecular weight, higher surfactant concentrations, and higher ammonium nitrate content in the aqueous phase. The structures observed in the oil phase of the emulsions were close to those observed in related inverse micellar oil solutions, except for a slight tendency to higher structural curvature in ammonium nitrate solution-based emulsions. Nanostructure in the oil phase, where present, extended to cylindrical and or spherical micelles with varying water content but constant hexadecane content. As with the phase diagram, the micelles are of higher curvature (spheres rather than rods, or smaller rather than larger spheres) when surfactant molecular weight and concentration are increased and when ammonium nitrate is used as the aqueous component. The surfactants, used singly, have mostly shown spherical inverse micelles in the oil phase, except where there are particular combinations of low concentration and low molecular weight of surfactant, and a water aqueous phase.

10.1021/jp903475z CCC: $40.75  2009 American Chemical Society Published on Web 08/14/2009

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Figure 1. Structure of the “MW 1200 PIBSA” surfactant.

It has often been observed in inverse micelles made from mixed surfactants that micelle properties vary smoothly and approximately linearly between the single components.4-10 In most cases, for concentrated surfactant mixtures, there is a single type of micelle with a surfactant composition the same as that of the mixed ingredients. For these systems, the surfactant components are not demixed. There are exceptions, a notable one being the use of mixed surfactants to produce disk shaped micelles.11 These observations have often been complicated by phase changes (e.g., sphere to rod transitions), or complex morphology as ellipsoidal micelles.12-16 Rocca and Stebe17 have examined by use of SANS a series of mixed surfactants and oils in fluorinated high internal phase emulsions. However, there are complications due to the immiscibility of pure fluorocarbons and hydrocarbons. This may be why they did not extract quantitative structural information on micelle geometry and droplet characteristics from their results. In one case, however, an observation of high structural nonlinearity has been observed by time-resolved fluorescence quenching.18 With these points in mind, a number of mixtures have been examined in this paper using SANS. These are emulsions and related inverse micellar oil solutions made with binary mixtures of monodisperse MW 750, MW 1200, and MW 1700 PIBSAs; mixtures of polydisperse MW 450 and 1000 PIBSAs; and mixtures of sorbitan monoleate (SMO) and MW 1200 PIBSA. This was to determine whether the structures formed by the surfactants in both emulsions and inverse micelles can be predicted by interpolation from the structures found in single surfactant systems. A related question is whether a mixture of two monodisperse surfactants performs the same functions as a surfactant mixture with a continuous polydisperse molecular weight distribution. Experimental Section Surfactants. Three monodisperse polyisobutylene N-(2hydroxyethyl)succinamide (PIBSA) samples (Mw/Mn ) 1.1) were prepared and characterized as described previously.2 The number of isobutylene repeats was estimated as integer n ) 10, 16, and 27, respectively. The middle member is illustrated in Figure 1. Rounded molecular weights of 750, 1200, and 1700 have been used throughout to label these three surfactants. Commercial sorbitan monooleate (Aldrich) (SMO), a mixture with the oleic acid/sorbitan in the ratio ca. 1.5, and two polydisperse PIBSA mixtures with Mw/Mn ) 1.5, labeled as MW 450 and MW 1000 to reflect the mean molecular weight, were used to compare the effects of polydispersity with the results from the monodisperse mixtures. Preparation of Emulsions and Inverse Micelles. To prepare the emulsions, the n-hexadecane (D/H mixtures) and dissolved surfactant were preheated in a water bath at ca. 80 °C. While stirring, water (D/H mixtures), or D2O almost saturated in ammonium nitrate (53 wt %), also preheated to 80 °C, was added slowly. This was followed by 5 min of rapid stirring at a speed to ensure thorough mixing.1 This method reproducibly forms 10 g quantities of emulsion. Sample 1009 was more free-

Reynolds et al. flowing, while 1010 initially did not form, and required 10 min of stirring to prepare. Inverse micelles were formed in oil by adding a drop of H2O or D2O, either pure or saturated in ammonium nitrate, to ca. 300 mg of dry surfactant solution in hexadecane, and left for several days. A clear hexadecane phase forms as an upper layer with a much smaller amount of a second denser white waterrich phase. We presume the white phase is water/surfactant, perhaps even emulsion. The majority clear phase was examined by SANS. If the mixture were agitated, there was a tendency toward emulsification ending in a single cloudy, mostly hexadecane phase. The inverse micellar solutions produced are listed in Table 1 and the emulsions in Table 2. SANS Experiments and Modeling. SANS experiments were performed on the LOQ instrument at the Rutherford Appleton Laboratory, United Kingdom.19 For the emulsion samples, the experiment, data correction, reduction, and background correction were as previously described.1-3 The inverse micellar solutions and emulsions were run in 1 mm sample thickness quartz cells. 35 new emulsion and 15 inverse micelle mixture samples will be discussed, in conjunction with previously published data using single surfactants.1-3 The emulsions were studied in pairs. One member contrast matched (CM), so that mainly deuterated oil and water phase scattering length densities are matched at high values, to contrast strongly with the low scattering length density of the hydrogencontaining surfactant. The other member, called contrast unmatched (UM), made up to highlight the aqueous droplet-oil phase interface. The surfactant content used in discussion will be the percentage figure for the weight percentage of surfactant in the whole emulsion or inverse micellar solution. The emulsions had an 89-92% aqueous phase content. The contents of the emulsions and inverse micellar solutions from which new SANS was measured are listed in Tables 1 and 2. The contents of others from previous papers are listed there. As an aid to the reader, the text sample labels are font-coded to correspond with the color-coded figures. Ordinary type labels indicate that the fits are not shown in the figures in this paper, bold font that they are illustrated in the figures. Thus, it is possible to easily track in the tables and text which experimental results are illustrated. We have modeled the total scattered intensity as the sum of a contribution from a flat aqueous-oil interface (locally rough but long-range flat layered interfaces), a contribution from the inverse micelles themselves (a Schultz polydisperse PercusYevick hard sphere fluid of spherical micelles with core and shell internal structure), and a flat background. On occasion, we also tried to model the inverse micelle scattering as from an assembly of monodisperse rod-like micelles. We incorporate experimental resolution functions, as summarized in the previous three papers in this series.1-3 For the emulsions, we have fixed aqueous droplet/oil phase roughnesses at 25 Å for water droplets and 5 Å for saturated ammonium nitrate droplets. These are estimated from previous SANS results.1-3 Results The neutron scattering functions presented in Figures 2-7 illustrate how very sensitive the method is to the structural changes produced by the permutations and combinations of the experiments, for both the micellar solutions and the whole emulsions. The success of the simple models of structure is apparent from the quality of fitting to the data. The quantitatively determined parameters from these fits, on an

Structure of High Internal Phase Emulsions

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TABLE 1: Compositions of Inverse Micellar Systems Generating Mixtures (in mg)a label

surf.

wt surf.

surf.

wt surf.

C16D34

64m 63m 85m 84m 87m 86m 88m 1000m 1001m 1002m 1003m 1004m 1005m 1006m 1007m

750 750 750 750 750 750 750 750 750 750 750 750 750 750 750

16 75 49 38 47 16 48 41 42 59 61 72 71 83 83

1200 1200 1200 1200 1700 1700 1700 1200 1200 1200 1200 1200 1200 1200 1200

14 37 54 78 50 17 54 40 41 19 20 9 10 0 0

339 338 327 342 333 343

a

C16H34

D2 O

H2O

AN-D2O

phases

42 40 32 34 44 39 39 39

2 ? 2 2 2 2 2 2 2 2 2 2 2 2 2

AN-D2O

match

11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830 11830

CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM UM CM

40

333 291

37 43 38 41 47 40

325 288 329 296 323 285 328

AN ) ammonium nitrate, surfactant PIBSAs labeled by molecular weight, unless SMO.

TABLE 2: Compositions of Emulsions (mg)a label

surf.

wt surf.

surf.

wt surf.

C16D34

57 58 59 60 61 1003 1004 1005 1006 1007 1008 1009 1010 1019 1020 1021 1022 1023 1024 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535

750 750 750 750 750 750 750 750 750 750 750 750 750 SMO SMO SMO SMO SMO SMO 450 450 450 450 450 450 450 450 450 450 450 450 450 450 450 450

75 75 41 37 39 101 103 149 150 175 175 200 200 105 101 150 150 173 177 0 0 50 50 100 100 150 150 200 200 0 0 25 25 50 50

1200 1200 1200 1200 1700 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000

74 75 114 113 154 101 104 51 51 26 24 0 0 102 101 51 52 25 25 200 200 150 150 100 100 50 50 0 0 50 50 25 25 0 0

1140 1120 1130 1120 1130

a

827 820 816 805 811 809 805 815 815 815 815 815 815 815 815

C16H34

D2 O

H 2O

8400

270 8710 290 8760 820

8400 8340 837 110 818 100 821 102 821 105 806 103 808 104 813 106 815 106 815 106 815 106 815 106 815 106 815 106 815 106 815 106

In the last column, CM refers to aqueous/oil contrast matched, UM to unmatched for each pair of similar emulsions.

absolute intensity scale, are summarized in the tables because of the large number of experiments conducted. The broad features of the micellar systems are discussed below in terms of these parameters against the questions posed below. The related properties of the emulsions and the role of polydispersity are presented. Inverse Micelles. Because inverse micelles are a key component of emulsions made from PIBSA, and because of the differences found in the inverse micelle phase with single PIBSA surfactants of different molecular weights, we examine

first the inverse micelle composition balances of the mixtures. The results from inverse micelles were designed to answer four questions: (1) Do surfactant mixture samples have the same structure as samples made with single surfactants? (2) Are the properties of samples made with a 50/50 mixture of surfactants different from those of samples made with 100% of an intermediate molecular weight surfactant? (3) As relative percentages of surfactant components change in a sample made with a mixture of surfactants and

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Figure 2. Comparison of molecular weight equivalent mixed and pure PIBSA surfactant inverse micelle scattering in the water-based emulsion for contrast matched oil and aqueous components. Concentrated surfactants: (red b) (87m) 50% MW 750 and 50% MW 1700; (red 9) (3m) 100% MW 1200 surfactant. Dilute surfactants: (blue O) (86m) 50% MW 750 and 50% MW 1700; (blue 0) (50m) 100% MW 1200. Solid lines are fits.

TABLE 3: Structural Parameters Derived from Fitting the Inverse Micelle Data from a Variety of Surfactant Samplesa surfactant nature (fractional wt %) sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles shell SLD (×10-6 Å-2) radius of core (Å) core SLD (×10-6 Å-2) a

50% 750, 50% 1700 50% 750, 50% 1700 50% 750, 50% 1700 50% 750, 50% 1200 50% 750, 50% 1200 50% 750, 50% 1200 87m 52.08(4) 0.123(1) 0.283(1) 3.19(1) 25 7.32(1)

88m 52.1 0.123 0.283 -0.46(1) 24.3(5) 3.3(2)

86m 64.96(8) 0.123(1) 0.112(1) 2.89(2) 33.2(4) 5.60(4)

64m 41.7(1) 0.141(2) 0.083(1) 0.90(4) 13 -1.2(3)

85m 46.5(4) 0.119(1) 0.259(1) 2.58(1) 13 7.86(9)

1001m 30.9(1) 0.26(1) 0.25 1.94(1) 13 3.28(3)

Parameters constrained from other refinements are listed without an attached error.

ammonium nitrate, how does the structure change? Can we interpolate linearly? (4) Does a change to water-based emulsions change the conclusions from (3)? (1) InWerse Micelles: Single and Mixed Surfactant Comparisons. [Samples 64m, 85m, 86m, 87m, 88m, 1001m]. Table 3 shows the results of refinements of the scattering functions from inverse micellar solutions made with 50 wt % pairs of the three monodisperse PIBSA surfactants. Satisfactory refinements were found for a model in which the surfactant resides in the oil as a Percus-Yevick fluid of polydisperse spherical inverse micelles. The micelles have a water core covered by a mixed surfactant tail/hexadecane shell. This results in six refinable parameters (micelle radius, polydispersity in size, volume fraction of micelles in the fluid, the scattering length density of the micelle shell and core, and a flat background). The quality of the fits is illustrated for samples 86m and 87m in Figure 2. As can be seen from the standard errors in Table 3, the correlation between these parameters is small, and the fitting has not been overparametrized. As explained in ref 2, dilution data on these systems confirms that this model of spherical micelles is the only reasonable one that will fit all data. Taken in pairs, the effect of change in surfactant molecular weight, surfactant concentration, substitution of saturated ammonium nitrate for water, and details of the location of water and hexadecane in the inverse micelles are given in Table 3. To calculate the composition of the micelle, we compare the fitted values of the shell (or core) scattering length densities with those for pure, bulk, hexadecane, and PIBSA. This assumes

that the mixing in the shell is ideal with the molecular volumes of each component constant. For example, comparison of 88m with 87m shows that the shell contains ca. 40% by volume of hexadecane, with some infiltration into a core region. This is indicative of some disorganization of the shell and is also observed with pure SMO samples.3 The only H2O sample, 64m, with its low core scattering length density (SLD) compared to all the others, shows that there is indeed water in the core. Reducing the surfactant concentration (87m and 86m) increases the micelle size, and increases the water content at the micelle core, just as for single surfactant samples. Similarly, substitution of saturated ammonium nitrate for water (85m and 1001m) decreases the micelle radius and water content, again just as for single surfactant samples. Thus, inverse micelles composed of surfactant mixtures behave structurally in a similar way to single surfactant inverse micelles. (2) Are the Properties of Samples Made with a 50/50 wt % Mixture of Surfactants Different from Those of Samples Made with 100% of an Intermediate Molecular Weight Surfactant? [Samples 2m, 3m, 50m, 85m, 86m, 87m, 88m]. In Table 4, we have the results from three 50/50 weight percentage surfactant mixtures of 750 and 1700 PIBSA compared to pure 1200 samples of otherwise similar composition. With some constraints, all refine satisfactorily to a spherical micelle model. The constraints involve fitting, where necessary, the core radius, micelle radius, polydispersity, and volume fraction. In some samples, the SLD contrasts are insufficient to define all of these parameters. When the parameters are allowed to vary freely, the associated standard errors become unacceptably large. The



TABLE 4: Structural Parameters Derived from Fitting the Inverse Micelle Data from Mixed 750/1700 Surfactants Compared with Analogous Pure 1200 Inverse Micelles surfactant nature (fractional wt %) 50% 750, 50% 1700 100% 1200 sample identification 87m 3m micelle radius (Å) 52.1(1) 31.21(4) micelle polydispersity 0.123(1) 0.164(1) volume fraction of micelles 0.283(1) 0.332(1) shell SLD (×10-6 Å-2) 3.19(1) 2.47(1) radius of core (Å) 25 13 core SLD (×10-6 Å-2) 7.32(1) 3.82(6)

50% 750, 50% 1700 100% 1200 86m 50m 64.96(8) 38.4(3) 0.123(1) 0.135(6) 0.112(1) 0.090(2) 2.89(2) 2.94(5) 33.2(4) 13 5.60(4) 3.75(3)

50% 750, 50% 1700 100% 1200 88m 2m 52.1 30.46(4) 0.123 0.168(2) 0.283 0.327(2) -0.46(1) 2.29(1) 24.3(5) 13 3.3(2) 2.24(6)

TABLE 5: Structural Parameters Derived from Fitting the Inverse Micelle Data from Mixed MW 750/ MW 1200 Surfactant Samples in Contact with Saturated Ammonium Nitratea surfactant nature (fractional wt %) 0% 750, 100% 1200 50% 750, 50% 1200 75% 750, 25% 1200 88% 750, 12% 1200 100% 750, 0% 1200 sample identification 99m 1001m 1003m 1005m 1007m 100m 1000m 1002m 1004m 1006m micelle radius (Å) 29.4(1) 30.9(1) 29.3(1) 28.4(1) 27.9(1) 28.7 30.9 29.3 28.4 27.9 micelle polydispersity 0.14 0.26(1) 0.36(1) 0.39(1) 0.43(1) 0.14 0.18(1) 0.26(1) 0.30(1) 0.34(1) volume fraction of micelles 0.232(1) 0.25 0.26 0.265 0.27 0.23 0.25 0.26 0.265 0.27 scattering length density of shell 2.44(3) 1.94(1) 1.55(1) 1.43(1) 1.33(1) -0.20(3) 0.12(2) 0.29(2) 0.47(2) 0.46(2) scattering length density of core 2.1(1) 3.28(3) 3.74(3) 3.94(3) 4.47(3) 1.5(2) 2.6(2) 1.8(2) 2.1(2) 1.7(2) a

Parameters without errors were fixed from other sources. Top row refers to oil phase C16D34; bottom row refers to C16H34. All core radii are fixed at 13 Å.

quality of the fitting is illustrated in Figure 2. Since 1200 is almost the average of 750 and 1700, this experiment examines the effect of surfactant molecular weight distribution on structural properties of the emulsion. The single surfactant endmember data are, mainly, reproduced from ref 3. Figure 2 shows two of these comparisons: one concentrated in surfactant (filled symbols), one dilute (open symbols). In all of the scattering plots, a log-log scale has been used, so the differences observed here are order of magnitude changes and are not small. Note that on the absolute scale the scattering intensity at maximum is about 10 cm-1 because the contrast has been adjusted to maximize the surfactant scattering. It is clear from the figure that the mixed surfactant inverse micelles are very different from the molecular weight equivalent single surfactant inverse micelles. The 50 wt % mixtures contain much larger micelles with higher water content than the samples from the mean molecular weight single surfactant. Samples 87m and 85m, in Table 3 allow us to see the result of changing the molecular weight of one component. For single surfactant samples, an increase in molecular weight favors lower micelle radii and water content. By contrast, 87m and 85m show a larger micelle radius and higher core water content when one component increases in molecular weight from 1200 to 1700 (Table 3). We conclude that inverse micelles made by mixing two monodisperse surfactants of very different molecular weights are larger than those generated from a single monodisperse surfactant of the same average molecular weight. We also see that the more disparate the molecular weights, the larger the expansion of micelles. (3) Effect of Surfactant Mixture Composition on InWerse Micelle Structure (Ammonium Nitrate Solutions). [Samples 99m, 100m, 1000m, 1001m, 1002m, 1003m, 1004m, 1005m, 1006m, 1007m]. Table 5 shows the results from saturated ammonium nitrate aqueous phase inverse micellar solutions, for which the surfactant mixture varied from pure MW 750 to pure MW 1200. Figure 3 shows examples of the data that can be

directly compared to related samples whose aqueous phase is water in Figure 2. For the samples 1006m and 1007m, a model of monodisperse micellar rods gives a refined value of the rod length approximately equal to the diameter. A monodisperse spherical micelle model gives, as one might then expect, a similar quality fit. However, a polydisperse spherical micelle model gives a significantly better fit. Modeling this polydisperse sphere system, allowing all variables to refine freely, results in parameters that are poorly defined, even when the data from contrast related pairs of samples (e.g., 1006m and 1007m) are used as constraints. To reduce the number of parameters per pair of samples by two, we note that the systems are well above the surfactant critical micelle composition (cmc), the micelle composition is known, as is the amount of PIBSA in the sample. Assuming no PIBSA is present as dissolved molecules, the volume fraction of micelles in the system may thus be calculated and no longer regarded as a variable. An internal consistency check is that the refined values of the shell and core SLDs should agree with those assumed for the micellar composition. Assuming a hexadecane content varying from 40% for MW 1200 samples to 60% for MW 750, as determined in our previous paper,3 then we can calculate shell SLDs varying from 3.9 to 2.4 × 10-6 Å-2. The refined values are 2.4 and 1.3 × 10-6 Å-2. These values are close to but lower than the calculations, showing a noticeable but small fraction of the added PIBSA remains dissolved as molecules in the oil. These constraints enable the smoothness of change with composition to be examined by use of a polydisperse sphere model for the inverse micelles. The pure MW 1200 inverse micelles 99m and 100m were previously modeled3 as an assembly of monodisperse spherical micelles with an outer shell of surfactant incorporating ca. 30% hexadecane, and a well-defined small inner core incorporating water. However, the saturated ammonium nitrate solution inverse micelles 89m-94m made from only MW 750 surfactant of the previous paper3 required a model of rod-like micelles. For those

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Figure 3. Comparison of molecular weight equivalent mixed and pure PIBSA surfactant inverse micelle scattering in ammonium nitrate-solutionbased emulsion: (red b) (1005m) 88% MW 750 and 12% MW 1200; (red 9) (1007m) 100% MW 750 surfactant; (blue O) (1001m) 50% MW 750 and 50% MW 1200 surfactant; (blue 0) (1003m) 75% MW 750 and 25% MW 1200 surfactant. Solid lines are fits.

TABLE 6: Structural Parameters Derived from Fitting the Inverse Micelle Data from Mixed MW 750/MW 1200 Surfactant Samples in Contact with D2O and with C16D34 Oil Phase surfactant nature (fractional wt %) sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell radius of core scattering length density of core

0% 750, 100% 1200 33% 750, 67% 1200 50% 750, 50% 1200 67% 750, 33% 1200 100% 750, 0% 1200 3m 84m 85m 63m 83m 31.21(4) 40.7(1) 46.5(4) 83.2(1) sheet 0.164(1) 0.121(1) 0.119(1) 0.341(2) structure 0.332(1) 0.283(1) 0.259(1) 0.301(2) 2.47(1) 2.52(1) 2.58(1) 3.41(1) 13 13 13 56.3(3) 3.82(6) 4.62(9) 7.86(9) 4.73(1)

samples, containing only 4% surfactant, the rods exceeded 1000 Å in length. For the 13% surfactant sample, the rods were shorter (90-190 Å in length) but still appreciably larger than the rod diameter of 56 Å. In our new samples, 1006m and 1007m, the surfactant concentration was 20% and a polydisperse spherical micelle model gave a good fit. Thus, increasing surfactant concentration has induced a rod to sphere micellar transition. The high polydispersity observed, however, shows that the resulting spherical micelle system is still not quite stable. As the fraction of MW 750 surfactant in the mixtures was increased, the micellar radius decreased slowly, but the major effect is that the polydispersity markedly increased. We note that the polydispersity for the samples containing hexadecane, C16H34, was slightly less than that for those containing C16D34. The former have scattering dominated by the core and the latter by both core and shell because of the contrast. There is no reason that the variability in the core should be the same as that for the shell. The four values for the SLDs of the shell and core may be used to define the micellar composition. For the C16H34containing samples, high values of the SLD reveal the location of deuterated water. Consistently, there was an approximately constant core, about 50% by volume water, together with a steadily increasing water content in the shell as the MW 750 surfactant content was increased. The C16D34 samples showed a significant hexadecane content of the shell, decreasing as the MW 750 content was increased. Comparison of the two core SLDs shows a steadily increasing hexadecane content in the core with increased MW 750 surfactant concentration. Thus, when the MW 750 content is increased, not only do the spherical micelles become unstable (or polydisperse) in size,

but also the core and shell regions become less clearly defined. This is reflected in Figure 3, where the large oscillation in the fit at high Qswhich is due to the core-shell modelsis not reflected in the data, which is much smoother. The polydispersity increase is required to fit the downward slope of the data at rather lower Q values, about 0.08-0.1 Å-1. The discrepancy becomes worse as the MW 750 content increases. The core-shell model is increasingly inaccurate as the division in the micelle between core and shell becomes increasingly blurred. We see, but will not comment on, this effect in some later samples in the high Q region. (4) Effect of Surfactant Mixture Composition on InWerse Micelle Structure (Water-Only Samples). [Samples 3m, 63m, 83m-85m]. More limited inverse micelle results using water as the aqueous phase are given in Table 6. The scattering data and the quality of the model fitted are shown in Figure 4. These curves are from hydrogenous surfactant dissolved in C16D34 in contact with D2O. They thus show the location of the surfactant. The sample 3m was 26% weight fraction of surfactant; 63m, 84m, and 85m were 23%; and 83m was 4%. The scattering from a sample made with pure MW 1200, using water as the aqueous phase, resembles that with saturated ammonium nitrate as the wetting agent. The notable difference is that SLDs of the core differ because the water in 99m was partly hydrogenous (Table 5). As the MW 750 concentration increases, the micelle radius increases strongly, in contrast to the saturated ammonium nitrate samples. The gross changes are qualitatively obvious in Figure 4. The water content and radius of the core increased markedly for 63m, but the shell hexadecane content was relatively constant. The pure MW 750 sample no longer contained spherical micelles (as for the ammonium nitrate



Figure 4. Instability of inverse micelles with increasing low MW PIBSA surfactant content: (red 9) (3m) 100% 1200; (sand O) (84m) 33% 750 and 67% 1200; (green +) (85m) 50% 750 and 50% 1200; (teal 0) (63m) 67% 750 and 33% 1200; (blue b) (83m) 100% 750. Solid lines are fits.

wet sample), but a sheet structure best fitted the scattering.3 Figure 4 illustrates how the behavior of sample 63m tends toward the sheet structure of 83m. However, note that the lower surfactant content, as noticed with the ammonium nitrate samples above, predisposes the system to the formation of rods and eventually sheets. Introduction of MW 750 decreases the curvature of the micelles, by increasing both external and core radii. As with the ammonium nitrate, when close to the transition in sample 63m, the polydispersity also blows up, reflecting incipient instability. These observations on the effects of surfactant concentration and quality tally with those found for single surfactant inverse micelles.3 Changing the surfactant properties allows the approach of instability of different inverse micelle systems to be studied and controlled. Emulsions Using MW 750 and MW 1200 Monodisperse Mixtures. Water-Based Emulsions [Samples 27, 30, 32, 57-60, 61]. Table 7a shows the effect of moving from 100% MW 1200 surfactant to 50% MW 750 plus 50% MW 1200 with a water aqueous phase. The data and model fits are plotted in Figure 5a and b for the contrast matched and unmatched samples, respectively. The constraints and fitting procedures here and in subsequent mixtures are as in ref 3, except where otherwise noted. The major difference from that work is that the polydispersity has been refined, which, given the number of parameters, means that the core SLD has been fixed at the value for water. For samples 30, 27, 59, and 60, the polydispersity remains around 0.14, so it was fixed at this value. For sample 58, fixing the core SLD was still not enough to ensure physically possible values for parameters at the minimum, and so the micellar volume fraction was fixed at the value obtained for sample 57. Figure 5a and b shows the large effect of surfactant substitution. Table 7a indicates that increased content of MW 750 causes a marked increase in micellar radius, from 31 to 50 to 75 Å, accompanied by an increase in the radius of the water-containing core and the volume fraction of micelles. The latter is a consequence of the increasing water content of the micelles. At 50 wt % MW 750 content, the micellar polydispersity also increases. These observations agree well with the observations for the water inverse micelles reported in Tables 3 and 6.

The aqueous droplet size of the emulsion also reaches its minimum at a 50 wt % MW 750 content, as indicated by the increasing droplet surface area and with corresponding increased quantities of surface-adsorbed surfactant. The surfactant loading at the aqueous-oil interface measured in milligrams of surfactant per square meter of the interface does not change appreciably. This means that the area occupied at the interface per surfactant molecule is relatively unchanged, in the assumption the contrast matched and unmatched samples have the same structure because of the constancy of the preparative method. A decreasing droplet size is an indication of approaching emulsion instability. In the previous paper, we have shown that pure MW 750 samples do not form a water emulsion but a sponge phase instead. Emulsions Using MW 750 and MW 1200 Monodisperse Mixtures. Ammonium Nitrate-Based Emulsions [Samples 64, 65, 1003, 1004, 1005-1009, 1010]. Table 7b shows a similar series for emulsions with a saturated ammonium nitrate aqueous phase but extending to 100% MW 750 emulsions, which are now stable. Figure 6 shows the SANS from three contrast matched emulsions (cf. Figure 5a for the water-based emulsions). The refinement differed from that of ref 3 in that the core radius was refined. Its SLD value was fixed at 4.8 × 10-6 Å-2, which was the same value as that for the aqueous ammonium nitrate phase; this value is due to H/D exchange with the ammonium nitrate used. Where the core radius was larger than about 20 Å, it was possible to refine the SLD as well as the radius. The SLD shifted little from the assumed value in the refinement. The comparability between contrast matched and unmatched samples broke down completely for samples 1009 and 1010, namely, the pure MW 750 surfactant samples. As already noted in the Experimental Section, both their treatment to obtain emulsions and the rheology of the resulting emulsions differed markedly. Thus, the headgroup area, in particular, since it is derived by combining results from the two emulsions, is not reliable. Trends in droplet surface areas are more reliable than headgroup areas or surface loadings, since they do not depend on combining data from pairs of emulsions; nonetheless, they were derived from the uncontrast matched sample data only.

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TABLE 7: Structural Parameters Derived from Fitting Emulsion Dataa (a) Water Emulsions surfactant concentration (wt %) and compositionsfraction and molecular weights sample identification inverse micelle radius (Å) scattering length density of shell core radius (Å) scattering length density of core volume fraction of micelles in oil phase micelle polydispersity surface area (m2/mL) surfactant adsorbed at interface (mg mL-1) mean headgroup area per molecule (Å2) % of total surfactant adsorbed at interface

1.8 0% 750 100% 1200 27 30 31.9(1) 30.6(1) -0.06(7) -0.06(8) 13 13 3.8(3) -0.54 0.137(2) 0.139(3) 0.14 0.14 0.35(1) 0.57(2) 122(4) 3

1.5 25% 750 75% 1200 59 60 53.7(1) 50.5(1) 1.12(5) 0.05(4) 23.4(2) 23.4 6.34 -0.54 0.172(2) 0.179(2) 0.14 0.14 0.39(1) 0.75(5) 94(9) 5

1.5 50% 750 50% 1200 57 58 73.0(1) 78.0(1) 2.88(1) 0.60(1) 33.9(2) 33.9 6.34 -1.60(4) 0.353(6) 0.353 0.346(6) 0.346 0.84(1) 1.4(2) 101(13) 9

2.9 0% 750 100% 1700 32

1.8 20% 750 80% 1700 61

33.9(1)

49.0(1)

0.84(3)

1.92(3)

13

13

4.1(1)

5.4(4)

0.337(2)

0.227(2)

0.14

0.14

(b) Saturated Ammonium Nitrate Emulsions surfactant concentration (wt %) and fraction and molecular weights sample identification inverse micelle radius (Å) scattering length density of shell core radius (Å) scattering length density of core volume fraction of micelles in oil phase micelle polydispersity surface area (m2/mL) surfactant adsorbed at interface (mg mL-1) mean headgroup area per molecule molecule (Å2) % of total surfactant adsorbed at interface

2.9 0% 750 100% 1200 64 65 30.0(3) 30.0 1.4(1) -0.4(1) 9(1) 11(1) 4.8 4.8 0.264(5) 0.264 0.14 0.14 0.50(4) 1.37(4) 73(2) 6

1.6 50% 750 50% 1200 1004 1003 35.6(2) 35.6 1.4(1) 0.04(5) 11(1) 16.4(5) 4.8 4.8 0.194(3) 0.194 0.14 0.14 0.72(2) 0.78(7) 150(15) 4

1.6 75% 750 25% 1200 1006 1005 37.3(2) 37.3 1.3(1) 0.17(5) 14(1) 17.6(3) 4.8 4.8 0.179(3) 0.179 0.14 0.14 0.95(2) 1.30(3) 105(3) 6

1.6 88% 750 12% 1200 1008 1007 37.7(2) 37.7 1.3(1) 0.09(5) 14(1) 20.4(2) 4.8 4.8 0.176(3) 0.176 0.14 0.14 0.68(1) 0.72(5) 126(13) 4

1.6 100% 750 0% 1200 1010 1009 42.0(4) 42.0 2.2(1) -0.04(4) 20(1) 23(1) 4.8 4.8 0.148(5) 0.148 0.14 0.14 0.12(1) 0.66(2)b 23(1)b 4

a For some entries, there are two values, the first from the contrast matched emulsion and the second from the unmatched. Entries with single values are derived by combination of CM and UM data. Absence of a quoted error implies that the parameter was not refined but fixed from the other contrast. There are no fits for the pure PIBSA 750/water system, since it does not form emulsions, as discussed in the text. b Refined parameters unreliable; see text.

When saturated ammonium nitrate was used instead of water as the aqueous phase, the micellar radii decreased. The ammonium nitrate-based system with 100% MW 750 produced emulsions rather than sponge phases. The radii increased from 30 to 42 Å, accompanied by an increase in the water core radii as the MW 750 fraction increased (Figure 6). However, even for 100% MW 750, the polydispersity in micelles remained low; we had no need to increase it from a fixed low value of 0.14 to produce a good fit to the data. As with the inverse micelle samples, substitution of saturated ammonium nitrate for water stabilized structures of higher curvature. Comparing the inverse micelle results of Table 5 with the emulsion results of Table 7b, we see, however, that in the inverse micelle solutions the micelles are less expanded. This may be understood if one notes that the volume fraction of micelles in the inverse micelle systems varied from 23 to 27%, whereas in the emulsions it decreased from 26 to 15% as the MW 750 content increased, accompanied by the increased in micelle radius. As the

oil-aqueous interface competes for surfactant, the result can be interpreted as a surfactant concentration effect. As we have already observed, lower surfactant content produces larger micelles, particularly for high MW 750 content. Emulsions Using MW 750 and MW 1700 Monodisperse Mixtures. [Samples 27, 32, 59, 61]. The results are given in the last two columns of Table 7a. Since there are only results for contrast matched emulsions, extraction of results for surface areas and loadings is not feasible. The results from pure MW 1700 are quite similar to those from pure MW 1200 surfactant. The radius was slightly larger, due to the larger tail size, even though the emulsion was more concentrated in surfactant. On dilution to 20% MW 750, the micelle radius increased substantially, together with some increase in hexadecane content in the micellar shell, and an increase in micellar core water content. Both are consistent with what is observed in emulsions made from MW 750/MW 1200 mixtures. For example, comparing the results from emulsion 61 with 59, substitution of MW 1200



Figure 5. (a) Typical contrast matched water-based emulsion scattering from PIBSA MW 1200/MW 750 mixtures. Data (markers) and fits (lines): (red 0) (27) 100% MW 1200 surfactant; (green b) (59) 75% MW 1200 and 25% MW 750; (blue 9) (57) 50% MW 1200 and 50% MW 750. (b) Typical contrast unmatched water-based emulsion scattering from PIBSA MW 1200/MW 750 mixtures. Data (markers) and fits (lines): (red 0) (30) 100% MW 1200 surfactant; (green b) (60) 75% MW 1200 and 25% MW 750; (blue 9) (58) 50% MW 1200 and 50% MW 750.

for MW 1700 produces only a minor effect. We should contrast this with the differences between 27 and 59, where substitution of about the same amount of MW 750 for MW 1200 produces a much greater effect. Emulsions Using 450 and 1000 Polydisperse Mixtures. [Samples 520, 521, 522-524, 525, 526-528, 529, 530-535]. The surfactants used for these experiments had broad molecular weight distributions peaking at MW 450 and MW 1000. Figure 7 shows examples of the scattering from contrast matched emulsions (showing the surfactant) formed from these polydisperse surfactants individually and at a 50%/50% mixture. We note that the pure polydisperse MW 450 surfactant still forms an emulsion with the saturated ammonium nitrate aqueous phase, just as the monodisperse MW 750 surfactant does. This contrasts with the water-based emulsions where reduction of MW from 1200 to 750 induces emulsion instability toward a

sponge phase at MW 750. Evidently, the destabilizing effect of MW reduction from 750 to 450 is here outweighed by the stabilizing effect of ammonium nitrate and the greater polydispersity of the MW 450 surfactant. All of the data are smoother with little or no interference fringes compared to the data of Figures 2-4. The contrast unmatched samples show no obvious bump at higher Q associated with micelles; in this case, if there were substantial water incorporation into a core, we would expect such a feature, since water is the only deuterated material added. The relative smoothness of the contrast matched samples suggests high polydispersity in micelle dimension. Refinements show polydispersities varying from 0.35 to 0.5. The large number of variables produces large errors in the polydispersity, so we have fixed all at 0.4. With a deuterium-rich water core, we might expect an SLD difference from the shell. When a core was

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Figure 6. Typical contrast matched ammonium nitrate-based emulsion scattering from PIBSA MW 1200/MW 750 mixtures. Data (markers) and fits (lines): (blue 0) (1010) 100% MW 750 surfactant; (green b) (1004) 50% MW 1200 and 50% MW 750; (red 9) (64) 100% MW 1200.

Figure 7. Polydisperse PIBSA contrast matched emulsions: (red 9) (521) 100% MW 1000 surfactant; (green 0) (525) 50% MW 1000 and 50% MW 450; (blue b) (529) 100% MW 450.

allowed to refine separately from the shell, the SLDs remained very similar. There appears therefore to be no well-defined core and so a uniform micelle has been refined. The results for surfactant-concentrated emulsion samples are shown in Table 8a. For the surfactant-diluted emulsions, there is little trace of scattering from micelles, so we have refined only micelle volume fraction, remembering that at these low volume fractions only the product of volume fraction and micelle SLD difference from oil phase is significant. The results are given in Table 8b. The micellar information is omitted, as it is of no physical significance. However, we note that the refined values of the above product imply reasonable micellar volume fractions of ca. 1/8-1/4 of those in the more concentrated samples, assuming similar micellar compositions for both. An immediate difference from previous mixtures is the high polydispersity of inverse micelles and lack of a well-defined core. Both of these may be a consequence of the large polydispersity in both the MW 450 and MW 1000 surfactants, since we notice that mixing monodisperse MW 750 and MW 1200 does not result in similar micellar disorganization and

variability. The lack of a well-defined core does not imply no water content. The micellar SLDs may be transformed to measure water contents of 17, 32, 32, 31, and 23 vol % as the surfactant composition was changed from 100% MW 1000 to 100% MW 450. There are also implied contents of hexadecane, which are relatively constant at 36, 24, 28, 24, and 29 vol %, respectively, for the same change. We note that the micelles from single surfactants (MW 1000 or MW 450) have lower water content but that all hexadecane contents are relatively constant. The most interesting trend is in micelle radius. This is at a maximum for the 50/50 mixture and decreases from here on both sides to the single surfactants (Table 8a). We notice a similar trend for the volume fraction of micelles, indicating a substantial nonlinear effect associated with polydisperse surfactant mixing. This is obvious in Figure 7 where the mixed surfactant data (open squares) are clearly not intermediate between data from the two single surfactant samples (closed symbols). If we turn to the droplet surface area and interfacial loading, we note from Tables 8a and b that there is a smooth progression from MW 1000 to MW 450 in which both surface area and loading increase with MW 450 content, with a relatively constant surfactant molecular footprint. Dilution of surfactant decreases surface areas, with relatively unchanged footprint. This is consistent with the data discussed previously. Emulsions Using SMO and MW 1200 Mixtures. [Samples 4, 65, 1015, 1016, 1019-1024]. The scattering from these emulsions was fitted in a similar way to that for emulsions made from the MW 450/MW 1000 polydisperse mixtures. By comparison with, for example, the MW 1200 surfactant, which is pure and relatively monodisperse, SMO is quite mixed. Refinement of polydispersities in the scattering from spherical micelles in emulsions made from SMO and MW 1200 mixtures produces values from 0.18 to 0.25. Again, the large number of variables produces large errors in the polydispersity, so for all modeling, it was fixed at 0.2. With a deuterium-rich water core, an SLD difference from the shell might have been expected, but in fact, when a core is allowed to refine separately from the shell, the SLDs remain very similar. There appears to be no well-defined core, so we have refined a uniform micelle. The



TABLE 8: Structural Parameters Derived from Fitting Emulsion Dataa (a) MW 450/MW 1000 Polydisperse PIBSA Mixtures, Emulsions Concentrated in Surfactant surfactant concentration (wt %) and fraction and molecular weights sample identification inverse micelle radius (Å) scattering length density of micelle volume fraction of micelles in oil phase micelle polydispersity surface area (m2/mL) surfactant adsorbed at interface (mg mL-1) mean headgroup area per molecule molecule (Å2) % of total surfactant adsorbed at interface

1.6 0% 450 100% 1000 521 520 23.0(8) 23.0 3.1(2) 0.45(3) 0.173(2) 0.173 0.4 0.4 0.22(1) 0.41(2) 89(3) 2

1.6 25% 450 75% 1000 523 522 26(1) 26 3.6(1) 1.22(1) 0.242(2) 0.242 0.4 0.4 0.47(2) 0.95(3) 71(2) 5

1.6 50% 450 50% 1000 525 524 35(1) 35 3.83(4) 1.21(1) 0.339(2) 0.339 0.4 0.4 0.46(2) 0.89(3) 62(2) 5

1.6 75% 450 25% 1000 527 526 26.9(9) 26.9 3.5(1) 1.17(2) 0.240(2) 0.240 0.4 0.4 1.01(2) 1.25(4) 79(3) 6

1.6 100% 450 0% 1000 529 528 24.9(9) 24.9 3.3(1) 0.76(2) 0.226(2) 0.226 0.4 0.4 1.23(2) 1.50(4) 61(2) 7

(b) MW 450/MW 1000 Polydisperse PIBSA Mixtures, Emulsions Dilute in Surfactant surfactant concentration (wt %) and fraction and molecular weights

0.4 0% 450 100% 1000 531 530 0.19(1) 0.38(1) 83(3) 7

sample identification surface area (m2/mL) surfactant adsorbed at interface (mg mL-1) mean headgroup area per molecule molecule (Å2) % of total surfactant adsorbed at interface

0.4 50% 450 50% 1000 533 532 0.33(1) 0.61(1) 65(3) 12

0.4 100% 450 0% 1000 535 534 0.39(1) 0.65(2) 45(2) 12

a For some entries, there are two values, the first from the contrast matched emulsion and the second from the unmatched. Entries with single values are derived by combination of CM and UM data. Absence of a quoted error implies that the parameter was not refined but fixed from the other contrast. There are no fits for the pure PIBSA 750/water system, since it does not form emulsions, as discussed in the text.

TABLE 9: Structural Parameters Derived from Fitting Emulsion Dataa SMO/MW 1200 PIBSA Mixturesb surfactant concentration (wt %) and fraction and molecular weights sample identification inverse micelle radius (Å) scattering length density of micelle scattering length density of core volume fraction of micelles in oil phase micelle polydispersity surface area (m2/mL) surfactant adsorbed at interface (mg mL-1) mean headgroup area per molecule molecule (Å2) % of total surfactant adsorbed at interface

2.9 0% SMO 100% 1200 64 65 29.4(2) 29.4 1.28(9) -0.4(1) 3.4(3) 2.6(8) 0.312(5) 0.312 0.2 0.2 0.50(1) 1.14(2) 73(2) 6

1.6 50% SMO 50% 1200 1020 1019 27.2(4) 27.2 2.63(7) 0.3(1) no core 0.167(7) 0.167 0.2 0.2 1.23(1) 1.51(5) 119(4) 7

1.6 75% SMO 25% 1200 1022 1021 24.4(5) 24.4 2.6(1) 0.5(1) no core 0.126(10) 0.126 0.2 0.2 1.01(1) 1.24(4) 97(3) 6

1.6 88% SMO 12% 1200 1024 1023 23.5(6) 23.5 2.6(2) 0.6(1) no core 0.110(11) 0.110 0.2 0.2 1.26(1) 1.69(4) 79(3) 8

1.6 100% SMO 0% 1200 1016 1015 22.9(6) 22.9 2.5(2) 0.7(1) no core 0.094(13) 0.094 0.2 0.2 1.36(1) 1.64(4) 77(3) 8

a For some entries, there are two values, the first from the contrast matched emulsion and the second from the unmatched. Entries with single values are derived by combination of CM and UM data. Absence of a quoted error implies that the parameter was not refined but fixed from the other contrast. There are no fits for the pure PIBSA 750/water system, since it does not form emulsions, as discussed in the text. b Core radius for samples 64 and 65 fixed at 13 Å, no core for the remaining samples.

exception is the pure MW 1200 sample where the UM sample (65) gives a significant water content in a well-defined core, which is confirmed in the CM refinement (64). This comparative point again suggests that surfactant variability, or polydispersity in the mixed surfactant case, disrupts well organized water cores. The results of modeling are given in Table 9. These mixtures show characteristics intermediate between the MW 450/MW 1000 polydisperse surfactant mixtures and pure MW 1200 surfactants. The SMO/MW 1200 micelles are

polydisperse but less so than MW 450/MW 1000 surfactant systems; there are no well-defined cores; the water content increases from 1% for pure MW 1200 surfactant to 14, 18, 20, and 22 vol % with increasing SMO. The hexadecane content remains almost constant: 26, 35, 31, 29, and 25 vol % across the series. Unlike for MW 450/MW 1000 mixtures, the micellar radii decrease as the surfactant changeover from MW 1200 to SMO occurs. It should be noticed, however, that the SMO/MW 1200 surfactant micellar radii in the emulsion systems are

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distinctly higher than values interpolated from the single surfactant emulsions, whether we interpolate using weight or mole fractions. The nonlinearity observed is in the same direction as for MW 450/MW 1000 mixtures but less extreme. Again, as with MW 450/MW 1000 mixtures, both surface area and loading at the droplet interface increase with SMO content but less dramatically. The headgroup area per molecule is relatively constant but with a suggestion it is larger for mixtures than single surfactant emulsions. In these emulsions, there is a clear trend downward in micelle volume fraction as the SMO content increases. An increasing fraction of surfactant is not accounted for either in micelle or at the droplet interface, although the latter does seem to increase a little. This effect also occurs in most of the other series but is often obscured by changes in micellar radius and water content. It appears that the more hydrophilic surfactantssSMO, MW 750, and MW 450sare aggregating in the oil phase more strongly than the more hydrophobic MW 1000, MW 1200, and MW 1700 compounds. Aggregation rather than dissolution is being observed, since the surfactant solubility in oil runs in the opposite direction, and solubility in the aqueous phase droplets would be negligible. In addition, when similar emulsions are cooled, the SANS patterns must be interpreted as ALL the surfactant being in large aggregates in the oil phase.1 In our studies of mixed surfactants at the air-water interface,20 we also see aggregation, except that this time, rather than “losing” hydrophilic surfactant from an oil phase, aggregation of the hydrophobic component from the aqueous subphase explains the phenomena. Discussion We have shown that emulsions and inverse micelle systems composed of mixed surfactants are structurally similar to those made from single monodisperse surfactants, and respond in similar ways to changes in composition; e.g., low molecular weight surfactant gives increased micellar radius, ammonium nitrate content (decreased micellar radius), or surfactant dilution (increased micellar radius) which have been discussed before.2 Diffuseness of the micellar core with dissimilar surfactants such as two different polydisperse PIBSA or a PIBSA and SMO is novel. We present no further discussion of the mapping of the current experiments on those already published but focus on the interesting question of synergistic effects in mixed systems. For the micellar solutions, the mixed surfactant experiments show that large changes in oil phase inverse micellar radii occur with changes in mixture composition, and that these are accompanied by changes in micellar water content but not fractional hexadecane content. For the emulsions, trends in aqueous droplet surface areas and loadings are less strong. In the following discussion, we will discuss only micellar radius trends, recognizing that water content trends follow these. The instability of the emulsions to a sponge phase transition, for some compositionssparticularly low molecular weight surfactant and a pure water aqueous phasesindicates that the micellar changes observed are related to the proximity of a particular emulsion composition to the emulsion/sponge phase boundary. It is suggested that micellar radius changes are indicators of that proximity. That we observe nonlinear effects at all, while many others have not in other systems, is connected to the existence of this nearby phase boundary. To simplify discussion, we will assert four conclusions arrived atsthe latter three from the current datasand discuss our results successively in terms of these principles:

Reynolds et al. (1) Lower molecular weight PIBSAs increase micellar radii, but ammonium nitrate in the aqueous phase results in its decrease. (2) The inverse micelles in emulsions for a given surfactant composition are smaller than those in the equivalent inverse micelle solution. (3) Polydisperse surfactants produce polydisperse micelles without clear “core” signatures. (4) Mixtures of polydisperse surfactants of well separated molecular weight produce a maximum micelle radius at 50:50 wt % composition. Consider the inverse micellar solutions first of all. The data of Table 4b concern the mixing in of MW 750 with MW 1700 to simulate the properties of inverse micelles made from MW 1200 alone, which has the average molecular weight of the pair. The data do not support the interpretation of separate micelles derived from the MW 750 and the MW 1700, despite the bimodal distribution of surfactant molecular weight and their monodispersity. There is apparently no demixing of the MW 750 and the MW 1700. The marked difference between pure and mixed surfactant emulsions can be attributed to the strong effect of MW 750 causing an increase in the micelle radius. For the mixtures of MW 750 with MW 1200 (Table 5), there is a slight increase in radius at the 50/50 ratio, a similar effect. The effect here is, however, much suppressed, since use of saturated ammonium nitrate rather than water contracts the micelles, even for pure MW 750. In Table 6, we again have water-based inverse micelles and, again, see a large expansion from the micelle size for pure MW 1200, on adding MW 750, until the instability of the micelle with respect to sheet structures occurs. We are thus seeing a combination of bimodality, which increases micellar radius, tending to more emulsion instability, and instability due to lowering of average molecular weight. Lastly, we compare inverse micelles 85m and 87m (Table 3) in which 50% MW 1200 and 50% MW 1700 surfactant mixtures with 50% MW 750 were used. The micelles in the MW 1700-containing mixture are larger than those in the MW 1200-containing mixture. The change in average molecular weight when MW 1700 was included is expected, from pure emulsions, to increase the micelle radius from about 31 to about 34 Å. In fact, the micelles expanded to 47 and 52 Å. Both expansions are attributed to the effect of bimodality, and we notice that the more disparate in molecular weight the components, the more expansion occurs. We suggest that this bimodality effect arises from unevenness in the micellar packing to incorporate the disparate molecular weights, since there is undetectable demixing of the surfactants. The polydispersities in this bimodal system were comparable at 0.12. This fact again helps to exclude a mixture of different sized micelles as the model. We turn now to emulsions. The scattering from micelles in the water-based emulsions of Table 7a is explicable in the same way as that from the inverse micelle solutions of Table 6; the micelles expand due to a combination of surfactant bimodality and the approach of phase instability due to lowering of the average molecular weight. The use of saturated ammonium nitrate (Table 7b), just as in Table 6, almost suppresses the phase instability. Thus, we see a large expansion in micelle size going from pure MW 1200 to 50/50 mixtures followed by slower expansion to pure MW 750 as the incipient effects of phase instability emerge. The PIBSA MW 450/MW 1000 mixture emulsions show the effect of the polydispersity in the two constituent

Structure of High Internal Phase Emulsions surfactants. The pure MW 450 emulsion had small micelles, with decreased water content compared to those in its mixtures with MW 1000. The MW 750 data of Table 7b would suggest that at such a low molecular weight as PIBSA 450 there would be much larger, water swollen micelles or even a sponge phase. The only factor different from the MW 750 system is the large polydispersity in the MW 450 surfactant, which must therefore stabilize the emulsion against phase instability. The synergistic swelling effects of the surfactant bimodality also appear, as seen for MW 1700/ MW 1200. Mixing MW 450/MW 1000 (Table 8a, samples 524-527) produced larger micelles than expected from linear interpolation between the radii for the emulsions made from the surfactants singly, and these micelles have a larger water content. However, the hexadecane fraction is unaltered. The deviation from linearity is at a maximum at about 50/50 wt % content. The deviations can be large, a tripling of the micelle volume, for example. The use of polydisperse PIBSA MW 450 surfactant not only suppresses the instability, but it also appears to enhance this nonlinearity compared to use of monodisperse MW 750 PIBSA. We conclude that polydispersity not only causes suppression of the instability but also enhances synergistic effects as revealed in micelle swelling. We can compare pure MW 750 emulsions (1009 and 1010) to MW 450/MW 1000 50/50 mixtures (524 and 525). As with Table 4, the average molecular weight of the mixture is almost that of the single surfactant emulsion. We observe micelle radii of 42 and 35 Å, respectively. From our previous discussion, the bimodality should increase micelle size, but here, unlike in Table 4, the mixed surfactants are also polydisperse. It appears that the contracting effect of polydispersity just outbalances the expanding effect of bimodality. Another illustration of the contracting effect of polydispersity is to compare the micelle sizes of pure monodisperse MW 750 (42 Å) and MW 1200 (30 Å) with pure polydisperse MW 450 (25 Å) and MW 1000 (23 Å). We can see they are not collinear, and that polydispersity contracts the average micelle size. In Table 9, a less extensive, though clear, nonlinearity can be seen in the SMO systems. Pure SMO, unlike MW 750 PIBSA, forms stable emulsions with water, containing spherical micelles. This is possibly connected with SMO being a mixture, and therefore showing the same effect as MW 450 PIBSA polydispersity in suppressing the phase instability. The incipient instability of micelle solutions to sheet, and of emulsion to sponge phases, as the molecular weight of the surfactant decreases is easily explicable in terms of “Wedge” theory. However, the sensitivity to surfactant molecular weight distribution is not, and superficially, the two effects observed are incompatible with each other. Is not bimodality just a type of polydispersity? Thus, why should bimodality and polydispersity have opposite effects? However, the comparison above with 10 sets of data establishes this empirical conclusion. Our observation can be re-expressed as follows: when the separation in average molecular weights of each component is large compared to the polydispersities of each component, micelle expansion occurs, whereas, when this separation in molecular weights is comparable to polydispersities, there is contraction of the micelles. Expressed in this way, our conclusions are not incompatible but remain theoretically puzzling to us. Conclusions We have previously observed emulsions made with MW 1200 PIBSA surfactant at various dilutions3 and found that all can

J. Phys. Chem. B, Vol. 113, No. 36, 2009 12255 be fitted to a single model with the aqueous droplet-oil interface coated with slightly less than a monolayer of surfactant. The remainder of the surfactant, whether large or small in amount, is distributed in the oil phase of the emulsion as an inverse micellar phase, whose structure is relatively constant. At high surfactant concentrations, some surfactant was aggregated into large structures in the oil phase. This micellar phase can be assembled separately as a one-phase system, which has been studied by SANS. The micelles in the inverse micelle system and emulsion were of radius ca. 32 Å and contained a small core of water, surrounded by an oil shell, mainly PIB surfactant tail, but with some 30% hexadecane. Other emulsions formed with MW 1700 molecular weight PIBSA, sorbitan monooleate, and sorbitan isostearate are qualitatively similar, although the inverse spherical micelles of the sorbitan surfactants have a poorly defined watery core. The 100% MW 750 surfactant forms a bicontinuous sponge bilayer L3 phase when emulsions with water are attempted, at all surfactant contents. The bilayer is a pair of thin surfactant layers sandwiching a thin, ca. 15 Å, oil layer. The bilayers are separated by 200-300 Å of water. When saturated ammonium nitrate is used, an emulsion is formed with MW 750 surfactant. Emulsions are also stabilized by use of higher surfactant concentrations. All of the mixed surfactant emulsions and inverse micelle solutions examined here fall into this qualitative pattern. All of the emulsions contain an oil inverse micellar system, with similar dependence on ammonium nitrate content and surfactant concentration. At high concentrations of MW 750, the instability toward a phase change to L3 is manifested by large increases in micelle size, water content, and core radius. We also notice that use of polydisperse surfactants increases micellar size polydispersity, and discourages formation of a well-defined aqueous micellar core compared to the use of more monodisperse surfactants. By comparing 10 systems, several conclusions emerge. The latter two from these current data, while the former have already been deduced from samples containing single surfactants: (1) The lower molecular weight of monodisperse surfactant destabilizes spherical micelle-containing phases in emulsions. (2) Ammonium nitrate content in the aqueous phase stabilizes spherical micelle-containing phases in emulsions. (3) Polydispersity in surfactants stabilizes micellar phases in emulsions. (4) Bimodality in surfactant polydispersity destabilizes micellar phases in emulsions. Increasing disparity in the average molecular weight of the two components increasingly destabilizes micellar phases, thus reconciling the last two conclusions. Mixing monodisperse PIBSA surfactant molecules very different in molecular weight destabilizes spherical micelles, while mixing those close in size has the opposite effect. The effects are most marked for approximately equal mixtures. Acknowledgment. Our thanks to the Rutherford Appleton Laboratory for access to the LOQ instrument at ISIS. Travel grants, through the Australian Government ISTAC/ANSTO Access to Major Facilities Program, are gratefully acknowledged, as is access to ISIS through the AINSE/ARC collaborative program. This work was financed by the Australian Research Council, under SPIRT and SRF awards joint with Orica Ltd. and ICI UK Ltd. We would also like to thank Dr. Deane Tunaley and Dr. Richard Goodridge of Orica Ltd. and Dr. D. E. Yates of Yates Consulting Ltd. for useful discussions.

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References and Notes (1) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2000, 104, 7012. (2) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2001, 105, 6925. (3) Reynolds, P. A.; Gilbert, E. P.; Henderson, M. J.; White, J. W. J. Phys. Chem. B. DOI: 10.1021/jp903484j. (4) Staples, E.; Penfold, J.; Tucker, I. J. Phys. Chem. B 2000, 104, 606. (5) Cates, M. E.; Roux, D.; Andelman, D.; Milner, S. T.; Safran, S. A. Europhys. Lett. 1988, 5, 733. (6) Griffiths, P. C.; Whatton, M. L.; Abbott, R. J.; Kwan, W.; Pitt, A. R.; Howe, A. M.; King, S. M.; Heenan, R. K. J. Colloid Interface Sci. 1999, 215, 114. (7) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204. (8) Pedone, L.; Chillura Martino, N.; Caponetti, E.; Floriano, M. A.; Triolo, R. J. Phys. Chem. B 1997, 101, 9525. (9) Borbely, S.; Cser, L.; Vass, S.; Ostanevich, Yu. M. J. Appl. Crystallogr. 1991, 24, 747.

Reynolds et al. (10) Aswal, V. K.; Goyal, P. S. Physica B 1998, 245, 73. (11) Zemb, T.; Dubois, M.; Deme, B.; Gulik-Krzywicki, T. Science 1999, 283, 816. (12) Pilsl, H.; Hoffmann, S.; Kalus, J.; Kencono, A. W.; Lindner, P.; Ulbricht, W. J. Phys. Chem. 1993, 97, 2745. (13) Brasher, L. L.; Kaler, E. W. Langmuir 1996, 12, 6270. (14) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. B 1997, 101, 5639. (15) Lusvardi, K. M.; Full, A. P.; Kaler, E. W. Langmuir 1995, 11, 487. (16) Caponetti, E.; Chillura Martino, N.; Floriano, M. A.; Triolo, R. Langmuir 1993, 9, 1193. (17) Rocca, S.; Stebe, M. J. J. Phys. Chem. B 2000, 104, 10490. (18) Bales, B. L.; Ranganathan, R.; Griffiths, P. C. J. Phys. Chem. B 2001, 105, 7465. (19) Heenan, R.; King, S. In ISIS User Guide; Boland, B., Whapham, S., Eds.; ISIS, Rutherford Appleton Laboratory: Didcot, U.K., 1992. (20) Reynolds, P. A.; McGillivray, D. J.; Gilbert, E. P.; Holt, S. A.; Henderson, M. J.; White, J. W. Unpublished data.

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