Structure of High Internal Phase Aqueous-in-Oil Emulsions and

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Structure of High Internal Phase Aqueous-in-Oil Emulsions and Related Inverse Micelle Solutions. 3. Variation of Surfactant 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 16, 2009; ReVised Manuscript ReceiVed: July 1, 2009

The small angle neutron scattering from high internal phase water-in-hexadecane and saturated ammonium nitrate-in-hexadecane emulsions is compared with that from related hexadecane-based inverse micellar solutions. Three molecular weights of the monodisperse polyisobutylene acid amide (PIBSA) surfactant 750, 1200, and 1700 were studied over a range of surfactant concentrations. As an additional comparison, emulsions based on sorbitan monooleate and isostearate surfactants were investigated. The scattering from molecular weight 1200 water-based PIBSA emulsions can be fitted at all concentrations to a model with a surfactant coated aqueous droplet-oil interface together with the majority of the surfactant in the oil phase of the emulsion in the form of inverse micelles. Variation of the molecular weight shows a variety of phases of increasing curvature: lamellar, sponge, and, most commonly, the emulsion structure described above. In addition, the molecular weight affects the oil component in the emulsions, which can contain either cylindrical micelles or spherical micelles of varying water but constant hexadecane content. Increased phase curvature is favored by both increased PIBSA molecular weight and ammonium nitrate dissolved in the water. These observations are consistent with “Wedge theory”. The structures observed in the emulsions are close to those observed in related inverse micellar solutions made from hexadecane, the surfactant, and water. Lower concentrations of surfactant in the micellar solutions decrease micelle curvature, except where the inverse micelles are spherical and small; here, there is little effect of dilution. Substitution of sorbitan surfactants for PIBSAs produces slightly less organized but similar structures, with smaller spherical micelles containing proportionally more water. The aqueous-oil droplet interface has a relatively invariant monolayer of adsorbed surfactant. For all emulsions, we can infer from the mass balance that micelle concentrations are depressed in the inverse micellar solutions because up to half the added surfactant is present as individually dissolved molecules. Introduction Many techniques1 have been used to study emulsion structure at submicrometer resolution, but there has been little investigation by scattering methods to probe the nanometer scale. In previous papers,2,3 small angle neutron scattering (SANS) results from two-phase aqueous/hydrocarbon high internal phase emulsions show the promise of the method. The high internal phase emulsions have about 90% aqueous phase, either water or ammonium nitrate solutions, dispersed as micrometer-sized droplets in a continuous hexadecane oil phase. The polyisobutylene acid amide (PIBSA) surfactants had polyisobutylene oligomer tails with mainly acid amide headgroups (Figure 1). The experiments, for defined surfactants and their concentrations, gave insight into the microstructure within the oil region, and the nature of the interface between the oil phase and the aqueous droplets by use of selective deuteration. In our first experiments,2 the key tool was variation of the neutron scattering contrasts between the components to identify the scattering from different components of the emulsion structure. Three series of emulsions were examined containing two polydisperse surfactant types, with both water and supersaturated ammonium nitrate as the aqueous phases. The SANS results were well described by a single model invoking the sum of the scattering from an assembly of micrometer-scale poly* 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: Bragg Institute, ANSTO, Private Mailbag 1, Menai, NSW 2234, Australia.

Figure 1. Structure of a typical “MW 1200 PIBSA” surfactant molecule.

disperse spherical aqueous droplets and the scattering from a continuous hexadecane surfactant/water inverse micellar structure. The amount of surfactant absorbed at the aqueous droplet interface, the roughness of that interface, and the micellar compositions in the oil phase were all measured quantitatively. The aqueous volume fraction in the whole emulsion was about 90%, greater than the 74% close packing limit for spheres. Even so, no evidence was seen for nonsphericity of the aqueous droplets due to flattening by mutual droplet interaction. This we ascribe to polydispersity in the size of the aqueous droplets allowing high internal phase densities. Dilution experiments3 in which surfactant concentrations were varied over a 75-fold range established that the oil phase component of the emulsion contains inverse spherical micelles. SANS data from both the emulsions and the corresponding pure inverse micellar solutions fit to a model with a compound micelle, in which a core region of radius a little less than 15 Å is surrounded by a shell of ca. 20 Å thickness. There is no hexadecane in the core and no water in the shell. Typical volume

10.1021/jp903484j CCC: $40.75  2009 American Chemical Society Published on Web 08/14/2009

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percentages in the inverse micelles of water, hexadecane, and surfactant are 6, 31, and 64% but vary somewhat between samples. The dilution data show that the surfactant loading at the oil-water droplet interface is almost independent of dilution, and at the highest concentrations only 5% of the surfactant is at the interface with the remainder in the oil phase. The headgroup area per molecule at the interface is 140 Å2. The oil phase contains inverse spherical micelles at low concentrations of surfactant, but at higher concentrations, there are also noticeable amounts of 500-5000 Å-scale surfactant aggregates. In this paper, we examine emulsions, inverse micellar solutions, and related phases by SANS to determine the effect of surfactant variation. We have used three different monodisperse PIBSA surfactants of differing molecular weights and two sorbitan-based surfactants. In addition, the aqueous phase was either water or almost saturated ammonium nitrate. Both variations cause large structural changes. Experimental Methods Surfactants. Three “monodisperse” PIB samples were prepared by living carbocationic polymerization of isobutylene, as previously described for one member.3,4 The PIB samples were characterized by molecular weights (Mn) 584, 1040, and 1698 (1H NMR); 510, 930, and 1530 (vapor phase osmometry); and 600, 960, and 1560 (size exclusion chromatography). The percentage of exo double bond was 97, 96, and 97, respectively. The PIB was reacted with maleic anhydride and 2-hydroxy ethanolamine to give the surfactant species polyisobutylene N-(2-hydroxyethyl)succinamide. The final purity was checked by 13C and 1H NMR. Given the uncertainty in Mn, we can estimate the number of isobutylene repeats as n ) 10, 16, and 27, respectively. The middle member is illustrated in Figure 1. In what follows, rounded molecular weights of 750, 1200, and 1700 have been used to label these three surfactants. Commercial sorbitan monooleate, SMO (Aldrich), and sorbitan isostearate, SIS (Dr. W. Kolb AG), were used, after checking by NMR and elemental analysis. Both are mixtures, as is well-known; for example, the oleic acid/sorbitan ratio is ca. 1.5 in the “monooleate”. Preparation of Emulsions and Inverse Micelle Solutions. The n-hexadecane (D/H mixtures to vary contrast) and dissolved surfactant were preheated in a water bath at ca. 80 C. Water (D/H mixtures) or D2O almost saturated in ammonium nitrate (53 wt %), also preheated to 80 °C, was then added slowly. This was followed by rapid stirring at a speed to ensure thorough mixing.2 This method reproducibly forms 10 g quantities of emulsion. Water free surfactant solutions were prepared by dissolving surfactants into hexadecane. Wet inverse micellar solutions were formed by adding a drop of H2O or D2O, either pure or saturated in ammonium nitrate, to ca. 300 mg of dry surfactant solution, and leaving for several days. A clear hexadecane phase forms with a small amount of a second denser white water-rich phase. We presume the white phase is water/surfactant. The majority clear phase was examined by SANS. Agitation of the mixture produced some emulsification, ending in a single cloudy hexadecane phase. This process began during transport in a few samples, rendering the clear phase slightly cloudy. For the lower molecular weight PIBSA samples, this progress to a single cloudy phase was rapid and often spontaneous, no agitation being required. SANS Experiments. SANS experiments were performed on both the LOQ instrument at the Rutherford Appleton Laboratory, United Kingdom, and the SAND instrument at the Intense

Reynolds et al. Pulsed Neutron Source, Argonne National Laboratory, USA.5,6 For the emulsion samples, the experiment, data correction, reduction, and background correction were as previously described.2,3 The emulsions and inverse micellar solutions were run in 1 mm sample thickness quartz cells, except for a few on SAND which incorporated relatively little hydrogen content which were run in 2 mm cells at a temperature of 25 °C. We will discuss 28 emulsion and 32 inverse micelle solution samples. The emulsions were often in pairs. One member was contrast matched (CM), so that mainly deuterated oil and heavy water phase scattering length densities matched at high values. These components then contrasted strongly with the low scattering length density of the hydrogen-containing surfactant. The other member of the pair was contrast unmatched (UM) so as to highlight the aqueous droplet-oil interface. The emulsions had an 89-92% aqueous phase content. The contents of the emulsions and inverse micellar solutions used are listed in Tables 1 and 2. As an aid to the reader, the text sample labels are fontcoded to correspond with the color-coded figures. Ordinary type labels indicate that the fits are not shown in the figures in this paper, varied font that they are illustrated in the figures. Bold indicates a dry, inverse micellar solution (green in figures), bold italic samples concentrated in surfactant (red in figures), and italic dilute in surfactant (blue in figures). Thus, it is possible to easily track in the tables and text which experimental results are illustrated. To maintain comparability from previous studies, all samples were run together, even though this duplicated some measurements in ref 3. However, inverse micelle solutions 1m, 2m, and 3m were not repeated. The repeated data closely resemble those in ref 3. Particle Sizing. The droplet size distribution was measured by dilution of some of the emulsion with hexadecane or decane in a laser sizing apparatus (Malvern Mastersizer) approximately 4 weeks after production. Experiments on emulsions up to 1 year old indicate that the droplet size increases little over the maximum 4 week period over which the emulsions were measured by SANS. All emulsions showed a large polydispersity in aqueous droplet sizessthe left and right half-maximum in the distribution being relatively constant for all emulsions at 0.6 and 1.7 of the droplet size of the medianswhich was generally 4-10 µm. More detailed sizing experiments involving optical and electron microscopy and laser light scattering on original, undiluted emulsions will be discussed and compared with SANS measurements in a later paper. Modeling. We have modeled the unconvoluted total intensity as the sum of a contribution from a flat interface between the aqueous droplets and the external oil/ inverse micellar solution phase, a contribution from the inverse micellar solution itself, and a flat background. Ravey et al.7 have shown that a simple addition of intensities is appropriate, provided that the relative curvatures of the inverse micellar solution, the related phase microstructure, and the interface are sufficiently different, which our previous paper established was so in this case.5 Depending on the needs of the system, the scattering has been modeled as (a) locally rough but long-range flat interfaces, (b) a polydisperse hard sphere fluid of spherical micelles with core and shell internal structure, (c) internally structured sheets with and without intersheet correlation, and (d) internally structured monodisperse cored cylinders. The experimental resolution functions have been incorporated, using standard theories and methods as summarized in the previous two papers in this series.2,3 To maintain comparability between LOQ and SAND results, all refinements (unlike in ref 3) have been made in the

Structure of High Internal Phase Emulsions

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TABLE 1: Compositions of Inverse-Micelle-Generating Mixtures (Weights in mg)a label

surf.

wt surf.

7m 8m 9m 1m 2m 3m 92m 93m 94m 98m 99m 100m 1010m 1011m 57m 58m 83m 50m 49m 54m 89m 90m 91m 95m 96m 97m 1008m 1009m

MW 750 MW 750 MW 750 MW 1200 MW 1200 MW 1200 MW 750 MW 750 MW 750 MW 1200 MW 1200 MW 1200 SMO SMO MW 750 MW 750 MW 750 MW 1200 MW 1200 MW 1200 MW 750 MW 750 MW 750 MW 1200 MW 1200 MW 1200 SMO SMO

40 37 40 79 81 79 61 57 60 106 100 95 83 84 18 16 14 24 24 25 15 14 15 25 27 25 20 20

11m 82m 81m 6m

MW MW MW MW

C16D34

C16H34

D2 O

223

36

219 213

H 2O

AN-D2O

AN-H2O

26 198

184 182 344 339

38 44 38 38 53 52 51

331 343 344

32 108 54 36 44

330 289 332 292

41

296

28 40 35

337 326 336

42

330 327 335

44 43 78 37

337 345 333

49 37 340 296

49 37 37

327

phases 1 1 1 2 2 2 2 2 2 2 2 2 2 2 1 1 1 2 2 2 1 1 1 2 2 2 2 2

“Dry” Inverse Micelle Solutions 750 750 1200 1200

76 13 23 80

190 326 332 187

1 1 1 1

a AN ) ammonium nitrate. Surfactant PIBSAs labeled by molecular weight unless SMO. Each composition is (mostly) grouped in the three contrasts then the five surfactant types and whether concentrated or dilute in surfactant. Lastly, four dry inverse micelle solutions are listed.

same Q range 0.008-0.25 Å-1. One sample (539) was made up, in error, slightly contrast unmatched. A simple correction, based on an asymmetric slab, was applied.8 Results and Discussion The results for the inverse micellar solutions are first discussed and then compared with those from related emulsions. The changes in the oil phase have been followed by using the structures determined from inverse micelle solutions. These have been prepared so as to mimic the oil phase subsequently examined in the emulsion samples. PIBSA/Water Inverse Micellar Solutions. [Samples 1m, 2m, 3m, 7m, 8m, 9m, 49m, 50m, 54m, 57m, 58m, 83m]. The data from the concentrated and dilute MW 1200 inverse micellar solutions illustrated in Figure 2a have been discussed before.3 The data in Table 3a indicate spherical micelles, with a welldefined water core which is hexadecane free and a PIB surfactant tail/hexadecane shell which is water free. The micelle water content was 3-6% by volume and the hexadecane content 30-40% by volume for the concentrated surfactant systems. On almost 10-fold dilution of surfactant, the micellar external radius increases by about 8 Å, the hexadecane content of the micellar hydrophobic coating increases, and the water content and the radius of the hydrophilic core increase, as implied from the decrease in core scattering length density (SLD). The polydispersity index of 0.14-0.17 indicates quite monodisperse micelles. Some surfactant is not accounted for in the micelles but only in the surfactant-concentrated samples. For the concentrated samples, 29% of the added surfactant is

unaccounted for, whereas 0% is unaccounted for in the dilute analogues. In the previous paper,3 we speculate that this material could reside as aggregates in the oil phase but dissolution to the molecular level is more likely. The data from the concentrated and dilute MW 750 systems (Figure 2b) are quite different. An immediate indication that there is a different surfactancy is that the MW 1200 system exhibits the almost spontaneous formation of a single cloudy phase in mixing. This must be oil-surfactant rich and water poor. The MW 750 surfactant, wet system, mixtures, at high surfactant concentration, show up to four peaks spaced at multiples of 0.0293 Å-1 in all three contrasts. This indicates a lamellar phase of period 215 Å. If the lamellae did not have substructure, then the peaks would decrease in intensity monotonically with Q, and the different contrasts would only vary in scale. This is not observed, so the lamellae must have internal structure. The two mixes, in which water and surfactant plus water are highlighted, show four peaks steadily decreasing in intensity with Q. The mix highlighting the surfactant shows anomalously strong third- and particularly fourth-order peaks. This indicates a substructure favoring fourth-order intensity. An obvious substructure is a 215/4 Å separation of surfactant layers in a surfactant-water-surfactant bilayer. If we assume a 30 Å thickness of surfactant and oil, we obtain a structural picture of a bilayer consisting of 30 Å surfactant/oil layers sandwiching a 25 Å slab of water, with these bilayers separated by ca. 215 Å. This implies a water/surfactant/oil volume ratio of 12%/19%/ 74% in good agreement with the known content of the mix,

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TABLE 2: Compositions of Emulsions (Weight in mg)a number

surf.

35 34 25 29 32 72 62 63 64 65 1018 1017 537 536 56 70 50 39 55 71 68 69 66 67 1014 1013 539 538

MW 750 MW 750 MW 1200 MW 1200 MW 1700 MW 1700 MW 750 MW 750 MW 1200 MW 1200 SMO SMO SIS SIS MW 750 MW 750 MW 1200 MW 1200 MW 1700 MW 1700 MW 750 MW 750 MW 1200 MW 1200 SMO SMO SIS SIS

wt surf. C16D34 C16H34 D2O H2O AN-D2O match 153 149 290 290 290 297 152 148 293 285 307 301 200 200 36 40 36 36 36 35 36 36 37 35 50 50 50 50

718 697 670 670 670 814 822 811

812 106 815 983 1020 984 924 991 1030 829 817 816 106 815

6950 900 7970 6780 2260 9040 6780 2260 8710 160 1014 15 830 725 10 815

11830 11810 11830 11810 11830 11830 11830 11830

45 7000 8710 52 8980 9040 44 8960 8710 270 1095 280 1071 264 972 815

11830 11830 11820 11830 11830 11830 11830 11830

CM UMb CM UMb CM UMb CM UM CM UM CM UM CM UM CM UMb CM UMb CM UMb CM UM CM UM CM UM CMc UM

a In the last column, CM refers to aqueous/oil contrast matched, UM to unmatched, for each pair of similar emulsions. Surfactantconcentrated emulsions (six types) are listed first followed by dilute. b Note that these emulsions are contrast unmatched by use of C16D34/ surfactant/H2O mixtures, rather than C16H34/surfactant/AN-D2O as in the remainder. c Slightly contrast unmatched.

13%/13%/74%. We remark that, in nearly all micellar or monolayer situations, the micellar radii or surfactant thicknesses are distinctly less than what would be expected from the surfactant extended chain length, implying some chain coiling and/or canting away from the normals to the interfaces. The MW 750 surfactant dilute system can be fitted to a simple uncorrelated sheet bilayer model. The refined thicknesses and SLDs of the three components give a total bilayer thickness of ca. 110 Å, with a ca. 50/50 mix of surfactant and hexadecane sandwiching a 50 Å thick water layer. The increased water content and swollen water layers, relative to the concentrated mix, is consistent with the known relative content in the preparation. The MW 750 oil-rich systems contain bilayers, in which all the water is absorbed into a sheet, sandwiched between surfactant layers. In the more concentrated system, these bilayers become correlated into a lamellar structure. Reducing the molecular weight from MW 1200 to MW 750 produces a much more hydrophilic surfactant able to absorb all of the water present and form low curvature planar structures. The MW 1200 system by contrast retains only the water strongly hydrated to the headgroup and forms high curvature spherical structures. There is thus an important difference in the behavior of these two surfactants, indicating that their mixtures may show unusual effects. PIBSA-Only Dry Inverse Micellar Solutions. [Samples 6m, 11m, 81m, 82m]. One could question whether the addition of water to the PIBSA/oil solution does in fact change the structures. Table 3a and b shows that dry PIBSA/oil solutions are indeed very different from the wet solutions. For both MW 1200 solutions, we observe spherical micelles of much smaller radii than the wet solutions. For the concentrated MW 750 system, we now observe spheres rather than sheets, while the

sheets in the dilute dry MW 750 solution are much thinner. We can generalize to say that addition of water produces larger spheres or more swollen sheets. PIBSA/Ammonium Nitrate Solution Inverse Micellar Solutions. [Samples 92m, 93m, 94m, 89m, 90m, 91m, 95m-100m]. When the same PIBSA/hexadecane mixtures as above are exposed to saturated ammonium nitrate solution rather than water, changes in the partial molal free energy of the water in the ammonium nitrate solution have consequences. Table 3c shows these for the micelles in the oil phase for 1200 molecular weight PIBSA. For concentrated PIBSA solutions, a micellar external radius of 29 Å (compared to 31 Å for the water samples) was found. The hydrophobic micelle shell still contains 40% hexadecane, but the amount of water in the core of each micelle has dropped from 6800(100) to 3300(30) Å3. Note that, when ammonium nitrate in D2O was used, the scattering length density of the water in the micelle core became 4.82 × 10-6 Å2, rather than 6.34 × 10-6 Å2 for D2O itself. This difference reflects the exchange of protons from the added ammonium nitrate. At low concentrations of PIBSA in the inverse micellar solution, the micellar radius increases to 30 Å. This is far less than the 39 Å in the corresponding water inverse micellar solution, due to the lower availability of water in the micelle interior. The water content of the core increased to 6000(1000) Å3 but was again less than the 8300(800) Å3 for the corresponding water sample. As for the water samples, 31 and 24% of the surfactant is unaccounted for in the mass balance of the micellar composition. Qualitatively, the effect of adding ammonium nitrate to the water was to form micelles of higher curvature and lower water content. Later, we will show that in emulsions, while micelles of smaller radius are formed, the water content increases. For the MW 750 PIBSA, the change from the water-only systems shown in Figure 2c is more dramatic. The data no longer fit planar lamellar models, but a model of micellar cylinders is required. For the data with samples concentrated in PIBSA, both the rod radius and length are accessible, since the rod length is short enough to be defined by the Q range of the data. The rods are 27 Å in radius, with a hydrophobic shell containing 60% hexadecane and 40% PIBSA, surrounding a 12 Å radius core containing 50% by volume of this core of water and 50% PIBSA headgroup. The hydrophobic shell of thickness 15 Å is comparable to the thicknesses observed for films of this surfactant spread at the air-water interface.9 The rod lengths vary from 89 to 190 Å for the three samples. Such variation for very similar samples is to be expected, since it is well-known that rod lengths of cylindrical micelles of finite length are extremely sensitive to small differences in sample conditions. The sphere to infinite length rod transition in generic phase diagrams contains only a small region of finite length rods for pure surfactants. This sensitivity of rod length is further illustrated in the samples diluted in MW 750 PIBSA. The surfactant dilution has produced rods longer than about 1000 Å. The Q range of the instrument cannot define longer rod lengths, and at low Q, the intensity varies at Q-1. One of the samples shows a rod radius of 30 Å, larger than that for the concentrated samples, with an increased water content of a small radius core compared to the corresponding surfactant-concentrated sample. The sample 89m shows a rod radius of 44 Å, due to a, much expanded, core of water of radius 23 Å. The trends are as expected. Use of ammonium nitrate has produced structures (rods) of higher curvature than the lamellae



Figure 2. SANS from pure PIBSA surfactant inverse micelle solutions at contrasts to show: (circles) water; (squares) surfactant; and (triangles) water plus surfactant. The inverted green triangles show “dry” surfactant solution, the red filled symbols show concentrated surfactant, and the blue open symbols show dilute surfactant. The samples used [...] are detailed in Table 1. Fits are shown as continuous lines. (a) 1200 mol wt surfactant/ water [1m, 2m, 3m, 6m, 49m, 50m, 54m, 81m]. (b) 750 mol wt surfactant/water [7m, 8m, 9m, 11m, 57m, 58m, 82m, 83m]. (c) 750 mol wt surfactant/ammonium nitrate [89m, 90m, 91m, 92m, 93m, 94m].

in water-only samples. Dilution of surfactant produces longer rods, i.e., lower curvature, with increasing water content of a central rod core. Indeed, our lower concentration samples appear to be close to an instability in the rod structure, in radius as well as length, perhaps to lamellar structures. Sorbitan Surfactant/Ammonium Nitrate Inverse Micelle Solutions. [Samples 1008m-1011m]. In Table 3e, it can be seen that the scattering from sorbitan monooleate (SMO) inverse micellar solutions is fitted well by a spherical micelle model. There is, however, a significant difference from the PIBSA inverse micellessa central core of different composition to the surrounding hydrophobic shell was not needed in the model to obtain satisfactory fits to the data. Examining the results for samples concentrated in SMO, for 1010m, where the contrast arises only from an AN-water component, a sphere of radius 15 Å containing 30% water fits the data well. For 1011m, where there is contrast from both hexadecane and water, a sphere of radius 22 Å containing 31(2)% hexadecane was required to fit the data when the micelle was constrained to possess a 30% water, 15 Å radius core. The model for the micelle is thus spherical, with 22 Å radius and containing 30% hexadecane, and with a core of 15 Å containing 30% by volume of water. However, 1011m alone can be equally well fitted by a uniform sphere containing surfactant, hexadecane, and water. It is only by combining 1010m and 1011m that the need for a core arises. This is because in 1011m the extra 7 Å surfactant/hexadecane

shell is not resolvable in a single profile with the Q range of this experiment. The small, 7 Å, tail region compared to the larger, 15 Å, head region is quite reasonable. The sorbitan headgroup is comparable in volume to the ethanolamine-succinic anhydride headgroup of the PIBSAs. The tail volume arising from the ca. C24 portion of SMO is less, however, than half the C64 chain of MW 1200 PIBSA. The head to tail volume ratio for SMO is thus much larger than that for MW 1200 PIBSA, or even MW 750 PIBSA. This raises the question of why SMO does not exhibit a sponge phase like MW 750 PIBSA. A sponge phase is one in which both oil and aqueous components are intertwined and both continuous. A partial answer is that not only are the headgroups different but also the presence of the double bond in the oleate tail of SMO is known to drastically affect surfactant properties such as HLB number. It is well-known that HLB numbers are not solely dependent on tail-to-head volumes. There is also a suggestion of more structural variability in the sorbitan micelles; a polydispersity of 0.26(1) was needed to fit the data. This could arise from several sources: a genuine variability in radius due to variable headgroup packing, the greater polydispersity of the SMO relative to PIBSA, or an incipient transition to short, almost isotropic, rodlike micelles. We have shown earlier2 that the use of either rods or polydisperse spheres model some data equally well.

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TABLE 3: Structural Parameters Derived from Fitting the Pure Surfactant Inverse Micelle Solution Dataa (a) PIBSA/Water Mol Wt 1200 C16D34 dry

isotopic composition of oil and water concentrated mol wt 1200 sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell scattering length density of core

C16D34 H2 O

C16H34 D2 O

sphere 6m 2m 21.56(6) 30.46(4) 0.272(2) 0.168(2) 0.327(3) 0.327(2) 2.30(2) 2.29(1) 2.58(6) 2.24(6)

C16D34 D2 O

sphere 1m 3m 31 31.21(4) 0.166 0.164(1) 0.33 0.332(1) 0.10(1) 2.47(1) 4.87(6) 3.82(6)

C16D34 dry

isotopic composition of oil and water dilute mol wt 1200 sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell scattering length density of core

C16D34 H2 O

C16H34 D 2O

sphere 81m 54m 22.84(7) 40.97(9) 0.253(6) 0.141(2) 0.087(9) 0.103(1) 2.83(8) 2.04(3) 2.81(9) -0.40(2)

C16D34 D 2O

sphere 49m 50m 39 38.4(3) 0.140 0.135(6) 0.095 0.090(2) -0.34(2) 2.94(5) 5.7(5) 3.75(3)

(b) PIBSA/Water Mol Wt 750 C16D34 dry concentrated mol wt 750 sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell scattering length density of core

sphere 11m 27.36(7) 0.154(2) 0.146(2) -0.32(3)

C16D34 H2 O

C16H34 D2 O

C16D34 D2 O

8m

lamellae 7m

9m

lamellar repeat distance 215 Å surfactant layer separation in each lamellar bilayer 55 Å

dilute mol wt 750 sample identification sheet totalthickness(Å)

C16D34 dry

C16D34 H 2O

C16H34 D2 O

C16D34 D2 O

sheet 82m 34.6(2)

58m 140(2)

sheet 57m 110

83m 88(8)

0

6.7

6.5(4)

SLD of core

-2.15(2)

(c) PIBSA/Ammonium Nitrate Mol Wt 1200 isotopic composition of oil and water

C16D34 AN-H2O

C16H34 AN-D2O

C16D34 AN-D2O

isotopic composition of oil and water

C16D34 AN-H2O

C16H34 AN-D2O

concentrated mol wt 1200 sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell scattering length density of core

98m 28.26(6) 0.140 0.229(1) 2.38(3) 1.79(10)

sphere 100m 28.7 0.140 0.23 -0.20(3) 1.53(16)

99m 29.36(6) 0.140 0.232(1) 2.44(3) 2.13(10)

dilute mol wt 1200 sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell scattering length density of core

95m 30.3(1) 0.140 0.070(1) 2.48(8) 0.4(2)

sphere 97m 30.4 0.140 0.075 -0.36(4) 3.0(5)

C16D34 AN-D2O N/A

(d) PIBSA/Ammonium Nitrate Mol Wt 750 isotopic composition of oil and water

C16D34 AN-H2O

C16H34 AN-D2O

C16D34 AN-D2O


C16D34 AN-H2O

C16H34 AN-D2O

C16D34 AN-D2O

concentrated mol wt 750 sample identification rod radius (Å) rod length (Å) volume fraction of rods scattering length density of shell radius of core (Å) scattering length density of core

92m 26.3(1) 190(1) 0.39 3.67(3) 12 3.5(1)

cylinder 94m 28 160(8) 0.39 -0.44 12 2.2(1)

93m 28.2(1) 88.9(3) 0.39 3.82(3) 12(2) 3.5(1)

dilute mol wt 750 sample identification rod radius (Å) rod length (Å) volume fraction of rods scattering length density of shell radius of core (Å) scattering length density of core

89m 44(1) >1000 0.089(3) 3.64 23(6) -0.04

cylinder 91m 30 >1000 0.092 -0.44 6.3(1) 3.5(2)

90m 30.1(2) >1000 0.092(2) 3.64 10 6.3

(e) SMO/Ammonium Nitrate isotopic composition of oil and water

C16H34 AN-D2O

concentrated SMO sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell radius of core (Å) scattering length density of core

1010m 21.7 0.26 0.151 -0.44 15.1(4) 1.3(1)

C16D34 AN-D2O


C16H34 AN-D2O

1011m 21.7(2) 0.26(1) 0.151(9) 2.2(1) 15.1 2.2

dilute SMO sample identification micelle radius (Å) micelle polydispersity volume fraction of micelles scattering length density of shell radius of core (Å) scattering length density of core

1008m 22.7 0.26 0.055 -0.44 16.3(9) 0.9(1)

sphere

C16D34 AN-D2O sphere 1009m 22.7(5) 0.27(2) 0.055(8) 2.7(2) 16.3 2.7

a Parameters without standard errors were fixed from other sources. All core radii were fixed at 13 Å when not otherwise specified. In this and subsequent tables, scattering length densities are in units of 10-6 Å-2.

The two models may be distinguished by varying the surfactant concentration. Rod lengths, as discussed above, change with surfactant dilution, and if the apparent polydispersity and sphere radius are insensitive to dilution, the system is polydisperse spheres. On dilution, the polydispersity increases by only 0.01 and the radii in both scattering contrast situations by only 1 Å. These small changes show that the SMO inverse micelle solutions are quite polydisperse spherical micelles. PIBSA/Water Emulsions. [Samples 25, 29, 32, 34, 35, 39, 50, 55, 56, 70, 71, 72]. Figure 3a-c illustrates the SANS from

six emulsion types: concentrated surfactant (1.5 and 1.8 wt %) and dilute surfactant (0.35 and 0.40 wt %) with surfactants of molecular weight 1200 (Figure 3a), 1700 (Figure 3b), and 750 (Figure 3c). For each sample, a contrast matched sample (circles), in which the surfactant is contrasted against the oil plus water, and an unmatched sample (squares), in which the oil is contrasted against surfactant plus water, is shown. We also show the model fits to these 12 samples. For the 1200 and 1700 molecular weight



Figure 3. SANS from 12 emulsions composed of pure PIBSA surfactants and water. Red solid points are emulsions concentrated in surfactant, blue open points are dilute in surfactant, squares are contrast unmatched, circles are contrast matched, and solid lines are the fits. (a) 1200 molecular weight [25, 29, 39, 50]. (b) 1700 molecular weight [32, 55, 71, 72]. (c) 750 molecular weight [34, 35, 56, 70].

samples, the values of the fitted parameters are given in Table 4, those for the 750 molecular weight samples will be discussed below. The MW 1200 and 1700 PIBSA Emulsions. [Samples 25, 29, 32, 39, 50, 55, 71, 72]. As already discussed,3 for the MW 1200 samples, the oil phases mostly contain inverse spherical micelles of radius 32(1) Å. There is a core smaller than 15 Å radius, which contains only water and surfactant with no hexadecane. This core is coated by a shell, which is dominated by PIB, contains negligible water, but incorporates some hexadecane. The contrast matched (CM) and unmatched (UM) mixtures used have shell and core contrasts of ca. 2 × 10-6 and 3 × 10-6 Å-2 for the CM samples and 2 × 10-6 and 0 × 10-6 Å-2 for the UM samples. Both contrasts pick up similar SLDs from the deuterated hexadecane in the shell, while in the core the CM has D2O of high SLD and the UM has H2O of low SLD. The hexadecane content in the shell is calculated from the four refined scattering length densities (including the MW 1200 data discussed further below). The calculated values vary from -15 to 38%. -15% is an obviously unphysical content, with 0% being the nearest physical value. This figure illustrates the systematic errors of the analysis. In particular, we notice that this systematic variation arises from a correlation with the amount of aqueous-oil interface scattering. Where this interference is reduced, either by use of extended Q-range data or inverse micellar solutions (ref 3), a relatively constant hexadecane content of ca. 30% is obtained.

The micelle content and radii are comparable with those in the micelle structure deduced from the concentrated inverse micellar solution data (Table 3a). A few emulsion samples, including sample 50, have micellar radii around 6-8 Å larger. This radius agrees with the dilute inverse micellar solution values. As we see further down Table 4b, the observed volume fraction of micelles in sample 39 is double that for sample 50. The apparent inconsistency is thus resolved, and arises from a minor difference in surfactant concentration in the initial emulsion mix. Overall, we can see that inverse micellar solution and emulsion data show a consistent picture. As the high surfactant concentration was reduced, the micellar radius and core water content decrease slowly. However, at the lowest surfactant concentrations, there is a more sudden and larger increase in both radius and core water content. At the lowest MW 1200 PIBSA concentrations, the system appears to approach an emulsion/sponge phase phase boundary. This will be further discussed below for the MW 750 PIBSA emulsions. The MW 1700 samples are very similar to the MW 1200 samples in the oil phase structure (Table 4). The micellar radius is only about 4 Å larger than for comparable MW 1200 PIBSA emulsions, and has a similar core water content. There is a slight decrease in both micelle radius and water content in the core as the surfactant is diluted. The 4 Å increase in micellar radius is therefore not caused by the molarity of MW 1700 surfactant in the emulsion being lower than that in the MW 1200 samplesthey have similar wt % surfactant contentsbut by

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TABLE 4: Structural Parameters Derived from Fitting Emulsion Dataa (a) Samples Concentrated in Surfactants surfactant concentration (wt %) and (molecular weights)

2.9 (1200) water 25 29 31.9(1) 30.2(1) 1.35(4) 0.7(1) 13 13 4.0(2) -0.3 0.311(2) 0.262(4) 0.331(3) 0.75(3) 88(4) 25 59(2) 3

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 surface area (m2/mL) surfactant absorbed at interface (mg mL-1) mean headgroup area per molecule (Å2) roughness of interface (Å) % of total surfactant absorbed at interface

2.9 (1700) water 32 72 33.8(1) 35.1(2) 0.84(3) 2.35(3) 13 13 4.1(2) 1.35 0.337(2) 0.302(4) 0.23(1) 0.67(3) 97(4) 25 20(4) 2

1.5 (750) AN-water 62 63 29.8(7) 30.2 1.9(4) -0.4 13 21(3) 2(2) 2(1) 0.097(6) 0.097 0.51(1) 1.33(2) 48(2) 5 13(3) 12

2.9 (1200) AN-water 64 65 29.6(2) 29.6 2.2(1) -0.4 13 18(2) 3(1) 3(1) 0.247(4) 0.247 0.60(1) 1.78(4) 67(2) 5 12(3) 8

3.0 (SMO) AN-water 1018 1017 21(1) 21 2.4(5) -0.4 none 16.5 none 2.3(1) 0.192(8) 0.192 1.75(1) 2.68(5) 61(1) 5 5 8

2.9 (SIS) AN-water 537 536 19(1) 19 1.5(7) none none 19 none 2.0(1) 0.08(2) 0.08 1.00(2) 1.64(4) 57(1) 5 5 8

(b) Samples Dilute in Surfactants surfactant concentration (wt %) 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 surface area (m2/mL) surfactant absorbed at interface (mg mL-1) mean headgroup area per molecule molecule (Å2) roughness of interface (Å) % of total surfactant absorbed at interface

0.36 (1200) water 50 39 37.0(3) 28.0(2) -0.3(9) 0.7 13 13 3.7 -0.3 0.025(7) 0.056(1) 0.250(5) 0.76(3) 66(2) 25 -3(23) 21

0.36 (1700) water 55 71 32.2(4) 32.8 -1.4(15) 0.7 13 13 2.6 -0.3 0.015(6) 0.0213(6) 0.140(5) 0.45(2) 88(4) 25 19(5) 13

0.36 (750) AN-water 68 69 28.1(7) 28.1 2.1 -0.4 13 21 3.1 4.3(7) 0.022(2) 0.022 0.48(1) 0.90(3) 66(2) 5 18(2) 33

0.36 (1200) AN-water 66 67 26.1(7) 25.7 2.1(1) -0.4 13 18 2(1) 4.2(4) 0.047(2) 0.047 0.247(4) 0.77(2) 64(2) 5 12(5) 29

0.50 (SMO) AN-water 1014 1013 21 21 2.4 -0.4 none 16.5 none 4.8(8) 0.007(1) 0.007 0.81(1) 2.5(1) 30(1) 5 5 47

0.36 (SIS) AN-water 539 538 19 19 1.5 none none 19 none 3.8(1) 0.014(1) 0.014 0.54(2) 0.96(2) 53(1) 5 5 19

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 an error implies that the parameter was not refined but fixed from the other contrast. There are no fits for PIBSA MW 750/water, since it does not form emulsions, as discussed in the text.

changes in tail volume and conformation. There is no evidence for a sudden increase in both radius and water content at low surfactant concentration, in contrast to the MW 1200 PIBSA emulsion. This relative insensitivity of micelle structure to increasing molecular weight is quite surprising. “Wedge theory” suggests that the MW 1700 micelles might have greater curvature and thus be smaller than those for MW 1200. Conversely, if micelle structure were determined by “magic” numbers of either head or tail for packing purposes, we might expect larger micelles for the MW 1700 system than the MW 1200 system. The implication of the above results is that there is an effectively constant PIB tail length, independent of molecular weight. This is consistent with our observations on the thickness of surface films of these surfactants using neutron reflectometry at the air-water interface.9 Those results indicate that the MW 1700 surfactant’s tail is distinctly more coiled than that of the MW 1200 system, and thus shows approximately the same interfacial thickness. For the MW 1200 and MW 1700 surfactants, the observed two to three molecules of water per headgroup are probably water molecules strongly ligated to the acid and amide frag-

ments, or in the case of deuterated water samples, deuteration of the headgroup hydrogen atoms. These hydrated headgroups pack into a small core in some “optimal way”, which we assume is common to both MW 1200 and MW 1700 micelles. The PIB shell then packs around this, adjusting coiling, to produce complete cover of the core. Thus, if we assume that the radius of the MW 1200 micelle is 31 Å, then this argument implies a MW 1700 micellar radius by calculation ((1700/1200)1/3) of 34.8 Å, an expansion of 3.8 Å, the same as that actually observed in Table 4a. The interplay of complete PIB core coverage and the requirement of fixed water/headgroup numbers in the core thus determines the overall micelle size. However, if it becomes thermodynamically favorable to include more water in the core, then the micelle expands dramatically, and as seen at low MW 1200 concentrations and as shown below for MW 750 emulsions. The droplet surface areas as measured from the Porod-like scattering at low Q with high water droplet/oil contrast show trends with molecular weight. Perusal of Table 4 shows that for the same preparative protocol greater surface areas (and smaller average aqueous droplet sizes) were obtained for both lower molecular weights and more concentrated surfactant mixtures. The amount of surfactant adsorbed at the aqueous-oil

Structure of High Internal Phase Emulsions interface corresponds to a monolayer as expected; the footprint per molecule varies from 66 to 97 Å2, with the MW 1700 PIBSA having the larger values as expected. While the samples diluted in surfactant show between 0.0 and 0.2%, i.e., a negligible amount, of surfactant missing from the micellar mass balance (probably aggregated into larger structures), the more concentrated MW 1700 emulsion shows 0.7% unaccounted for, an increase over the 0.4-0.6% found for the MW 1200 concentrated emulsions. The MW 750 PIBSA Emulsions. [Samples 34, 35, 56, 70]. The scattering patterns from emulsions made from MW 750 PIBSA were quite different from those for the higher molecular weight surfactant. Cursory inspection shows the absence of the characteristic “bump” at medium Q, indicating the presence of small micellar structures in the oil phase. Instead, there is a ca. Q-2 dependence of the scattered intensity for all of the scattering curves, indicating sheet-like structures rather than emulsions. The very high intensity for the high surfactant concentration, unmatched, emulsion implies very high specific surface areas for these sheet-like structures. There are also modulations in the scattering functions from the unmatched data, indicating internal structure in these sheets. A flat sheet model was fitted to all of these data. The contrast unmatched data fit a model in which a thin sheet is partially correlated at a distance to another sheet. A 15 Å thickness for the sheets was assumed in the calculation, being about the maximum thickness that LOQ could not resolve due to Q limitations. For the concentrated sample, a correlation coefficient of 0.51 was found with another sheet 180 Å away. In the dilute sample, we obtain a correlation of 0.24 at 280 Å. The high water composition leads us to assign the sheets as surfactant plus oil separated by a large thickness of water. The change from 180 to 280 Å on dilution by a factor of 3 rules out a lamellar phase but suggests a 3D sheet phase, i.e., an L3 sponge phase.10-12 The decrease in correlation on dilution also makes sense under this hypothesis. The fitted model captures the essence of the structuressheet-like, high surface area, long-range sheet correlationsbut is clearly not as adequate a model as for those emulsions (MW 1200 and MW 1700)swhich contain spherical micellar structures. The external evidence for this phase being L3, not emulsion, is threefold. First, the phase has a very low, water-like viscosity. This is quite different from the thick emulsions for higher molecular weight surfactants. Second, when diluted with hexadecane, this phase does not disperse, unlike the MW 1200 and MW 1700 samples. It does, however, disperse in water. Third, the electrical conductivity is high, characteristic of an aqueous continuous phase. This is unlike the oil continuous emulsions whose conductivity is low. We therefore conclude that no waterin-oil emulsion is formed but rather a water continuous L3 sheet phase. The contrast matched data give some more support to the L3 assignment. Both dilutions fit the same model: two interfering perfectly correlated thin slabs separated by 18 Å, with no longrange correlation. A single slab is a distinctly poorer fit. If the phase is indeed an L3 sponge, we would expect the thin sheets to be approximated by a layer of hexadecane and PIB tails insulated from the water by a layer of headgroups on either side of the slab. Contrast variation resolves the structuresmatched data, emphasizing the surfactant, would be expected to be two close sheets, while the unmatched, emphasizing the hexadecane, would be a single, thinner, slab. This is what we observe, further supporting our hypothesis of a water bicontinuous L3 sponge

J. Phys. Chem. B, Vol. 113, No. 36, 2009 12239 phase composed of three-dimensionally arranged surfactant bilayer sheets. A further subtle point emerges: Why is a long (200-300 Å) correlation observed in the contrast unmatched samples (observing hexadecane + surfactant) but not in the matched samples (observing surfactant only)? This implies that the hexadecane slabs are correlated, but the surfactant molecules are much less so. A possible explanation is that the very high surface areas of ca. 35 m2 mL-1 imply a headgroup area of up to 800 Å2. As the surfactant headgroup has an area of ca. 85 Å2 at the air-water interface, the 800 Å2 value indicates that much less than a monolayer of surfactant supports the structure. Thus, the hexadecane molecules are close packed in each slab, and the slabs are correlated. The surfactant molecules are however very diffusely spread on either side of each slab, enough to stabilize the phase, but still diffuse enough that mutual lateral clumping on the surface of each slab may be enough to destroy the interslab surfactant correlations. By lowering the surfactant molecular weight, the system has gone from a well behaved oil continuous emulsion (MW 1700) to one that is well behaved, except at the lowest surfactant concentrations when there is a suggestion of instability to lower curvature micelles (MW 1200), to a water continuous sponge phase (MW 750). Thus, as the molecular weight has decreased, the positive curvature of surfactant against oil has decreased. This we might expect from wedge theory on lowering the tailto-headgroup relative volumes. PIBSA/Saturated Ammonium Nitrate Solution Emulsions. [Samples 62-69]. In this system, inverse micelle solutions are again observed in the oil component. Use of saturated ammonium nitrate reduces the micelle radius by about 2 Å compared to water-only emulsions for both concentrations of MW 1200 PIBSA as surfactant. There is also a larger core, 18(2) Å, which contains more water (15% of the micelle volume for concentrated and 30% for the dilute samples). Thus, on dilution, the external micelle radius decreases and the core water content increases. The shell again contains about 30% hexadecane, as with both the inverse micelle solutions and water-only emulsions. We note that, as we have used C16H34 and AN-D2O in the unmatched (UM) mixtures both here and for the sorbitan emulsions, only the water content and not the hexadecane content for the inverse micelle can be estimated. Thus, the SLD of the shell changes from the ca. 2 × 10-6 Å-2 of the wateronly emulsions to ca. 0 × 10-6 Å-2, and that of the core from ca. 0 × 10-6 to ca. 3 × 10-6 Å-2, for these UM systems, as is observed. The MW 750 PIBSA-saturated ammonium nitrate emulsions can be fitted with spherical micelles, and the dilution dependence confirms that a model of rods is not appropriate. Just as with the inverse micelle solutions, addition of ammonium nitrate has produced structures in the oil of higher curvature, even higher than that of the rods in the ammonium nitrate inverse micelle solutions. The micelles produced are larger than those for MW 1200 PIBSA, but this is due to a 4 Å increase in the size of the water-containing core for both concentrated and dilute emulsions. The aqueous droplet surface areas are generally larger than those for the concentrated water-only emulsions. This is accompanied by an increased surfactant loading at the interface when expressed in mg/mL of emulsion, though addition of ammonium nitrate had no strong effect on the calculated area per headgroup. The MW 750 PIBSA tends to smaller area than MW 1200 PIBSA as expected. Table 4a shows that the roughness at the interface is less for the ammonium nitrate solution. This might be explained by different capillary wave

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Figure 4. SANS from four emulsions made with SMO and ammonium nitrate solution [1013, 1014, 1017, 1018].

effects. The increased physical density difference between oil and ammonium nitrate solution (650 kg m-3) as compared to oil and water (227 kg m-3) would reduce roughness, independent of any changes in interfacial tension which may also occur. Sorbitan Surfactant/Ammonium Nitrate Emulsions. [Samples 536-539, 1013, 1014, 1017, 1018]. Both SMO and SIS form emulsions easily with saturated ammonium nitrate. The SMO emulsions are not as stable as PIBSA emulsions, decomposing in a few weeks if the SMO concentration is less than ca. 1%. The SMO data and model fits are shown in Figure 4. In the SMO emulsions, certain parameters, such as core radii, have been fixed at values deduced from the inverse micelle solution data; this is a result of the large core size relative to micelle size and the aqueous-oil interfacial scattering preventing the level of detail of these parameters from being resolved. Similarly, for the SIS emulsions, no core was required to provide a good fit to the data. The external radius of the micelles for the concentrated SMO emulsions refined to the same value as for the inverse micelle solutions, 21 Å; the SIS emulsions refined to an external micellar radius of 19 Å. The hexadecane content of the micelles refined for the concentrated systems to 20% by volume of the micelles for both SMO and SIS emulsions. The water contents refined to 25 and 50% for concentrated and dilute SMO emulsions, respectively, and 46 and 80% for concentrated and dilute SIS emulsions. Though there must be systematic errors in these numbers, the trends are clear. Just as with PIBSA-based emulsions, there is a substantial hexadecane content in the micelles. However, the core region is relatively much more pronounced, and its water content is larger, and increases with dilution. Refinement of polydispersities in emulsions 1018 and 537 gave 0.24(3) and 0.21(6). These values support the idea derived from the inverse micelle solution results that the micelles are quite polydisperse. Both the SMO content of the micelles and particularly the SIS content are reduced from comparable PIBSA systems. In the dilute SIS case, taking account of errors, the SIS content of the micelles is less than 20%. We are led to a model of the sorbitanbased micelles in which there is a larger core size in a smaller micelle than for PIBSA emulsions, and that the water content is larger and the packing less well organized.

The aqueous-oil interfacial areas are also larger than those for the corresponding PIBSA emulsions, but the footprint indicates a monolayer of both SMO and SIS given the expectation of footprint sizes of around 60 Å2.9 The slightly lower SIS micellar radius than that for SMO (both having C17 tails) may perhaps reflect a “straightening” effect of the oleate double bond, and tail shortening by branching in isostearate. Surfactant Distribution, Molecular Dissolution, and LargeScale Aggregation. In the previous sections, the structures formed and their compositions, in a variety of inverse micelle solutions and emulsions, have been measured from the scattering. Table 5 summarizes the observed amounts of surfactant adsorbed at the aqueous-oil interface and that in the inverse micelles. The remainder of the surfactant has been assumed to be in the form of large-scale aggregates or fully dissolved. The trends in these numbers are clear, but there is a good deal of scatter in the values. In all of the emulsions, a relatively constant amount of surfactant is adsorbed at the aqueous-oil interface in an amount corresponding to about a monolayer. For the concentrated emulsions, this amount is only a small fraction, 12% or less, of the total amount of surfactant. Even in the inverse micelle solutions for PIBSA, over 75% of the surfactant is in the form of inverse micelles, but the SMO inverse micelle solutions only have 50-60% of the surfactant in the form of micelles. This “missing” surfactant may be in the form of large aggregates. The observation of noticeable light scattering in the PIBSA inverse micelle solution indicates that some surfactant is in the form of large-scale aggregates.13 The light scattering is weak and only visible with strong illumination, so the amount of largescale aggregates is small. This agrees with our Ultra Small Angle Neutron Scattering (USANS) results, where for most PIBSA emulsions, including those here with hexadecane as the oil, the amount of aggregate observed is usually small. We can conclude that, in the PIBSA emulsions, about one-fourth of the total surfactant is fully dissolved in the oil. For the sorbitan emulsions, no USANS measurements are available but the amount of “missing surfactant” is larger, ca. 50%. A suggested explanation for this result comes from USANS from related systems. These indicate that more hydrophilic surfactants exhibit greater aggregation and, in addition, there should be an enhanced aggre-



TABLE 5: Summary of Derived and Inferred Surfactant Percentages in the Various Surfactant Structures Surfactant Conc. Emulsions surfactant concentration (wt %) and (molecular weights)

2.9 (1200) water 3 77 20

surfactant absorbed at aqueous-oil interface surfactant in micelles surfactant unaccounted for

2.9 (1700) water 2 70 28

1.5 (750) AN-water 12 57 31

2.9 (1200) AN-water 8 64 28

3.0 (SMO) AN-water 8 49 43

2.9 (SIS) AN-water 8 22 70

0.36 (1200) AN-water 29 106 -35

0.50 (SMO) AN-water 47 9 44

0.36 (SIS) AN-water 19 25 56

Surfactant Dilute Emulsions surfactant concentration (wt %) and (molecular weights) surfactant absorbed at aqueous-oil interface surfactant in micelles surfactant unaccounted for

0.36 (1200) water 21 103 -24

0.36 (1700) water 13 50* 37*

0.36 (750) AN-water 33 50 17

Surfactant Conc. Inverse Micelles surfactant concentration (wt %) and (molecular weights) surfactant in micelles

31 (1200) water 71

15 (750) water lamellar

15 (750) AN-water 123(cylinder)

23 (1200) AN-water 69

22 (SMO) AN-water 53

6.7 (1200) AN-water 76

6.3 (SMO) AN-water 60

Surfactant Dilute Inverse Micelles surfactant concentration (wt %) and (molecular weights) surfactant in micelles

6.7 (1200) water 100

gation effect, since the more hydrophilic sorbitans would be expected to be less soluble in oil. However, direct measurement of aggregate is required to be certain. Conclusions Structure of Water-Based Emulsions and Micellar Solutions. High molecular weight (MW 1700) PIBSA forms high internal phase emulsions with water in hexadecane at concentrations of both 2.9 and 0.36 wt % of surfactant. The neutron scattering data from both can be fitted to a model with the aqueous droplet-oil interface coated with slightly less than a monolayer of surfactant. The remainder of the surfactant is distributed in the oil phase of the emulsion as inverse micelles, each of relatively constant structure. At high surfactant concentrations, the micelles were of radius ca. 34 Å and contained a small core of water, surrounded by an oil shell, mainly PIB tail, but with some 30% hexadecane. At lower concentrations, the radius decreases to ca. 32 Å with a lower water content of core. When the PIBSA surfactant molecular weight is decreased to 1200, there are only small changes in structure. The micellar radius generally decreases 2-4 Å from the MW 1700 values with similar water core contents. That the radius decreases more slowly than the molecular weight is explicable by uncoiling of the PIB tail chain in the MW 1200 sample. At the lowest MW 1200 PIBSA concentrations in both emulsions and inverse micelle solutions, there is an increase in micellar radius of ca. 9 Å accompanied by a distinct increase in core water content. This incipient instability to structures of lower curvature probably reflects the higher head to tail ratio of the MW 1200 system compared to the MW 1700 system. The aqueous-oil interfacial area is larger for MW 1200 than MW 1700 at the same weight percentage content, indicating greater ease of emulsification to smaller droplets. The interface in both cases is stabilized by a monolayer of surfactant with the MW 1700 footprint being slightly larger than MW 1200 as expected.

4.5 (750) water sheet

4.4 (750) AN-water 100 (cylinder)

As the average molecular weight is further reduced, the head to tail ratio increases, favoring micelles of smaller curvature in the emulsion. The micelles increase in size, while the much larger aqueous droplets decrease in size, reflecting the improved ease of emulsification as the molecular weight decreases, at least down to 750 and at constant weight percentage. If we disentangle the changing surfactant molarity, it appears that this effect results in poorer emulsification as the molecular weight decreases. There is a steadily decreasing thickness of oil between the structural features. It is possible to imagine that, taken to the extreme, neighboring surfactant layers, one at the aqueous interface and one at the micelles, both with the PIB tail pointing into the hexadecane, could collapse to a bilayer L3 sponge phase. This actually does happen. The MW 750 surfactant forms a bicontinuous sponge bilayer L3 phase when we attempt to form an emulsion, at high water and 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. As we might expect from this scenario, the pure MW 750 inverse micelle solutions of much lower water content are not strictly relevant to this story. They do not form a spherical inverse micelle solution but instead, at high surfactant concentration, form a lamellar phase and, at low concentration, uncorrelated bilayer sheets which can be modeled as two 50% PIB/50% hexadecane 30 Å thick sheets sandwiching all of the water, in this case a 25 Å thick slab. These surfactant emulsions and inverse micelle solutions have provided results consistent with previous experiment and theory concerning the phase diagrams and stabilities of single phase surfactant solutions, where similar patterns of spherical micellar, flow birefringent L3, and lamellar LR phases are seen.8-10 Our observations require only that as the average molecular weight is decreased, at constant surfactant weight content, the high curvature at the micelles decreases and the large aqueous droplet size decreases. The substantially higher hydrophilicity of the MW 750 surfactant at higher concentrations causes a cessation

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of emulsion formation, and sponge L3 phases (or at other water concentrations other planar bilayer phases) are then formed. Effect of Ammonium Nitrate on PIBSA Emulsions. The effect of adding ammonium nitrate to the water is to form micelles of higher curvature and lower water content in both inverse micelle solutions and emulsions containing 1200 molecular weight PIBSA. Structures observed when using water are replaced by cylindrical structures in the inverse micelle solution and spherical inverse micelles in the emulsion, albeit unexpectedly large and with a substantial water core for AN-water. This is the only place where we have noticed a qualitative difference between inverse micelle solution and emulsion. This is explicable in that the structures at this point are clearly changing rapidly for small changes in conditions. The inverse micellar solution is probably in equilibrium, while the emulsion is only metastable, with a history dependent structure. Thus, it is not self-evident that the micellar structures within the oil of each system should be the same. However, they are indistinguishable, so the aqueous droplets evidently do not perturb the oil structure in the emulsions significantly. The droplet size produced in the emulsification process is evidently smaller than that in the water-only emulsions, and again smaller for lower molecular weight. The latter is again an effect of the increases in surfactant molarity. This overwhelms the opposite effect from molecular weight change. The surface is again a monolayer of molecules with reasonable molecular footprints. Sorbitan-Based Micellar Solutions and Emulsions. Both inverse micelle solutions and emulsions of SMO and emulsions of SIS contain spherical inverse micelles of smaller external radius than that for PIBSAs and with a relatively larger water/ headgroup core. The micelles contain about the same hexadecane content as the PIBSA systems. Both are predictable, given the smaller absolute tail sizes and the relatively larger headgroups. The SIS emulsions are not significantly different in structure to the SMO, in spite of the difference in branching and unsaturation of the tail chains. A degree of increased disorganization in both is indicated by higher polydispersities in micelle sizes. The emulsification process produces even smaller droplet sizes than PIBSAs, but these droplets are still stabilized by a monolayer of sorbitan surfactant with a footprint indicating near close packing. Structure Curvature and Emulsifiability. We can draw these conclusions together into two strands: curvature of structures and emulsifiability/water distribution. A variety of structures in order of increasing curvature have been observed: lamellar, sponge, long cylindrical micelles, short cylindrical micelles, and last larger and then smaller micelles. We have observed that increasing curvature is favored by both an increase in surfactant molecular weight and addition of ammonium nitrate. This is in accord with the predicted behavior of Wedge theory concerning tail-to-head volume ratios, if we assume the salt effect is to reduce effective head size by tighter binding. The structures observed in emulsions are close to those observed in related inverse micelle solutions, except for a slight tendency to higher curvature in emulsions. Surfactant dilution decreases curvatures, except where the inverse micelles are spherical and small when there is little effect of dilution. Substitution of sorbitan surfactants for PIBSAs produces similar structures, perhaps a little less organized. This is surprising given

Reynolds et al. the smaller tail/head ratio for these, which is, however, reflected in the size and internal structure of the micelles. Increase in surfactant molecular weight at constant molarity, use of ammonium nitrate, use of sorbitans, and increase in surfactant concentration all increase “emulsifiability”, i.e., result in smaller drop sizes. That this is not purely a kinetic effect is indicated by an accompanying increase in water content of the micelles in the oil phase; the water-oil interface is becoming more stable. Lastly, we can infer the presence in some cases of small amounts of large-scale surfactant aggregates, as are directly observed by USANS.13 However, for the PIBSA emulsions, about one-quarter of the added surfactant is in the form of fully dissolved molecules. For the sorbitan surfactants, even more surfactant is unaccounted for, and may be either fully dissolved or aggregated or both. Acknowledgment. We would like to thank Dr. P. Thiyagarajan and Mr. D. Wozniak at Argonne National Laboratory, USA, and Drs. R. Heenan and S. King at ISIS facility of the Rutherford-Appleton Laboratory, U.K., for experimental assistance and advice on modeling; Prof. R. Faust, Harvard University, and Dr. George Adamson, ANU, for their parts in the surfactant syntheses; Dr. Deane Tunaley and Dr. Richard Goodridge of Orica Ltd. and Dr. D. E. Yates of Yates Consulting Ltd. for useful discussions; and Prof. Simon Biggs, Leeds University, and Prof. Graeme Jameson, Newcastle University, for access to the Malvern Mastersizer. This work has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory, which is funded by the U.S. Department of Energy, BES-Materials Science, under Contract W-31-109ENG-38. Travel grants through the Australian Government Access to Major Research Facilities Program are gratefully acknowledged as is access to ISIS through the AINSE/ARC collaborative program. This work was jointly financed by the Australian Research Council, Orica Ltd., and ICI UK Ltd. under LINKAGE, SPIRT, and SRF awards. References and Notes (1) Cameron, N. R.; Sherrington, D. C. AdV. Polym. Sci. 1996, 126, 163. (2) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2000, 104, 7012. (3) Reynolds, P. A.; Gilbert, E. P.; White, J. W. J. Phys. Chem. B 2001, 105, 6925. (4) Balogh, L.; Faust, R. Polym. Bull. 1992, 28, 367. (5) Heenan, R.; King, S. In ISIS User Guide; Boland, B., Whapham, S., Eds.; ISIS, Rutherford Appleton Laboratory: Didcot, U.K., 1992. (6) Thiyagarajan, P.; Urban, V.; Littrell, K.; Ku, C.; Wozniak, D. G.; Belch, H.; Vitt, R.; Toeller, J.; Leach, D.; Haumann, J. R.; Ostrowski, G. E.; Donley, L. I.; Hammonds, J.; Carpenter, J. M.; Crawford, R. K. ICANS XIV - The Fourteenth Meeting of the International Collaboration on Advanced Neutron Sources, June 14-19, 1998, Starved Rock Lodge, Utica, IL, Vol. 2, pp 864-878. (7) Ravey, J. C.; Sauvage, S.; Stebe, M. J. J. Phys. IV 1993, 3, 141. (8) Reynolds, P. A.; Henderson, M. J.; White, J. W. Unpublished data. (9) Reynolds, P. A.; McGillivray, D. J.; Gilbert, E. P.; Holt, S. A.; Henderson, M. J.; White, J. W. Langmuir 2003, 19, 752. (10) Gazeau, D.; Bellocq, A. M.; Roux, D.; Zemb, T. Europhys. Lett. 1989, 9, 447. (11) Cates, M. E.; Roux, D.; Andelman, D.; Milner, S. T.; Safran, S. A. Europhys. Lett. 1988, 5, 733. (12) Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. (Paris) 1988, 49, 511. (13) Baranyai, K. J.; Reynolds, P. A.; Jackson, A. J.; Henderson, M. J.; Zank, J.; Barker, J. G.; Kim, M.-H.; White, J. W. Unpublished data.

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