Some Aqueous Solution and Surface Properties of Dialkyl

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J. Phys. Chem. 1994, 98, 11512-11518

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Some Aqueous Solution and Surface Properties of Dialkyl Sulfosuccinate Surfactants M. E. Karaman and B. W. Ninham Department of Applied Mathematics, RSPhysSE, The Australian National University, Canberra, Australia

R. M. Pashley* Department of Chemistry, The Faculties, The Australian National University, Canberra, Australia Received: January 26, 1994; In Final Form: July 19, 1994@

Ion exchange of sodium didodecyl sulfosuccinate to the lithium form produces a dramatic increase in aqueous solubility. Solutions of the lithium salt were studied using transmission electron microscopy which showed that stable, multiwalled vesicles formed spontaneously with an average size of 500-700 A. Some larger liposomal structures with a size of about 2000 A were also present. Individual bilayer spacings of about 35 A were observed via a negative staining technique. The effect of ion exchange on the head-group area was studied using the Langmuir-Blodgett technique. Exchange to the lithium form caused a 20% increase in head-group area. Another sulfosuccinate surfactant Aerosol-OT (AOT) for which both the sodium and lithium forms are soluble was used for a comparison of surface charge. Using microelectrophoresis, both the sodium and lithium forms of AOT were found to have similar surface charge densities. However, the slightly larger head-group area observed for the lithium form indicates an increase in counterion dissociation. These results suggest that it is the increased hydration of the lithium counterion that is responsible for increasing the headgroup area and hence change in aggregation state and solubility.

Introduction M) doubleEven at very low concentrations (e.g., chained surfactants often do not form micelles but aggregate into a variety of bilayer structures. This molecular organization depends on both intra- and intermolecular forces as well as on the fluidity of the surfactant tails. The balance of the attractive forces between hydrocarbon tails, electrostatic forces between the charged head groups, andor head-group hydration effects together influence architecture and stability of the aggregates formed. In a zeroth-order theory' that characterizes dilute surfactant association, details of intermolecular forces are ignored. If the tails are treated as an oil-like interior, which aggregate forms depends upon a packing parameter vlaol, characteristic for a given surfactant and determined by geometric arguments alone. In the packing parameter v is the volume of the hydrocarbon tail, 1, is the extended hydrocarbon chain length, and a0 is an optimal head-group area. The packing parameter provides a useful predictive guide to the molecular architecture of the aggregates formed for a particular surfactant. Thus if vlaol, < l/3, spherical micelles are expected, in the range l/3 > v/aol, < l/2 polydispersed cylindrical micelles form, and in the range l/2-1 vesicles, oblate micelles, or lamaller phases form. For values > 1 inverted phases are observed. At first sight tautological, this single characterization successfully subsumes a range of aggregate structure^.^,^ The theory was put on a firm statistical mechanical foundation by Mitchell and Ninham? Later work of H ~ d ethat ~ , extended ~ these ideas by imposing global as well as local packing constraints showed that the entire diverse range of cubic and other bicontinuous phases observed in sodium dodecyl sulfate, and also phospholipids, is embraced in a predictable way by the same parameter. The phase boundaries and microstructure7 for a three-component ionic microemulsion can be predicted if the effects of cosurfactancy and oil penetration into the tail regions are taken into account. @

Abstract published in Advance ACS Abstracts, September 1, 1994.

0022-3654/94/2098- 11512$04.50/0

These notions have been carried further, and in principle the theory has been put onto a quantitative basis8 that links the foundations directly to molecular forces assuming only the validity of the Poisson-Boltzmann equation. In this theory the ion binding model emerges as an asymptotic approximation. Critical micelle concentrations as a function of chain length, temperature, and salt, tail-tail interactions, and ion binding parameters all emerge self-consistently in agreement with simulation studies. In general, the role of molecular forces is extremely subtle. Although the volume (or effective volume with oil or cosurfactant) and chain length can be reasonably estimated, the headgroup area is determined by a complex interplay of forces and is best measured experimentally. The cloud point behavior with nonionic surfactants9 is an obvious example of the delicacy of the self-association phenomena. So too, and not less subtle, are specific ion effects with ionic surfactants. Changes in counterions can effect quite dramatic changes in aggregation properties. For example, in a series of studies on cationic surfactants, it has been shown that while the cationic double-chained surfactant didodecyldimethylammonium bromide is virtually insoluble, forming lamellar phases, counterion exchange to the hydroxideloor the acetate" produces spontaneous single-walled vesicles, in the first case, and few layered aggregates containing micelles or cubic phases in the second. For the corresponding single-chained surfactants, cmc's and (phenomenological)ion binding parameters change remarkably. While the observed phase changes can be accommodated to some extent by postulating different hydrated counterion size, so increasing electrostatic head-group repulsion, area, and hence interfacial curvature, and through awareness of correspondingly enhanced interaggregate force^,'^^'^ the story is by no means free from controversy. Quaternary ammonium ions at high pH (Le., for the OH- form) may undergo some degree of Hoffman degradation to amines and alcohols that may provide cosurfactants which would influence packing geometry of the aggregate. For the bromide/acetate exchange, it is not at all clear how much 0 1994 American Chemical Society

Properties of Dialkyl Sulfosuccinate Surfactants of the change in aggregate structure can be ascribed to hydration or electrostatic effects. This report is concerned with changes in aggregation induced by counterion exchange in double-chained anionic surfactants that may throw further light on origins of the phenomenon. The didodecyl sulfosuccinate surfactants that mirror the behavior of cationics were chosen because they are stable and show no significant hydrolysis, oxidation, or biochemical degradation over many months. They also have some structural similarities to charged natural phospholipids. Counterion effects observed are free from the possible ambiguities associated with the cationics.

Materials and Methods The water used in this study was produced from tap water which was fed into a Memtec Krystal Kleen unit using a threestage purification process. The first stage involves a prefilter treatment to remove suspended solids. Next the water was passed through a reverse osmosis membrane, where dissolved electrolytes were removed. Finally, it passed through an activated charcoal stage, by which dissolved gases such as chlorine were removed. The treated permeate was then distilled in Pyrex glassware and collected in a positive pressure dustfree laminar flow cabinet. Glassware used for critical experiments was thoroughly cleaned by rinsing in 10% NaOH, washed in warm Teepol solution, and followed by rinsing in tap water and AR grade BDH ethanol to remove any surface-active agent adsorbed to the glass surface. The diethylhexyl sulfosuccinate Na+ salt (AOT) was 99% pure as purchased from Sigma Chemicals. Surface tension measurements indicated that no further purification was necessary. The sodium 1,2-bis(dodecyl(oxycarbonyl))ethane-l-sulfonate (i.e., sodium didodecyl sulfosuccinate) purchased from Sogo Pharmaceuticals was also used without further purification. The didodecyl sulfosuccinate Li+ salt was prepared from the Na+ form obtained from Sogo Pharmaceuticals (Japan). This Na+/Li+ exchange was achieved by dialyzing the sodium form against a concentrated solution of LiCl and then against copious amounts of dilute LiCl and distilled water to eliminate any excess salt present. It was then freeze-dried and subjected to microanalysis. It was found to contain a trace of NaCl, but the level found was negligible considering the experiments were carried out in swamping electrolyte. The freeze-fracture/freeze-etch techniques involved fast snap freezing a droplet of solution on gold discs in liquid freon. The frozen specimen was then transferred as rapidly as possible via liquid nitrogen to a Blazer freeze-etch apparatus under vacuum and held at a temperature of -170 "C. The specimen holder was mounted on a cold stage where the frozen specimen was cut using a microtone at -120 "C. The microtoming step reveals a cleaved crisp surface, free of smearing and specimen damage. After fracturing, sublimation of ice from the exposed surface (etching) was carried out. At 2 nm thick platinum/ carbon coat was evaporated at a 45" angle to the freshly cut surface followed by a 25 nm thick carbon deposit evaporated directly above the specimen so providing a replica of the structures present in the original specimen. The replicas were removed from the evacuated chamber, allowed to thaw, and then lowered gently at a 45" angle into the surface of distilled water. The structural replica separates from the gold disc and floats on the water surface. The replica was then picked up onto a formvar (poly(viny1formal))-coated copper grid, air-dried, and then viewed under a Zeiss 109 transmission electron

J. Phys. Chem., Vol. 98, No. 44, 1994 11513

microscope. This technique is presumably free from artifacts induced by air-wing, chemical staining, ionic strength, and PH. The negative staining technique was carried out using M didodecyl sulfosuccinate lithium salt in distilled water. It was used as an independent technique to confirm aggregate structure. In negative staining spherical aggregates appear as white structures on a dark background. The results supported the aggregate size and structures seen by the freeze-fracture/ freeze-etch techniques. In addition, with negative staining the individual liposomal bilayers could be visualized and their thickness estimated. The negative staining technique involved the placement of a droplet of the surfactant solution onto a piece of paraffin wax on which a formvar-coated grid was applied for approximately 30 s. It was then blotted off and the process repeated using uranyl acetate solution (a negative stain). The specimen grid is allowed to &-dry and then viewed under the Zeiss 109 TEM. The U0z2+ acts as a counterion in the system and makes this ordered structure visible in the electron microscope. Artifacts induced by the use of such divalent counterions are wellk n o ~ n ' ~ 3but ' ~ are not important here for reasons that will become clear later. Solution conductivities were measured at 25 "C directly using a TPS conductivity meter. Mineral fluorspar (CaF2) used for electrophoresis studies was obtained in pure rock form from the Geology Dept, A.N.U., and crushed to a fine white powder. The resulting fluorite particles were examined using a Cambridge S360 scanning electron microscope and found to be angular in appearance and approximately 0.3-2 pm in diameter. The microelectrophoresis measurements were carried out using positively charged fluorspar particles on which a surfactant bilayer could be adsorbed, using a Rank Bros particle microelectrophoresis apparatus MK11. DSC thermograms were taken on aqueous solutions (10% w/w) of the sodium and lithium salts using a Perkin Elmer DSC 4 differential scanning calorimeter in an inert atmosphere of argon. Thermograms obtained were recorded over the range 7-90 "C at a rate of 1 "C/min. The Langmuir-Blodgett experiments were carried out using a rectangular Teflon trough enclosed in a filtered nitrogen environment. The film pressures were obtained using the rodin-free-surface (RIFS) method.I6 The surface tension measurements on AOT solutions were also obtained by this method. The surface tension of distilled water used was measured using the RIFS technique and was found to be 71.9 mJ m-2 at 25 "C, indicating an acceptable water quality.

Results and Analysis Preliminary studies showed that the sodium didodecyl sulfosuccinate salt was completely insoluble in water. In fact, it appeared to resist wetting. In an attempt to increase solubility, the effect of exchange with highly hydrated counterions (e.g., H+, Li+) was studied. A small amount (12 mg) of the sodium form was placed into 10 cm3 volumes of a number of acids and aqueous salt solutions at room temperature (-20 "C). If soluble, this would give a surfactant concentration in each case of 2.1 x M. Sulfuric, acetic, and hydrochloric acid (10 M) each produced a cloudy dispersion with a little residual sediment, whereas nitric acid produced an optically clear solution. On warming to 30 "C the acetic acid solution also cleared. The sodiumihydrogen exchange was not investigated further because acids are known to catalyze hydrolysis of the

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Karaman et al.

Figure 1. Freeze-fracture/freeze-etchtransmission electron micrograph of a typical liposomal structure present in

M aqueous Li+ didodecyl

sulfosuccinate solution.

ester groups present in sulfosuccinate. However, these results clearly show that proton exchange does produce increased solubility. In 1 M NaCl the same result was obtained as for water with no solubility at all, whereas in 1 M LiCl a dispersion was easily formed giving a blue/opalescent solution, indicating the presence of large aggregated structures. The solubility differences seen in this system are similar to those observed for quaternary ammonium surfactants on exchange of Br-/OH- and Br-/ acetate. Because of these initial results, we explored the didodecyl sulfosuccinate sodium/lithium exchange further. The DSC thermograms on the Na+/Li+ forms at 10% (w/w) in aqueous solution indicated that the Krafft temperature was reduced from about 60 "C to 15 "C. This value of 60 "C for the sodium form is consistent with earlier results,17 and a similar reduction for Na+/Li+ has been observed for single-chained surfactants.l8 The reduction in Krafft temperature is attributed to the increased hydration of the counterion. Electron Microscopy of Aggregate Structures. Transmission electron microscopy was used to study aggregates formed in solutions of the didodecyl sulfosuccinate lithium salt in both distilled water and LiCl solutions at room temperature. The solutions were prepared without the aid of sonication or any other mechanical means to induce aggregation and were stored over 6 months for long-term stability studies. Two independent techniques, freeze-fracture/freeze-etch and negative staining, were used to compare results and to minimize artifacts induced by either technique. The freeze-fracture/freeze-etchtechnique was developed by Shaefer and Harker19 in which the specimen details were transferred to a thin electron translucent replica. The replica, due to variations in thickness of the evaporated material, creates electron beam variations which through electron scattering produces a representation of the surface detail of the original specimen. Metal shadowing provides a relief of the

topography of the replica. Surfaces facing normal to the source of the evaporation accumulate more material than others subtending a lower angle to it. The shadowed replica was produced by simultaneous evaporation of platinum and carbon followed by a further carbon evaporation which binds the replica into a unified thin film. The structures seen in the freeze-fracture/freeze-etch transmission micrographs are presumably replicas of the structures present in the original solutions. They show that the didodecyl sulfosuccinate lithium salt aggregates spontaneously into polydispersed versicles and lipo!omal structures. The average aggregate size was 500-700 A with larger liposomal structures (multiclosed layer vesicles) of approximately 2000 A appearing as large well-defined depressions or dark spherical elevations (see Figure 1). White irregular conelike structures seen in Figure 2 are due to the Pt/C shadowing of structures protruding from the fracture plane. This shadowing effect indicates the direction of deposition which has been denoted by an arrow in the figures. In negative staining the selective uptake of a heavy metal enhances electron scattering producing an image of the sample. There are a number of excellent papers written on the question of artifact-laden fixation techniques in electron microscopy. Goodman and Clunie15 stated that "although negative staining appears capable of providing images of hydrated mesomorphic structures, it is still possible for phase changes associated with variations in ionic strength, pH and composition to occur during drying". Kilpatrick et al.14 looked at double-chained alkylaryl sulfonate surfactants using optical polarizing microscopy, 13C NMR, and electron microscopy to address the problem of staining and drying-induced artifacts. They demonstrated that both the crystalline and liquid crystalline structures, upon drying, were quantitatively different from the original hydrated state in both the presence and absence of the 14~15920-22

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Properties of Dialkyl Sulfosuccinate Surfactants

Figure 2. Freeze-fracture/freeze-etchtransmission electron micrograph (with Pt shadowing) of liposomal structures present in aqueous 1Ow3 M Li+ Y

I

didodecyl sulfosuccinate solution.

negative stain. Both macro- and microstructural differences were seen for the hydrated versus dried specimens, stained versus unstained, for different stains and pH's and for different pH adjustment reagents. It appears that in the hydrated state the presence of stain enhances the electron micrograph contrast without changing its microstructure. Uranyl acetate, a commonly used negative stain, does not appear to alter the phase behavior dramatically prior to drying, but 23Na NMR studies have shown that significant amounts of uranyl ion can replace the sodium counterion in sodium 4-( 1'-heptylnony1)benzenesulfonate (SHBS). In this negative staining study we have good evidence to believe the micrographs obtained were not due to induced artifacts because the strain was not allowed to react with the vesicular solution in situ but only left a short time in contact with preadsorbed vesicle structures, and the results obtained were also supported by the artifact-free freeze-fracture/ freeze-etch technique. Micrographs from the negative staining technique are shown in Figures 3 and 4. Spherical multiwalled vesicles or liposomes appear as concentric dark fringes. The aggregates appear to be polydispersed with the average aggregate size between 500 and 700 A together with some larger aggregates around 1500-3000 A diameter. The individual bilayer fringes can be seen and found to correspond to a spacing of about 35 A. The surface adsorption and solution properties of the Li+ form were studied further by measuring surface tensions and conductivities over a wide concentration range of aqueous solutions. The surface tensions, measured by the rod-in-free-surface technique, are given in Figure 5 and suggest that a critical aggregation concentration exists just below 10-4 M. Conductivity measurements, also given in Figure 5, support this value. At lower concentrations it appears that only monomers were present in solution. From these results it seems clear that the insoluble didodecyl sulfosuccinate sodium salt when ion exchanged with a highly

hydrated cation (namely Li+) changes spontaneously from an insoluble, lamellar phase to a highly soluble vesicle-liposomal aggregated state. Microelectrophoresis Study of Sulfosuccinate-Coated Colloids. A microelectrophoresis study was carried out in order to determine the charge on the sulfosuccinate surface, using a positively charged colloid as a substrate for surfactant adsorption. The mineral fluorspar (CaF2) was chosen as a substrate and was obtained in pure rock form and crushed to a fine white powder. The adsorption of a surfactant bilayer effectively produces a large particle with surface properties comparable to a sulfosuccinate vesicle. A SEM study of the fluorspar particles showed that they were angular in appearance and were mostly between 0.3 and 2 pm in diameter. With particles of this size, the Smoluchowski equation can be applied accurately at relatively high electrolyte concentrations ( 2 M). AOT solutions were prepared 24 h before investigation to ensure equilibration. A 50 mg sample of fluorspar was weighed directly, added to the equibrated solutions. The solutions were transferred immediately to a microelectrophoresis cell preequilibrated at 25 "C in a water bath and used within 20 min. The first AOT investigation involved measurement of the zeta potential as a function of AOT concentration in constant background electrolyte (0.1 M NaCl) to determine the concentration at which a monolayer and bilayer adsorbed. The fluorspar surface charge was approximately zero (+lo mV) in 0.1 M electrolyte and became negative with addition of AOT. At an AOT concentration above the cmc (5 x M), the fluorspar appeared to be fully covered with an adsorbed surfactant bilayer. Microelectrophoresisexperiments were carried out on fluorM AOT with 0.1 M of the added spar particles dispersed in electrolytes NaCl and LiCl. The zeta potentials inferred from the Smoluchowski equation were -57.7 and -58.8 mV for the Na+ and Li+ cations, respectively. These results indicate that

Karaman et al.

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Figure 3. Negative-stained (uranyl acetate) electron micrograph of liposomal structures present in aqueous solution. At this higher magnification the multibilayer structure is clearly visible.

M Li+ didodecyl sulfosuccinate

Figure 4. Negative-stained (uranyl acetate) electron micrograph of liposomal structures present in aqueous solution.

M Li+ didodecyl sulfosuccinate

there is little difference between the charge density at the Li+ and Na+ sulfosuccinate surface for AOT. However, analysis of surface tension measurements on AOT solutions in 0.1 M

LiCl and NaCl solutions (see Figure 6) using the Gibbs adsorption isotherm indicate a roughly 10% larger head-group area for the Li+ form (77 against 69 A* for the Na+ form). These

Properties of Dialkyl Sulfosuccinate Surfactants

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.Oi+t+-ti

-6

t -5 -4 -3 -2 -1 LOG [LITHIUM DlDODECYLSULFOSUCCINATE/M 1

Figure 6. A comparison of surface tension values as a function of AOT concentration in 0.1 M LiCl and 0.1 M NaCl aqueous solutions.

Figure 5. (a) The cmc value determined by surface tension as a function of lithium didodecyl sulfosuccinate concentration. (b) The cmc value determined by conductivity as a function of lithium didodecyl sulfosuccinate concentration.

results correspond to about a 15% larger degree of dissociation for the Li+ form. Langmuir-Blodgett Study of Sulfosuccinate Surfactants. The water-insoluble sodium didodecyl sulfosuccinate surfactant was dissolved in chloroform and applied dropwise to the surface of a Teflon Langmuir-Blodgett trough with a M NaCl subphase. After approximately 20 min, sufficient to allow evaporation of the solvent, compression curves were measured up to film pressures below which film rupture might be expected. The measurements were repeated over a 2 h period, variation and the results are shown in Figure - 7. No significant in the film pressure-area (FI-A) curves were observed.

0

-

1

Karaman et al.

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two forms. It is interesting to note that this relatively small change in surfactant packing can have a dramatic affect on solubility.

Conclusions

ARENMOLECULE (A)2

Figure 8. ITA isotherms for Na+ didodecyl sulfosuccinate spread onto an aqueous M NaCl or LiCl subphase.

with time to give a compressible head-group area about 20% larger than formerly. It is surprising that after ion exchange, which should be rapid under these conditions, the soluble Li+ form does not appear to desorb from the air-water interface. In order to further test this observation, we repeated the experiments with dihexadecyldimethylanium bromide surfactant spread on a subphase of M potassium acetate. Replacement of bromide for acetate similarly converts this surfactant from a lamellar form to the soluble vesicle form. Again no desorption of the bromide when spread on the acetate solution was observed over a 2 h period. Similar observations have been reported by Gaines for stearic acid.23 These results suggest that the effects induced by the ionexchange process to a soluble form may not be sufficient to desorb the surfactant from the air-water interface, at least for the vesicle forming double-chained surfactants. This may, perhaps, be caused by the high-energy barrier that exists between a spread monolayer and a spherical bilayer in solution. To further test this possibility, we spread AOT on a M NaCl subphase and found that the AOT rapidly desorbed. This surfactant is known to form micellar structures that create less of a barrier to formation from the spread monolayer. It seems that it is indeed the combination of low monomer solubility and complexity of aggregates that inhibits desorption of surfactants from the air-water interface. II-A curves measured up to film collapse pressures for the sulfosuccinate surfactant are given in Figure 8. From these results we estimate fully compressed head-group packing areas of 54 and 45 A* for the Lit and Naf forms, respectively, consistent with the relative change obtained for AOT. These areas correspond to packing parameters for the didodecyl surfactant of 0.77 for the Li' form and 0.93 for the Na+, which is consistent with the difference in aggregate structure of the

In this study it was found that when sodium didodecyl sulfosuccinate was ion-exchanged to the lithium form there was a dramatic change in its solution behavior. The insoluble, presumably lamellar, sodium salt becomes spontaneously soluble, forming subcolloid aggregates. These were studied using transmission electron microscopy which showed stable, polydispersed vesicles averaging 500-700 A in diameter and some larger liposomal structures -2000 A. Individual liposomal bilayer spacings of about 35 A were visualized using a negative staining technique. The structures observed using freeze etch were found to be similar to those obtained by negative staining. Differential scanning calorimetry carried out on the sodium/ lithium forms indicate a large decrease in Krafft temperature upon ion exchange for Li', consistent with the observed spontaneous aggregation of the Li+ form at room temperature. A study of Langmuir-Blodgett compression curves indicates that there was a 20% increase in head-group area on ion exchange to the lithium form, which is sufficient to account for the observed change in aggregation state. A microelectrophoresis and surface tension study of adsorbed AOT layers show that the lithium form had only a slightly higher degree of dissociation. These results suggest that the greater hydration of the lithium ion was responsible for an increase in head-group area and hence reduction in the critical packing parameter. This change in surfactant packing geometry from a planar lamellar to a highly curved liposomal state led to the increase in the solubility of the didodecyl sulfosuccinate surfactant.

References and Notes (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem., SOC. Faraday Trans 2 1976, 72, 1525. (2) Tanford, C. J . Phys. Chem. 1972, 76, 3020. (3) Tartar, H. V. J . Phys. Chem. 1955, 59, 1195. (4) Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1981, 77, 601. (5) Hyde, S. T. J . Phys. Colloq. 1990, C7 (Suppl. 23), 209. (6) Hyde, S . T. Pure Appl. Chem. 1992, 64, 1617. (7) Hyde, S. T.; Ninham, B. W.; Zemb, T. N. J . Phys. Chem. 1989, 93, 1464. (8) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J . Phys. Chem. 1984, 88, 6344. (9) Ninham, B. W. Structure and Reactivity in Reverse Micelles; Pilenic, M., Ed.; Elsevier: Amsterdam, 1989; p 3. (10) Ninham, B. W.; Talmon, Y.; Evans, D. F. Science 1983,221, 1047. ( I 1) Radlinska, E.; Ninham, B. W.; Dalbiez, J. P.; Zemb, T. N. Colloids Sur& 1990, 46, 213. (12) Ninham, B. W.; Evans, D. F. Faraday Discuss. Chem. SOC.1986, 81, (1). (13) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, I.; Evans, D. F. J . Phys. Chem. 1986, 90, 1637. (14) Kilpatrick, P. K.; Miller, W. G.; Talmond, Y. J . Colloid Interface Sci. 1985, 107, 146. (15) Goodman, J. F.; Clunie, J. S. Liquid Crystals and Plastic Crystals; Ellis Hanvood Ltd.: London, 1974; Vol. 2. (16) Padday, J. F.; Pitt, A. R.; Pashley, R. M. J . Chem. SOC., Faraday Trans. 1 1975, 71, 1919. (17) Okahata, Y.; Lizuka, N.; Nakamura, G.; Seki, T. J . Chem. Soc., Perkin Trans. 2 1985, 1591. (18) Shinoda, K.; Hato, M.; Hayashi, T. J . Phys. Chem. 1972, 76, 909. (19) Shaefer, V. J.; Harker, D. J . Appl. Phys. 1942, 13, 427. (20) Talmond, Y. J . Colloid Interface Sci. 1983, 92 (2), 366. (21) Franses, E. I.; Puig, J. E.; Talmond, Y.; Miller, W. G.; Scriven, L. E.; Davies, H. T. J . Phys. Chem. 1980, 84, 1547. (22) Kilpatrick, P. K.; Miller, W. G. J . Phys. Chem. 1984, 88, 1649. (23) Gaines, Jr., G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966.