Optical Microscopy of Lyotropic Mesophases in Dilute Solutions of

C. C. Ho,t R. J. Goetz, M. S. El-Aasser,* J. W. Vanderhoff, and F. M. Fowkes. Emulsion .... (7) Fukushima, S.; Takahashi, M. J. Colloid Interface Sci...
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Langmuir 1991, 7, 56-61

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Optical Microscopy of Lyotropic Mesophases in Dilute Solutions of Sodium Lauryl Sulfate-Lauryl Alcohol (or Cetyl Alcohol)-Water Systems C.C.Ho,t R.J. Goetz, M. S. El-Aasser,* J. W. Vanderhoff, and F. M. Fowkes Emulsion Polymers Institute, Departments of Chemical Engineering and Chemistry, Lehigh University, 111 Research Drive, Building A , Bethlehem, Pennsylvania 18015-4732, and Department of Chemistry, University of Malaya, Pantai Valley, 59100, Kuala Lumpur, Malasia Received February 12, 1990. In Final Form: June 7, 1990

The dilute gel phases formed from sodium lauryl sulfate (SLS)-lauryl alcohol (L0H)-water and sodium lauryl sulfate-cetyl alcohol (CAI-water systems were prepared at water concentrations greater than 97 wt %. Compositions were examined at a fixed SLS concentration of 20 mM with varying alcohol concentrations by using a polarizing microscope. Once the compositions were sonicated, patterns of the phase behavior were similar to the schlieren textures of the nematic liquid crystalline phase. The effect of flow-induced ordering was also noted for these solutions. The compositions outside the gel phase consisted of a two-phase mixture comprised of a clear micellar solution and sedimented alcohol. The crystalline shapes of the sediment consisted of needle-like and short thread-like particles which appeared to be the basic structures for the gel phase. When heated, the birefringent textures and rod-like particles disappeared at temperatures near the melting point of the alcohol, denoting the transition from an ordered to an isotropic phase.

Introduction Surfactant molecules aggregate in solution to form a variety of liquid crystalline structures. They aggregate in the form of spheres, cylinders, and bimolecular sheets, which at sufficient concentration can attain long-range order, forming a liquid crystalline phase. The four common phases are lamellar, hexagonal, reversed hexagonal, and cubic.’ Liquid crystalline phases are important in the stabilization of foams and emulsions2and as model systems for drug delivery vehicles and biomembrane dynamic^.^ Model systems typically consist of an aqueous surfactant and short-chain alcohol solution. The internal structure and physical properties of liquid crystalline phases are greatly influenced by the physical nature and aggregation properties of the surfactants. By altering the hydrophilic and/or lipophilic balance (HLB) of the surfactant or the nature of the cosurfactant, the size and location of the liquid crystalline phase in a ternary phase diagram can be adjusted. Liquid crystals initially form at asolids content of approximately 20 wt % ;thus, a majority of the research has focused on concentrated systems. However, few studies have been conducted on dilute systems. This study concerns the microscopic examination of the liquid crystalline phases consisting of a long-chain alcohol ( n > 12) and an ionic surfactant at concentrations less than 3 wt % surfactant and alcohol. Benton and Miller’v5have recently reported an optically anisotropic phase in the sodium octanoate-decanol-brine system at water contents exceeding 90 w t 76. The sequence of phases that were observed with increasing salinity was an isotropic aqueous solution, a single lamellar liquid crystalline phase, and an optically isotropic phase which

* To whom all correspondence should be directed. t

University of Malaya.

(1)Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (2)Friberg, S.Kolloid 2. 1971, 244, 233.

(3)Bader, H.; Dorn, H.; Hupfer, B.; Ringsdorf, H. Adu. Polym. Sci. 1985, 64,1.

(4)Benton, W. J.; Miller, C. A. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984;Vol. 1, p 205. (5)Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983,87, 4981.

scatters light and exhibits streaming birefringence. The low interfacial tension and viscosity render this system ideal for enhanced oil recovery. The alcohols used were of the short-chain type, the longest being decanol. Longer chain alcohols such as CA and octadecanol are often used in the preparation of pharmaceutical creams6 and as additives to enhance emulsion ~ t a b i l i t yMixtures .~~~ of an ionic surfactant and long-chain alcohol are also used in the formation and stabilization of miniemulsions for emulsion polymerization reactions and the preparation of “artificial” latexe~.~JO Miniemulsions are prepared with dilute solutions of an ionic surfactant and a long-chain alcohol, Le., LOH or CA, at a combined amphiphile concentration between 0.5 and 3 wt %.ll Because the long-chain alcohol is a solid at room temperature and relatively insoluble in water, the aqueous solutions are mixed above the melting point of the alcohol. Once the solution is cooled to room temperature, a gel phase will form depending on the molar ratio of alcohol to surfactant. Lack has shown that the stability of miniemulsions depends on the presence of the gel phase prior to emulsification.l2 The microstructure of the gel phase is similar to the lamellar liquid crystalline phase. The amphiphiles are arranged in bilayers separated by water.13 However, the hydrocarbon chains of the surfactant are hexagonally packed and in the solid state. This particular crystalline type is referred to as the a form and is common for longchain alcohols and other biological lipids. In addition to (6)de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1987. - ... - . 265. 167. I

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(7) Fukushima, S.; Takahashi, M. J. Colloid Interface Sci. 1976,57, 201;1977, 59,159. (8)Thadros, Th. F. Colloid Surf. 1980, 1, 3. (9)Vanderhoff, J. W.; El-Aasser, M. S.; Ugelstad, J. U.S. Patent 4 177 177,1979. (10)Ugelstad, J.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Lett. Ed. 1973, 1 1 , 503. (11)Lack, C.D.; El-Aasser, M. S.; Vanderhoff, J. W.; Fowkes, F. M. In Macro- and Microemulsions: Theory and Practiue; Shah, D. O., Ed.; A.C.S. Symposium Series 272;American Chemical Society: Washington, D.C., 1985;pp 345-356. (12)Lack, C. D. PhD. Dissertation, Lehigh University, 1985. (13)Vincent, J. M.; Skoulious, A. Acta Crystallogr. 1966, 20, 432.

0743-7463/91/2407-0056$02.50/0 0 1991 American Chemical Society

Langmuir, Vol. 7, No. 1, 1991 57

Optical Microscopy of Lyotropic Mesophases

the ct form, long-chain alcohols exist in rhombohedral crystals, referred to as the j3 form, which do not form a gel phase.14 The macrostructure of the gel is unique compared to the liquid crystalline phases previously mentioned. The macrostructure is comprised of a network of solidified bilayer aggregates. This network is the origin of the high consistency which is exploited in the formulations of pharmaceutical and cosmetic creams.6J6 At the dilute concentrations used in the formation of miniemulsions, the aqueous CA-SLS gel maintains the network structure. Investigations of the interfacial and rheological properties have demonstrated that the viscoelastic network of the dilute gel develops with aging and irreversibly breaks down with shear.11J6J7 Self-diffusion coefficients obtained from an NMR technique have shown that the phase volume of the gel is as much as 1order of magnitude greater than the solids content of CA and SLS.18 This illustrates the remarkable network structure of the gel phase which forms with low molecular weight components. Goetz et a1.'6 have constructed a ternary phase diagram in the dilute corner for the SLS-CA-water system, focusing on the compositions used in the formation of miniemulsions. The gel-phase region exists at compositions where the molar ratio of CA/SLS was greater than 1. At lower molar ratios, the solutions consisted of either the coagel phase or a clear micellar solution of SLS with sedimented alcohol. The coagel phase is amixture of the gel and alcohol crystals in the 0 form.13 Compositions used in the formation of miniemulsions were at the extreme dilute corner of the gel-phase region. It has been reported that an increase in miniemulsion stability was found if the dilute gel was sonicated prior to emulsifying the oil." Sonicating the dilute gel enhanced the birefringence, suggesting a rearrangement of the macromolecular gel structures forming a more ordered solution. NMR self-diffusion studies have suggested that sonication breaks up the network into smaller fragments.ls However, the NMR measurements were incapable of analyzing any change in the order and/or arrangement of the gel fragments. In order to understand the effects of sonication on miniemulsion stability, the macrostructure of the dilute gel must be examined. This report investigates the macrostructure of the sonicated surfactantalcohol-water solutions through polarized microscopy.The compositions were examined at a fixed SLS concentration of 20 mM (0.56-0.57 w t %) varying the LOH and CA concentration. Structural transitions as a function of temperature were also investigated.

Materials and Methods The SLS (Stepan Chemical Co.) used was purified by recrystallization from absolute ethanol followed by Soxhlet extraction with diethyl ether for 48 h. LOH and CA were used as received (Conoco Chemical Co.). Double distilled deionized water was used in preparation of aqueous solutions. The following procedure was formulated for preparing 50100-g samples of aqueous mixed surfactant solutions. The SLS concentration was kept constant at 20 mM (0.56-0.57 wt %) while the molar ratio of SLS to alcoholwas varied by varying the concentration of the alcohol. After the two components were added to the water, the sample bottle was heated in a double(14) Junginger,H.;Fuhre,C.; Ziegenmeyer,J.; Friberg,S.J . SOC. Coam. Chem. 1979,30,9. (15) Barry, B.W. Adu. Colloid Interface Sci. 1976, 5, 37. (16) Goetz, R. J.; El-Aasser, M. S. Langmuir 1989, 6, 132. (17) Lack, C. D.; El-Aasser, M. S.; Vanderhoff,J. W.; Fowkes, F. M.; Silebi, C. Langmuir 1987,3, 1155. (18) Goetz, R. J.; Khan, A.; El-Aasser, M. S. J. Colloid Interface Sci. 1990,137,395.

jacketed vessel maintained at 70 "C by a thermostated water bath. Thorough mixing was achieved by a short magnetic stirring bar. The progress of the solutions was checked by regular observation at selected time intervals in polarized light (i.e., in a simple light box arrangement where samples could be viewed between two crossed polarizing filters). Inhomogeneityappeared as dispersed particles in the aqueous medium. All solutions were stirred at 70 "C for 3-3.5 h, cooled to room temperature for 1h, and then subjected to pulsed ultrasonication for 2 min at power level 3 and 50% duty cycles by using a Heat Systems Ultrasonics Model W-350 sonifier cell disrupter and a 1.9-cm sonifier probe. All solutions were allowed to equilibrate at room temperature for at least 3 weeks before they were observed in the optical microscope. Reichert Zetopan and a Leitz Wetzlar microscopes were used for the optical microscopic observations, which were carried out at room temperature (21-23 "C). A Reichert HT1 heating stage was used for transition temperature determinations. The temperature scale was calibrated by using Reichert standards. Both circular culture cells (Fisher Scientific Co.) of 3-mm thickness and rectangular capillary cells of 0.4-mm thickness (Invitro Dynamics) were used for microscope studies. The culture cell was covered with a thin coverslip. A fine capillary tube was used to load the rectangular capillary cell. The cell was then sealed with Paraplast.

Results and Discussion General Phase Behavior of LOH-SLS-Water. A homogeneous solution could not be obtained at LOH/ SLS molar ratios of less than 1. Upon cooling to room temperature, the solution contained a large amount of dispersed, solidified alcohol, which was visible under polarized light but hardly discernible in ordinary light. After 2 weeks of aging, the solutions separated into an opalescent sediment of solid LOH and a clear supernatant, which consisted of SLS micelles. The LOH sediment likely consisted of some mixed crystals of LOH-SLS; however, the reduction in the bulk SLS concentration in the supernatant was not measurable. The alcohol sediment will herein be referred to as alcohol crystals. At a molar ratio of LOH/SLS greater than 1,the solutions exhibited similar appearance to the dilute gel phase. After sonication, the solutions became birefringent within ca. 15 min. The turbidity and viscosity increased with increasing LOH concentration. However, phase separation on standing was not observed. The LOH-SLS-water system exhibited similar phase behavior to the CA-SLS system.16 Although an extensive phase diagram was not constructed, the compositions examined extend from the alcohol crystals micelle to the gel-phase regions.

+

Optical Microscopy SLS-LOH-Watek. The optical microscopy studies were carried out in polarized light with the optic axes of the analyzer and the polarizer positioned at 90' with respect to each other. For these dilute solutions, the thickness of the sample cell, i.e., path length, must be greater than 0.2 mm. Birefringent textures were not observed when a sample was loaded into a 0.2-mm-thick rectangular capillary, because of the small amount of material between the walls of the cell. The opalescent sediment from the 20 mM (0.57 wt % ) SLS and 5 mM (0.08 w t % ) LOH aqueous solution was found to consist of a dispersion of rod-like particles (Figure 1). The 20 mM (0.57 wt 5%) SLS and 10 mM (0.16 wt %) LOH aqueous solution gave a stable dispersion of fine short thread-like particles, which were smaller in size compared to those given by the opalescent sediment with an LOH concentration of 5 mM (0.08wt %). The 20 mM (0.57 wt %) SLS and 20 mM (0.31 wt %) LOH solution

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Figure 1. Polarized optical micrograph of the opalescent sediment from an aqueous solution of 20 mM (0.57 wt %) SLS and 5 mM (0.08 w t 5%) LOH.

Figure 2. Polarized optical micrograph of (a)turbid supernatent and (b)translucent sediment from an aqueous solution of 20 mM (0.57 wt 90) SLS and 20 mM (0.16 w t 76)LOH.

separated into two different dispersions. The turbid supernatant consisted of a dispersiqn of ultrafine threadlike particles (Figure 2a). However, the translucent sediment was birefringent and consisted of larger and longer thread-like units (Figure 2b). At higher LOH/SLS molar ratios extending into compositions in the gel-phase region, the solutions exhibited birefringent woolly textures, which were dispersed throughout the microscope cell (Figure 3a). As the molar ratio of LOH increased, the woolly textures extended over the entire cell. Upon a closer examination, the woolly structures appeared to consist of the rod-like particles similar to those found in the micelle + alcohol crystal region found at low molar ratios of LOH/SLS. However, the microstructure of the rod-like particles is that of the gel phase. When the samples were first introduced into the rectangular capillary cells, elongated birefringent domains developed along the edges (Figure 3b). Within a few minutes, these became more compact and took on the appearance of a different birefringent pattern consisting

c b

Figure 3. Polarized optical micrographs: (a) birefringent woolly textures, (b) elongated birefringent domains, (c) herringbone texture, (d) domain pattern with alternating striations of light and dark from an aqueous solution of 20 mM (0.57 w t %) SLS and 30 mM (0.45 w t ?6) LOH.

of a band of nearly parallel striations built up of the needlelike particles stacked together side by side (Figure 3c). With continued aging, the band developed into a domain pattern with distinct alternating striations of light and dark (Figure 3d). Similar striated textures have been reported in the binary system consisting of an anionic surfactant and the tobacco mosaic virus at concentrations

Optical Microscopy of Lyotropic Mesophases

Figure 4. Polarized optical micrograph from an aqueous solution of 20 mM (0.57 wt % ) SLS and 60 mM (0.93 w t %) LOH formed after cooling the solution from 85 "C.

between 20 and 50 wt %.19 These domain patterns are typical of the birefringent textures found in nematic liquid crystalline phase.20 The change in the birefringent texture with time was likely due to the shear alignment of the rod-like particles, during the loading of the capillary cell. The solutions are first drawn into a fine capillary tube and then loaded into the cell. The solutions are further subjected to shear from the cell walls, during the loading process. Upon the cessation of flow, the particles lose their positional order with Brownian movement as the system becomes isotropic. Rheological studies on systems ranging from rod-like molecules to cylindrical micelles have shown that they readily align in shear f l o ~ . In ~ addition, ~ , ~ ~ phase transitions can be induced in rod-like micelles with ~ h e a r . ~ 3 Transition temperatures were investigated with the solutions placed in the culture cell, using the hot stage at a heating rate of approximately 0.3 "C/min. The 20 mM (0.57 wt %) SLS and 5 mM (0.08 wt %) LOH aqueous solution consisted of rod-like particles, which disappeared at a temperature of 31 "C. The needle-shaped particles also observed in these solutions vanished at 34 "C, at which the solution became dark. No further changes were observed when the temperature was increased to 85 "C. For the 20 mM (0.57 wt %) SLS and 20 mM (0.31 wt % ) LOH aqueous solution, the rod-like particles disappeared at 34 "C and, at the same time, the intensity of the woolly texture decreased rapidly. At 39 "C, all rod-like particles seemed to have disappeared. For the solution of 20 mM (0.57 wt 9;) SLS and 60 mM (0.934 wt 7670)LOH aqueous solution, the transition occurred at 43 "C. However, when the solution cooled to room temperature, a cloudy and woolly texture with low contrast reappeared which was slightly different from the original texture (Figure 4). Reheating the gel above the melting point of the alcohol and subsequent cooling to room temperature re-formed the gel network to the structure present prior to sonication. This resulted in different birefringent structures observed in the polarizing microscope. Bright colors beginning with blue, violet, brown, and yellow from the side wall of the culture cell were also observed with samples consisting of LOH/SLS molar ratio greater than 1/3. The different textures were due to the change in composition with the evaporation of water in the sample cells. These results suggest that liquid (19) Rogers, J.; Winsor, P. A. J. Colloid Interface Sci. 1969,30, 500. (20)Dreyer, J. F. In Liquid Crystals and Ordered Fluids; Johnson, J. F., Porter, R. S., Eds., Plenum Press: New York, 1970; p 311. (21) Lim, T.; Uhl, J. T.; Prudhomme, K. K. J. Rheol. 1984,28, 367. (22) Wissbrun, K. J. Rheol. 1981, 25, 619. (23) Rehage, H.; Hoffmann, H. Rheol. Acta 1982,21, 561.

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Figure 5. Polarized optical micrograph of the opalescent sediment from a 20 mM (0.57 wt % ) SLS and 5 mM (0.12 w t 96) CA aqueous solution.

Figure 6. Polarized optical micrograph of the opalescent sediment from a 20 mM (0.57 w t 70) SLS and 10 mM (0.24 wt %) CA aqueous solution.

crystalline phases exists at higher concentrations. However, concentrated solutions were not examined in this study. SLS-CA-Water. Similar to the LOH-SLS-water system, the compositions examined extended from the micellar + crystalline alcohol to the gel phase. Figure 5 shows that the opalescent sediment of the 20 mM (0.57 wt %) SLS and 5 mM (0.12 wt %) CA aqueous solution consisted of thread-like particles. When the alcohol concentration was increased to 10 mM (0.24 wt %), the opalescent sediment consisted of a mixture of rod- and thread-like particles (Figure 6). With aging, the thin rodlike particles aggreated into larger and more extended aggregates. The slightlyturbid supernatant of this solution consisted of finely dispersed thin thread-like particles. The turbid 20 mM (0.57 wt %) SLS and 20 mM (0.48 wt %) CA aqueous solution consisted of a stable dispersion of fine thread-like particles. However, the contrast was too weak to photograph. The crystalline particles that settled out in this solution were a mixture of relatively large spine-like particles which were highly birefringent and large flocs comprised of rod-like particles (Figure 7). At compositions in the gel phase, the turbid 20 mM (0.57 wt %) SLS and 40 mM (0.96 wt %) CA aqueous solution first showed birefringent batonnets shortly after it was loaded into the capillary cell (Figure 8a). These batonnets are similar to the nucleated domain texture of the nematic phase formed from p-a~ophenetole.~~ With time, there was a transformation of some of the batonnet particles into birefringent droplets exhibiting the familiar (24) Friedel, G. Ann. Phys. 1922, 18, 273.

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Figure 7. Polarized optical micrograph of the opalescent sediment from a 20 mM (0.57 w t %) SLS and 20 mM (0.48 wt 5%) CA aqueous solution.

I

Figure 8. Polarized optical micrograph from a 20 mM (0.57 wt %) SLS and 40 mM (0.96 wt 5%) CA aqueous solution: (a) birefringent batonnets, (b) uniaxial planar drop, (c) birefringent drops with extinction cross.

uniaxial interference figure25(Figure 8b). Additionally, Figure 8c shows birefringent droplets with the characteristics extinction cross parallel to the polarized planes (25) Rosevear, F. B. J . Am. Oil Chem. SOC.1954,31,628.

Figure 9. Polarized optical micrographs from a 20 mM (0.57 wt %) SLS and 40 mM (0.96 wt %) CA aqueous gel of the birefringent drops with extinction cross (a) after loading in the capillary cell and (b) 3 h later.

of the polarizers, which were also observed. Although this suggests the presence of a lamellar phase, it is more likely a manifestation of the schlieren textures deformed with shear. On prolonged standing under quiescent conditions, the bright birefringent texture relaxed and reverted back to a relatively dark isotropic solution. Similar to the aqueous LOH-SLS system, this deformation was due to the alignment of the charged rod-like aggregates with flow, while being loaded into the capillary cell. Figure 9 illustrates change in the birefringent texture with time for an aqueous solution of 20 mM (0.57 wt %) SLS and 40 mM (0.96 wt %) CA gel. Streaming birefringence or flow-inducedordering could also be observed by disturbing the sample cell. Adjoining birefringent streaks are formed in the wake of an air bubble which is forced to move through the solution. Bubbles moving in different directions with respect to the capillary cell would produce varying birefringent intensities, which was due to the positional ordering of the rod-like particles with flow. Thus, the nematic behavior likely originated from the positional alignment of the sonicated gel fragments, which was enhanced with flow. This effect was more intense for the aqueous solutions consisting of CA. An aqueous solution of 20 mM (0.56 wt %) SLS and 60 mM (1.43 wt % ) CA displayed birefringent woolly strands which were made up of rod-like particles. However, these particles were much smaller in size compared with those of the SLS-LOH-water system. While the birefringent woolly texture was relatively disperse throughout the solution, the textures became more extensive and compact at alcohol concentrations up to 80 mM (1.89 wt %). In addition, domains of bright elongated units stacked together to considerable length were found among the woolly structure (Figure 10). Examination of the nonsonicated gel phase through optical microscopy has shown that the gel consisted of rod-like gel aggregates at low CA/SLS molar ratios.18 As the molar ratio of CA/SLS increases, the gel structure

Optical Microscopy of Lyotropic Mesophases

100 ,,In

Figure 10. Polarized optical micrograph of the birefringent woolly structure from a 20 mM (0.56 wt %) SLS and 80 mM (1.89 wt % ) CA aqueous solution.

transforms into polyhedral aggregates, which appear to be comprised of aggregated rod-like particles. Similar aggregate structures and phase behavior were observed in the dilute gel consisting of a 60:40 mixture of CA-stearyl alcohol (SA) and a cationic surfactant (stearyldimethylbenzylammonium chloride).26 Although studies on the sonicated CA-SA gel have not been conducted, sonication appears to disintegrate the aggregates into rod-like particles. These appear to be the basic building blocks of the gel-phase aggregates. When the 20 mM (0.57 wt %) SLS and 5 (0.12 wt % ) mM CA aqueous solution was heated on the hot stage, the thread-like particles became stretched and drawn out and appeared woolly-like at a temperature of 32.5 "C. At 51.5 "C, these woolly structures disappeared completely, and the solution became dark, indicative of an isotropic (26) Benton, W. J.; Miller, C. A.; Wells, R. L. J . Am. Oil Chem. SOC. 1987, 64, 424.

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solution. No further change was noted when the temperature was raised to 85 "C. This is in agreement with Lack et a1.,12who found the disapperance of birefringence at 52 "C. Goetz et a1.16 were also able to observe these transitions calorimetrically. Increasing the alcohol concentration to 20 mM (0.48 wt %) revealed similar results. The thread-like particles began to melt and disappear at 32 "C, and at 60 "C all the thread-like shapes had vanished. For compositions in gel phase, the 20 mM (0.57 wt %) SLS and 40 mM (0.48 wt % ) CA, the larger battonets disappeared at 31.5 "C, whereas the other textures became drawn out, diffuse, and took on a "scaly" appearance. By 60 "C, all the structures disappeared and the solution became isotropic. No further change was noted at higher temperature up to 85 "C. For both the 1:3and 1:4solutions, the woolly texture disappeared at 63-64 "C, at which the solutions became isotropic. In all cases, upon cooling to room temperature, light yellow spherulites were noted to form. Conclusion Under the polarizing microscope, it was possible to distinguish between the various birefringent textures formed from the sonicated solutions of SLS-alcohol-water systems. These included the rod-like crystals which existed in the micelle + alcohol crystal region to the birefringent textures observed in the gel phase. The alcohol crystals found at low alcohol concentrations seemed to be the building units for the birefringent woolly textures observed in the gel phase. Upon heating, the crystals and birefringent textures disappeared at temperatures near the melting point of the long-chain alcohol, forming an isotropic solution. It is proposed that the nematic textures originated from the positional ordering of the rod-like aggregates of the sonicated gel phase. Ordering was enhanced by the shear stress in loading the solutions in the microscope cell or disturbing the cell by shaking.