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The cream floating precipitates contain densely packed multilamellar vesicles, which were determined by freeze-fracture transmission electron microsco...
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Phase Transition of Precipitation Cream with Densely Packed Multilamellar Vesicles by the Replacement of Solvent Yuwen Shen,† Heinz Hoffmann,‡ and Jingcheng Hao*,† †

Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, Shandong University, Jinan 250100, PR China, and ‡Universit€ at Bayreuth, Bayreuther Zentrum f u€r Kolloide und Grenzfl€ achen, D-95446 Bayreuth, Germany Received April 13, 2009. Revised Manuscript Received May 8, 2009 The swelling of lamellar phase can be induced by the replacement of solvent in a tetradecyltrimethylammonium bromide (TTABr) and sodium laurate (SL) aqueous mixed solution that contains cream floating precipitates on the upper phase and L1-phase (micelles) at the lower phase. The cream floating precipitates contain densely packed multilamellar vesicles, which were determined by freeze-fracture transmission electron microscopy (FF-TEM) images. Phase transition, from cream floating precipitates to swelling birefringent vesicle phase, to two-phase LR/L1, and finally to micelle phase, can be induced by adding glycerin as solvent in the aqueous solution. At first, densely packed multilamellar vesicles of cream floating precipitates on the upper phase swelled throughout the whole phase with increasing content of glycerin. The replacement of solvent lowers the turbidity of the dispersion and swells the interlamellar distance between the bilayers, which is explained by matching of refractive index of the solvent to the refractive index of the bilayers of the surfactant mixtures. With an increasing amount of glycerin, the swelling LR phase turned to two-phase LR/L1, and finally to L1 phase (micelles). This phase transition can also be explained because of the increasing critical micelle concentration of the cationic and anionic (catanionic) surfactant mixture (TTABr and SL) at high glycerin concentration. The phase transition induced by addition of sorbitol can also be studied and compared to the case of adding glycerin. These results may direct toward acquiring an understanding of the phase transition mechanism of catanionic surfactants induced by solvents.

Introduction Cationic and anionic (catanionic) surfactant vesicles in aqueous solution are the fascinating subject of extensive investigations,1 the results of which could provide a basic understanding of the fascinating phenomena in surfactant sciences, especially for mixed surfactants in solution. There has been much interest in studying the microstructures and properties of spontaneous vesicles at thermodynamic equilibrium since it was demonstrated for the first time by Kaler et al.2 Khan and Marques et al.3 presented significant contributions to the work of catanionic surfactant systems. Many investigations were focused on the catanionic surfactant vesicle phases because of the apparent and latent applications in exceptionally selective membranes, drug-delivery, microreactors for production of colloidal particles, and cosmetic applications. When a cationic surfactant solution and an anionic one are simply mixed, the strong reduction in area per headgroup resulting from ion pairing induces the formation of molecular bilayers at low concentrations, and, at the appropriate mixing ratios, vesicles may be established spontaneously and are thermodynamically stable species.4,5 The cationic and anionic surfactant systems can produce precipitates when the stoichiometry between the cationic and anionic surfactants is exactly 1.6 *Corresponding author. Phone/Fax: +86-531-88366074. E-mail: jhao@ sdu.edu.cn. (1) Hao, J.; Hoffmann, H. Curr. Opin. Colloid Interface Sci. 2004, 9, 279–293. (2) Kaler, E. W.; Murthy, K. A.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371–1374. (3) Khan, A. ; Marques, E. In Specialist Surfactants; Robb, I. D., Ed.; Kluwer: Dordrecht, The Netherlands, 1997; p 37. (4) Marques, E.; Khan, A.; da Graca Miguel, M.; Lindman., B. J. Phys. Chem. 1993, 97, 4729–4736. (5) Bergstrom, M.; Pedersen, J. S.; Schurtenberger, P.; Egelhaf, S. U. J. Phys. Chem. B 1999, 103, 9888–9897. (6) Horbaschek, K.; Hoffmann, H.; Hao, J. J. Phys. Chem. B 2000, 104, 2781–2784.

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Phase transition of surfactants in solution can be induced by additives, such as changing equilibrium temperature, the addition of cosurfactants to change the hydrophilic-lipophilic balance (HLB) value, changing the ionic strength via the addition of salts, and so forth. In order to follow our recent results on the reversible LR phase transition triggered by external conditions, such as the addition of salts7 and melting temperature,8 herein we investigated the phase transition induced by the replacement of solvent. Cationic tetradecyltrimethylammonium bromide (TTABr) aqueous solution (200 mmol 3 L-1) and an anionic sodium laurate (SL) one (200 mmol 3 L-1) were mixed to form the 100 mmol 3 L-1 catanionic surfactant tetradecyltrimethylammonium laurate (TTAL) and 100 mmol 3 L-1 NaBr. White cream floating precipitates on the upper phase with densely packed multilamellar vesicles (MLVs) form, and the lower phase is micelles. We added glycerin as solvent to the aqueous solution for observing the phase transition. It is well-known that the application of surfactants in glycerin/ water mixed solvent is extensive in everyday life.9 There are several reports about the effect of glycerin/water mixed solvent on the micellization and phase behavior of amphiphiles.10-17 (7) Shen, Y.; Hao, J.; Hoffmann, H. Soft Matter 2007, 3, 1407–1412. (8) Shen, Y.; Hao, J.; Hoffmann, H.; Wu, Z. Soft Matter 2008, 4, 805–810. (9) Martin, M., Swaebrick, J., Ananthapadamanabhan, K. P., Eds. Physical Pharmacy; Lea & Febiger Press: Philadelphia, PA, 1983. (10) Corrin, M. L.; Hrakins, W. D. J. Chem. Phys. 1946, 14, 640. (11) (a) Di Paola, G.; Belleau, B. Can. J. Chem. 1975, 53, 3452–3461. (b) Aramaki, K.; Olsson, U.; Yamaguchi, Y.; Kunieda, H. Langmuir 1999, 15, 6226–6232. (12) Martino, A.; Kaler, E. W. Colloids Surf. A 1995, 99, 91–99. (13) Lin, Y.; Alexandridis, P. J. Phys. Chem. B 2002, 106, 12124–12132. (14) Takisawa, N.; Thomason, M.; Bloor, D. M.; Wyn-Jones, E. J. Colloid Interface Sci. 1993, 157, 77–81. (15) Palepu, R.; Gharibi, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1993, 9, 110–112. (16) Bakshi, M. S. J. Chem. Soc. Faraday Trans. 1993, 89, 4323–4326. (17) Cantu, L.; Corti, M.; Degiorgio, V.; Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1987, 116, 384–389.

Published on Web 06/01/2009

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It has been found that the critical micelle concentration (cmc) values of surfactants are higher in the mixed solvent than those in pure water and even exponentially increases with increasing glycerin concentration.18-20 Recently, Hoffmann et al.21 reported the swelling of LR phase in siloxane surfactant in water/glycerol mixture and explained their observation by the matching of a refractive index, which is a brand new theory for the swelling of LR phase. In the present study, we found that similar phase behavior occurred in our system when a certain amount of glycerin was added to the aqueous solution, which also can be explained by the matching of the refractive index. Because of the higher cmc in high concentrations of glycerin, the swelled LR phase will be transformed into two-phase LR/L1 and an L1 phase. The phase transition was demonstrated by conductivity and rheological measurements and freeze-fracture transmission electron microscopy (FF-TEM).

Figure 1. Sample photographs of 100 mmol 3 L-1 TTABr and 100 mmol 3 L-1 SL in water/glycerin mixed solvents. Upper row is the samples without polarizers and lower row is the samples with polarizers.

Experimental Section Chemicals and Materials. TTABr was purchased from Sigma, and the purity was 99%. SL was synthesized by NaOH (Hedinger, Germany, g99%) and lauric acid (Henkel, Germany, g98%) in ethanol. Glycerin was 99.5% (wt %) with a density of 1.26 g 3 cm-3; sorbitol was 99%. Water used in the experiments was triply distilled by a quartz water purification system. Phase Diagram. Two phases of densely packed MLVs and L1-phase were formed by mixing 200 mmol 3 L-1 TTABr and 200 mmol 3 L-1 SL in aqueous solutions (v:v = 1:1). Different amounts of glycerin from 10% to 99.5% (volume %) were added into the solution to replace the water solvent. The phase diagram was established by observing the solution in test tubes at 25.0 ( 0.1 °C for at least four weeks. Conductivity. The conductivity was tested by a Microprocessor Conductivity Meter LF2000 with a glass electrode at room temperature. The samples of precipitates/two-phase L1 were stirred during the conductivity measurement. FF-TEM. A small amount of sample was placed on a 0.1 mm thick copper disk covered with a second copper disk. This sandwich was plunged into liquid propane, which had been cooled by liquid nitrogen, and the copper sandwich was frozen with the sample. Fracturing and replication were carried out at a temperature of -140 °C. Pt/C was deposited at an angle of 45°. The replicas were examined using a JEM-100CX electron microscope. Polarization Microscopy. The presence of birefringent lamellar structures was revealed by a polarizing microscopy Axioskop 40/40 FL (Germany, Zeiss) at room temperature. Rheological Measurements. The rheological measurements were performed with a Haake RS6000 rheometer with a cylinder sensor. Temperature was controlled to (0.1 °C by a thermal controller (Haake TC81) for RS6000. Refractive Index. The refractive index was measured by using a Julabo Abe refractometer with temperature controlled by an F30-C water bath system (Zeiss, Germany). Surface Tension Measurements. We measured the cmc of the cationic surfactant TTAL with different amounts of glycerin because the catanionic surfactant TTAL with the same amount of NaBr is a two-phase system. The surface tension measurement was carried out with a tensionmeter (Kr€ uss, Germany) using the plate method at room temperature.

(18) Ionescu, L. G.; Trindade, V. L.; de Douza, E. F. Langmuir 2000, 16, 988–992. (19) D’Erricl, G.; Ciccarelli, D.; Ortona, O. J. Colloid Interface Sci. 2005, 286, 747–754. (20) Yan, Y.; Hoffmann, H. J. Phys. Chem. B 2007, 111, 6374–6382. (21) Hao, J.; Hoffmann, H.; Horbaschek, K. J. Phys. Chem. B 2000, 104, 10144–10153.

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data of 100 mmol 3 L-1 TTABr/ 100 mmol 3 L-1 SL and 100 mmol 3 L-1 NaBr in water/glycerin mixed solvents.

Figure 2. Conductivity

Figure 3. Schematic phase diagram of 100 mmol 3 L-1 TTABr/ -1 -1

100 mmol 3 L SL and 100 mmol 3 L mixed solvents.

NaBr in water/glycerin

Results and Discussion Phase Behavior of 100 mmol 3 L-1 TTABr and 100 mmol 3 L SL with Glycerin from 0 to 99.5%. The photos of the sample with different amounts of glycerin are shown in Figure 1. The sample without any glycerin is two phases: white cream floating precipitates of the upper phase with the densely packed MLVs, and the lower L1 phase (micelles). With increasing glycerin, the white cream floating precipitates swelled throughout the whole phase and became more transparent with birefringence. It is presumed that the densely packed MLVs swelled throughout the whole phase. The phase became the most transparent when glycerin was increased to 40%. When the amount of glycerin reached 50%, the sample became two transparent phases without -1

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Figure 4. Sample photographs of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvents. Upper row is the samples without polarizers, and lower row is the samples with polarizers.

Figure 5. Polarizer micrographs of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in mixed water/glycerin solvents. VGlycerin (%): 0 (a), 10 (b), 30 (c), and 43 (d).

birefringence, with the upper phase showing flow birefringence. It means that a small amount of vesicles probably exists in the upper phase. It was found that a rich phase transition occurred during the increase of glycerin from 40% to 50%, which was studied in detail. With 60% glycerin, the volume of the upper phase with flow birefringence increased. The sample became one phase with flow birefringence when glycerin was between 70% and 90%. It is considered that the vesicles became less and less with the addition of glycerin and could not even exist as one phase. When the glycerin amount reached 99.5%, the surfactants could not totally dissolve. 10542 DOI: 10.1021/la901303a

We measured the conductivity of 100 mmol 3 L-1 TTABr and 100 mmol 3 L-1 SL in water/glycerin mixed solvents. The conductivity data are shown in Figure 2. We observed that the conductivity of the cationic and anionic surfactant mixed system is lower than that of the 100 mmol 3 L-1 NaBr in the same solvent even though the catanionic surfactant system can actually form the same amount of NaBr in solution. This could indicate that the vesicles can enwrap free salt in solutions to reduce the conductivity. Because of the complicated phase behavior, the schematic phase diagram with phase boundaries was shown in Figure 3. Langmuir 2009, 25(18), 10540–10547

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Figure 6. FF-TEM micrographs of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvents. VGlycerin (%): 0 (a) and 40 (b).

Figure 7. Rheograms of the oscillatory shear for samples of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvents at 25 °C. VGlycerin (%): 20 (a), 30 (b), and 40 (c).

Each phase region contains one typical sample. The sample in the L1/L1 region was added to a small amount of hydrophobic dye, Sudan II. The upper phase showed obvious red color, which means the hydrophobic dye was almost dissolved in the upper phase. Most of the micelles existed in the upper phase. The sequences would thus be as follows: precipitate/L1, turbid phase, LR, LR/L1, L1/L1, L1, precipitate. The phase transitions of LR to LR/L1 and LR/L1 to L1/L1 will be discussed in detail later. Phase Behavior of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with Glycerin from 40% to 50%. Figure 4 shows the phase behavior of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with Langmuir 2009, 25(18), 10540–10547

glycerin amounts from 40% to 50%. One can see that the birefringent LR phase transferred to two phases as the glycerin concentration increase from 40% to 50%. The birefringent LR phase is the upper phase, and the lower phase is not birefringent (L1 phase). The upper phase lost its static birefringence until the glycerin amount increased to 49%. Above 50% glycerin, the birefringence of the upper phase totally disappeared, and the volume of the upper phase increased with increasing glycerin. From the phase behavior and conductivity data (See Supporting Information), we can make sure that the whole phase transition process occurred with the addition of glycerin: (1) the densely packed MLVs swelled throughout the whole phase between 0 and 45.5% glycerin; (2) the birefringent swelled lamellar phase became two phases in which the upper phase is birefringent and the lower phase is the L1 phase between 45.6% and 48% glycerin; (3) the upper phase became flow birefringent, and the volume of the upper phase increased with the addition of glycerin between 49% and 60% glycerin; (4) the system became one phase until the surfactant could not dissolve in the solution between 70% and 99.5% glycerin. The phase transition of the sample with 0-99.5% glycerin was complicated. We only paid attention to the swelling of the lamellar phase with 0-45.5% glycerin. Several methods were used to characterize this phase transition between 0 and 45.5%. Polarization Microscope Observations of the Upper Phase of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in Different Solvents. The sample solutions with different amounts of glycerin were measured by a polarization microscope at room temperature, and the micrographs are shown in Figure 5. The micrograph of the precipitates of the upper phase without glycerin in Figure 5a shows typical Malta crosses, which represent lamellar structures existing in the precipitate phase. When the solvent contains 10% glycerin, the sample solution becomes a turbid phase, but one still can see Malta cross birefringence in Figure 5b, meaning that lamellar structures still exist. With increasing glycerin between 30% and 43%, the birefringent texture changed a little, as shown in Figure 5c,d. FF-TEM Observations of the Upper Phase of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in Different Solvents. FF-TEM micrographs in Figure 6a confirmed that the white precipitates on the top phase of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL aqueous solution consist of MLVs.7,8 The diameter of the MLVs is around 200-600 nm. MLVs have several bilayers like onions. With the addition of glycerin, the lamellar phase swelled gradually with higher energy. In order to obtain a low energy state, the average number of bilayers inside the MLV may decrease, and close-packed unilamellar vesicles were formed. As shown in Figure 6b, unilamellar vesicles were formed after the glycerin DOI: 10.1021/la901303a

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Figure 8. Rotation rheograms of the samples of 100 mmol 3 L-1 -1

TTABr/100 mmol 3 L at 25 °C.

SL in water/30% glycerin mixed solvents

Figure 11. Selmilogatithmic plot of surface tension of TTAL in water/glycerin mixed solvents. VGlycerin (%): 0 (a), 30 (b), 40 (c), 60 (d).

Figure 9. Rheograms of the oscillatory shear for samples of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvents at 25 °C. VGlycerin (%): 60.

Figure 12. Sample photographs of 100 mmol 3 L-1 TTABr/ -1

100 mmol 3 L SL in water/sorbitol mixed solvents. Upper row is the samples without polarizers, and the lower row is the samples with polarizers.

Figure 10. Rotation rheograms of the samples of 100 mmol 3 L-1

TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvents at 25 °C. VGlycerin (%): 60 (a), 90 (b).

was added to 40%. The diameter was around 50-100 nm, which was much smaller than that in Figure 6a. Rheological Measurements. The variation of microscopic structures induced by different amounts of glycerin can be reflected by the changing of the rheological properties. The rheogram in Figure 7a shows the sample solution of 10544 DOI: 10.1021/la901303a

100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with 20% glycerin, which presents the properties of a Bingham fluid. The storage modulus (G0 ≈ 2.0 Pa) is almost 10 times larger than the loss modulus (G00 ≈ 0.2 Pa), and they both remain constant with different frequency, which is typical for viscoelastic vesicle systems.22,23 The water/glycerin mixed solvent is a Newtonian liquid, which has much lower viscosity than the surfactant solution. The rheograms of the oscillatory shear for samples of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvent at 25 °C are shown in Figure 7b,c. When the amount of glycerin was increased to 30% (Figure 7b) and 40% (Figure 7c), the storage modulus G0 decreased, but the loss modulus G00 changed less. It is inferred that the bilayers inside the MLVs swelled as a result of the addition of glycerin. The swelled bilayers became flexible and lost their rigidity. The storage modulus G0 and the loss modulus G00 tended to increase with frequency when the glycerin went to 40%. G0 and G00 cross each other at high frequency, meaning that, with more glycerin, the vesicle phase at high frequency does not show the rheological properties of a Bingham fluid, which viscoelastic (22) Horbaschek, K.; Hoffmann, H; Thunig, C. J. Colloid Interface Sci. 1998, 206, 439–456. (23) Bruno, D.; Monique, D.; Zemb, Th. Biophys. J. 2002, 82, 215–225.

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Figure 13. Polarizer micrographs of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with different amounts of sorbitol. Vsorbitol (%): 20 (a), 40 (b), 50 (c), and 60 (d).

Figure 14. FF-TEM images of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with 40% sorbitol.

LR phases usually have, but show some properties of wormlike micelles. The rotation rheograms of the samples with 30% glycerin show the shear stress, τ, and the viscosity, η, with shear rate, which are shown in Figure 8. The rheograms are all of shear thinning fluid because of the damage process of vesicles by shear. For the case when the amount of glycerin is increased to 60%, the rheograms of the oscillatory shear for samples of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/glycerin mixed solvent at 25 °C are shown in Figure 9. One can see that the loss modulus G00 overcame the storage modulus G0 . G0 and G00 decreased several magnitudes relative to those at lower glycerin amounts. The complex viscosity (η*) remained constant at different frequencies. It shows the properties of the solvent, which is a Newtonian fluid. The rotation rheograms of the samples with 60% and 90% glycerin show the shear stress, τ, and the viscosity, η, with shear rate, which are shown in Figure 10, respectively. One can see that the rotation rheograms of the viscosity, η, keeps constant, but the shear stress, τ, increases with shear rate with 60% and 90% glycerin. Surface Tension Measurements. Surface tension measurements of TTAL were performed at room temperature with different amounts of glycerin. From the surface curves in Figure 11, it can be seen that the surface tension decreases logarithmically with the surfactant concentration near the cmc, and remains constant when the concentration is above the cmc. The cmc values of TTAL in water/glycerin mixed solvent can Langmuir 2009, 25(18), 10540–10547

be obtained: 0.021 mmol 3 L-1 in pure water, 0.026 mmol 3 L-1 in 30% glycerin, 0.048 mmol 3 L-1 in 40% glycerin, and 0.095 mmol 3 L-1 in 60% glycerin. It is considered that the cmc of TTAL only increased less at low concentrations of glycerin below 40%. The influence of cmc variation due to addition of glycerin could be ignored below 40% glycerin. At high concentrations of glycerin (50-90%), the cmc of TTAL increased distinctively, which could take most of the responsibility for the phase transition. Much more surfactants absorbed on the surface of the solution because of the higher cmc. Aggregates with high aggregation numbers such as vesicles can be formed. The density of TTAL is lower than that of the solvent, causing TTAL to float on the top phase. After the LR phase swelling process, which was manipulated by the matching of refractive index (0%-45.5% glycerin), the following two-phase system and the upper phase losing birefringence are the results of the cooperation of refractive index matching and increasing cmc of TTAL. The volume of the upper phase increased in the concentration region of glycerin of 49-60% after it lost the birefringence. It is presumed that the increasing cmc of TTAL reduced the amount of surfactant in solutions, so the number of vesicles was also reduced. The vesicles, because of large aggregation number, have lower density than solvent. When the vesicles transformed into lower aggregation number structures, the density of the upper phase increased. It explained the volume and the disappearing of the upper phase. From the data above, we can confirm that the lamellar phase can swell with the addition of the glycerin. In order to understand DOI: 10.1021/la901303a

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Figure 15. Rheograms of the oscillatory shear for samples of

100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL in water/sorbitol mixed solvents at 25 °C. VSorbitol (%): 10 (a), 30 (b), and 50 (c).

this phase transition, we also tried other solvents, including dioxane, tetrahydrofuran, N-methyl-2-pyrrolidone, dimethylformamide, and sorbitol. The lamellar swelling can be observed only when the solvent was replaced by sorbitol. It is inferred that this phase transition can be induced by a carbonhydrate, which was already reported by Zemb et al.23 in a lecithin lamellar system. Carbonhydrates such as glycerin and sorbitol, i.e., the polyhydric alcohols, can form hydrogen bonds with water. Thereby, these molecules compete with water molecules to bind to surfactants in aqueous solutions. It could be concluded that the replaced solvent, which can cause lamellar swelling, should satisfy three requirements. First there is no other strong chemical interaction between solvent and surfactant. Second, the solvent can form a hydrogen bond to combine with water. Finally, the solvent can not effect the cmc of the surfactant. Phase Behavior of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL Aqueous Solution Induced by Addition of Sorbitol. The process of lamellar phase swelling with the addition of sorbitol is shown in Figure 12. The white precipitate cream on the top phase became more and more transparent with increasing sorbitol. Typical polarization microscope images are shown in Figure 13. Through the polarization microscope, we can see that the clearly birefringent phase gradually filled the tube with increasing sorbitol. It is considered that the MLVs became unilamellar vesicles. The FF-TEM photographs, as shown in Figure 14, proved that the MLVs swelled with the addition of sorbitol and 10546 DOI: 10.1021/la901303a

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Figure 16. Rotation rheograms of the samples of 100 mmol 3 L-1

TTABr/100 mmol 3 L-1 SL in water/sorbitol mixed solvents at 25 °C. VSorbitol (%): 10 (a), 30 (b), 50 (c).

became small unilamellar vesicles. The rheograms, as shown in Figure 15, demonstrated that the storage modulus G0 decreased with the increasing sorbitol. The lamellar phase could become softer and lose its rigidity when it is swelled by the addition of sorbitol. The storage modulus, which reflects the rigidity of the lamellar phase, could decrease. The rotation rheograms, as shown in Figure 16, proving that the rheograms are all of shear thinning fluid because of the damage process of vesicles by shear.

Conclusions Glycerin can form hydrogen bonds with water and compete with water molecules to bind to surfactant in aqueous solution. The catanionic surfactant TTAL has a higher cmc at high glycerin concentration, which causes phase separation. Our research focused on the phase behavior of the swelling of a lamellar phase. Normally, the bilayers of a lamellar phase have defined thickness and interspace. It is equilibrium between van der Waals attraction and Helfrich undulation. The pressure of attraction per unit area for the situation, when two bilayers of thickness δ approach a distance d (d . δ), is given by24 " # A 1 2 1 ð1Þ PA ¼ þ 6π d 3 ðd þ δÞ3 ðd þ 2δÞ3 (24) Parsegian, V. A. In van der Waals Forces; Cambridge University Press: Cambridge, U.K., 2005.

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where A is Hamaker’s constant, which depends on the material of the bilayer and the medium property. It can be described by25   3 εd -εm 2 3 ðnd 2 -nm 2 Þ2 A ¼ kT þ pffiffiffi pω 4 εd þ εm 16 2 ðnd 2 þ nm 2 Þ3=2

ð2Þ

εd and εm are the dielectric constants of particles and medium, respectively. For visible light, ω is the frequency of the dominating UV absorption (about 1.7-2.4  1016 rad/s), and p = h/2π, where h is Planck’s constant. nd and nm are the refractive indices of particles and medium. The second part of eq 2 is much larger than the first one. So we only think about the second part. For our research, the addition of glycerin increases the refractive index of the aqueous medium and, as a consequence, decreases the (25) Hiemenz, P. C. In Principles of Colloid and Surface Chemistry, 2nd revised ed.; Marcel Dekker, Ltd.: New York, 1985.

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difference in polarizabilities between particles and medium. This reduces the contribution of visible frequencies to the total van der Waals interaction. So the decreased van der Waals induces the swelling of the lamellar phase with unsteady high energy. The swelling lamellar pack closed into smaller unilamellar vesicles to obtain a low energy state. Acknowledgment. The authors are thankful for the financial support by the NSFC (Grant No. 20625307) and the National Basic Research Program of China (973 Program, 2009CB930103). Supporting Information Available: Phase behavior of 100 mmol 3 L-1 TTABr/100 mmol 3 L-1 SL with glycerin from 45.1 to 45.9% and from 50 to 60%. This material is available free of charge via the Internet at http://pubs.acs.org.

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