Effects of Hydrophobic Counterions on the Phase Behavior of

The absorbance of Sudan III in the micelle solution strongly depends on the β ... A schematic picture showing various states of the C14DMAOH·NphS solu...
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Effects of Hydrophobic Counterions on the Phase Behavior of Tetradecyldimethylhydroxylammonium Chloride in Aqueous Solutions Hideya Kawasaki, Ryota Imahayashi, and Hiroshi Maeda* Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Received May 29, 2002 We investigated the phase behavior of cationic tetradecyldimethylhydroxylammonium chloride (C14DMAOH‚Cl) (a fixed surfactant concentration, 20 mM) in the presence of sodium 2-naphthalenesulfonate (SNphS) as a function of the molar ratios β () SNphS/C14DMAOH‚Cl) by turbidity, viscosity, dye solubilization, and polarizing microscopy. The C14DMAOH‚Cl/SNphS system formed aggregates with a lower curvature than cetyltrimethylammonium bromide (C16TAB)/aromatic counterion systems, despite the shorter alkyl chain. Addition of SNphS into the C14DMAOH‚Cl solution at 25 °C produced precipitates of solid crystals above β ∼ 0.5, in contrast with long fibrous micelles of many other cationic micellearomatic counterion pairs. The crystalline solids (tetradecyldimethylhydroxylammonium naphthalenesulfonate, C14DMAOH‚NphS) at 20 mM concentration were transformed into the lamellar liquid crystalline phase dispersion (lamellar droplets) above 54 °C. The lamellar phase was not spontaneously converted into vesicles upon dilution to 1 mM, in contrast to spontaneous vesicle formation of cetyltrimethylammonium hydroxynaphthalenecarboxylate (C16TAHNC). The stability of the lamellar droplets was rather limited in the low surfactant concentration of 1 mM. The phase separation into the lamellar liquid crystalline phase and the clear isotropic solution was observed within several hours. By gentle shaking by hand, on the other hand, the lamellar phase could be dispersed to lamellar droplets at 60 °C again. The fact that the C14DMAOH‚NphS prefers the planar bilayer structure (a precipitate or a lamellar liquid crystalline phase) to the curved bilayer structure (vesicles) may manifest the rigid surface of the bilayers. Compared with C16TAHNC, the rigid nature of the C14DMAOH‚NphS bilayer is likely due to the proposed hydrogen bond formation between the two neighboring cationic headgroups, where their charge repulsion was now suppressed by the counterion binding. To examine effects of the hydrophobicity of the counterion on the phase behavior, similar experiments were carried out for sodium benzenesulfonate (SBzS) and Na2SO4. In the case of the SBzS, the interaction was much weaker than that of SNphS and no lamellar droplets were observed at elevated temperatures. Na2SO4 was found to be ineffective to induce a structural change of the micelle, despite consisting of divalent counterions.

Introduction Amphiphilic molecules such as surfactants self-assemble to form micelles in aqueous media above a critical micellar concentration (cmc). The structures of these micelles depend on the intensive variables (e.g., temperature, ionic strength, and pH) as well as the surfactant geometry (e.g., length of the hydrocarbon chains, size of the polar headgroups, and counterions species). In dilute solutions, the structure of these micelles is explained in terms of the surfactant packing parameter p ) v/Al: spherical micelles for p < 1/3, cylindrical micelles for 1/3 < p < 1/2, vesicles or flexible bilayers for 1/2 < p < 1, and planar bilayers for p ∼ 1,1 where v is the volume of the surfactant hydrophobic part, A is the surface area per headgroup, and l is the extended chain length of the hydrophobic part. As for ionic surfactants, the structures of these micelles are sensitive to the ionic strength of the solution, influencing the effective headgroup area (A) due to the screening of the electrostatic repulsion between the headgroups. As a result, the micelles can undergo a structural transition from the spherical micelle to the rodlike micelle by addition of salts.2,3 Under certain (1) Israelachvili, J. N.; Mitchel, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) (a) Ikeda, S.; Ozeki, S.; Tsunoda, M. J. Colloid Interface Sci. 1980, 73, 27. (b) Ozeki, S.; Ikeda, S. Colloid Polym. Sci. 1984, 262, 409. (3) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7917.

conditions, the rodlike micelle can grow extremely long (flexible wormlike micelles) due to a high end cap energy, which is defined as the energy required to create two hemispherical end caps.4 Such micelle growth is known to be more pronounced in the case of cationic surfactant/ aromatic counterion systems due to the strong binding of the counterions to the micelles.5-12 The wormlike micelles were observed in various kinds of aromatic counterion systems, such as sodium salicylate (SS),5-7 3,5-dichlorobenzonate,8 p-toluenesulfonate,9 and benzenesulfonate.10 One of the most extensively studied systems is the mixture of cetyltrimethylammonium bromide (C16TAB) with SS.5-7 In equimolar solutions with the surfactant concentration as low as 10-3 M, the system forms long wormlike micelles, which behave like a living polymer with continuous breaking and reformation of the micelles. (4) Cates, M. W.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (5) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (6) Rehage, H.; Hoffmann, H. Faraday Discuss. Chem. Soc. 1983, 76, 363. (7) (a) Shikata, T.; Hirata, H. Langmuir 1987, 3, 1081. (b) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1989, 5, 398. (8) Caver, M.; Smith, T. L.; Gee, J. C.; Delichere, A.; Caponetti, E.; Magid, L. Langmuir 1996, 12, 691. (9) Soltero, J. F. A.; Puig, J. E.; Manero, O. Langmuir 1996, 12, 2654. (10) Bunton, C. A.; Michael J. M.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (11) Brown, W.; Johansson, K.; Almgren, M. J. Phys. Chem. 1989, 93, 5888. (12) Bhat, M.; Gaikar, V. G. Langmuir 1999, 15, 4740.

10.1021/la020502l CCC: $22.00 © 2002 American Chemical Society Published on Web 10/05/2002

Effect of Counterions on Phase Behavior

In the case of the mixture of C16TAB with sodium 3-hydroxynaphthalene-2-carboxylate (SHNC) which almost completely binds to the micelle surface, a vesicular phase was found in the equimolar solution,13-17 instead of enormous micelle growth. At a fixed C16TAB concentration (60 mM), continuous addition of SHNC gave the following transitions: I f N f precipitate of aggregated vesicles (1:1 mixture of C16TAB with SHNC) f N f I.13,17 Here a highly viscoelastic optically isotropic liquid is denoted as I, and N is the liquid crystalline nematic phase. Cetyltrimethylammonium hydroxynaphthalencarboxylate (C16TAHNC), which was obtained from the equimolar mixture of C16TAB with SHNC, formed vesicles at room temperature for concentrations less than 60 mM and the lamellar liquid crystalline phase above 60 mM. Interestingly, the vesicular phase changed into the micellar phase when increasing temperature.14-17 From differential scanning calorimetry (DSC) and NMR experiments, the temperature-induced micelle-vesicle transition at low concentrations has been suggested to originate from the melting of the solidlike counterions on the vesicular surface.15,17 Sodium 2-naphthalenesulfonate (SNphS) is another aromatic counterion with a naphthalene ring, which is expected to have similar hydrophobicity to SHNC. Contrary to the case of SHNC, the addition of SNphS does not produce vesicles however. The equimolar mixtures of C16TAB with SNphS exhibited the wormlike micelles at 25 °C.11 The reason vesicles are not formed for the C16TAB/ SNphS system is still unclear. Alkyldimethylamine oxides (CnDMAO) exist as either nonionic [CnH2n+1(CH3)2NfO] or cationic (protonated) species [CnH2n+1(CH3)2N+OH]. The relative population of the two components (i.e., the degree of protonation R) depends on pH of the solution. In a pH lower than about 3, the CnDMAO behaves as a cationic surfactant. In good analogy with acid soaps of fatty acids, a stable complex of 1:1 composition has been shown to be formed not only in the solid state18,19 but also for the micelles in solution.20,21 The hydrogen bond between the nonionic and the cationic headgroups has been suggested as the interaction responsible for the complex formation.22-25 The proposed hydrogen bond has been supported recently by infrared spectroscopy.26 In a previous study on the cationic dodecyland tetradecyldimethylhydroxylammonium chloride (C12DMAOH‚Cl and C14DMAOH‚Cl), we reported that the cmc of the cationic species is lower than that of the nonionic one at ion strengths higher than 0.2 M NaCl.27 (13) Mishra, B. K.; Samant, S. D.; Pradhan, P.; Mishra, S. B.; Manohar, C. Langmuir 1993, 9, 894. (14) Hassan, P. A.; Narayanan, J.; Menon, S. V. G.; Salkar, R. A.; Samant, S. D.; Manohar, C. Colloids Surf., A 1996, 117, 89. (15) Salkar, R. A.; Hassan, P. A.; Samant, S. D.; Valaulikar, B. S.; Kumar, V. V.; Kern, F.; Candau, S. J.; Manohar, C. Chem. Commun. 1996, 1223. (16) Narayanan, J.; Manohar, C.; Kern, F.; Candau, S. J. Langmuir 1997, 13, 5235. (17) Horbaschek, K.; Hoffmann, H.; Thuing, C. J. Colloid Interface Sci. 1998, 206, 439. (18) Kawasaki, H.; Fukuda, T.; Yamamoto, A.; Fukada, K.; Maeda, H. Colloids Surf., A 2000, 169, 117. (19) Miyahara, M.; Kawasaki, H.; Fukuda, T.; Ozaki, Y.; Maeda, H. Colloid Surf., A 2001, 183, 475. (20) Maeda, H.; Yamamoto, A.; Souda, M.; Kawasaki,H.; Hossain, K. S.; Nemoto, N.; Almgren, M. J. Phys. Chem. B 2001, 105, 5411. (21) Maeda, H.; Kakehashi, R. Adv. Colloid Interface Sci. 2000, 88, 275. (22) Goddard, E. D.; Kung, H. C. J. Colloid Interface Sci. 1973, 175, 497. (23) Ikeda, S.; Tsunoda, M.; Maeda, H. J. Colloid Interface Sci. 1979, 70, 448. (24) Warr, G. G.; Grieser, F.; Evans, D. F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1829. (25) Maeda, H. Colloids Surf., A 1996, 109, 263. (26) Kawasaki, H.; Maeda, H. Langmuir 2001, 17, 2279.

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A hydrogen bond formation between the two neighboring cationic headgroups was, thus, suggested for the cationic C12DMAOH‚Cl and C14DMAOH‚Cl micelles. It is likely that the cationic headgroup forms hydrogen bonding to the neighboring headgroup either directly or through intervening water molecules. In the present study, we investigated the phase behavior of new systems, the cationic C14DMAOH‚Cl in the presence of the aromatic counterion SNphS with varying molar ratios β (β ) SNphS/C14DMAOH‚Cl). The phase behaviors were examined by turbidity, viscosity, dye solubilization, differential scanning calorimetry, and polarized microscopy. To examine effects of the hydrophobicity of the counterion, similar experiments were also performed for C14DMAOH‚Cl in the presence of sodium benzenesulfonate (SBzS) and Na2SO4. The aromatic counterions can strongly bind to the surface of cationic C14DMAOH‚Cl micelles, and the electrostatic repulsion between headgroups is strongly screened. As a result, the possible effects arising from a hydrogen bond formation between the two neighboring cationic headgroups on the surfactant aggregates are expected to be found in the phase behavior of the cationic C14DMAOH‚Cl/aromatic counterion systems. Materials and Methods Materials. Tetradecyldimethylammine oxide (C14DMAO) (Gerbu Co.) was recrystallized three times from hot acetone. After the recrystallization, there was no minimum in a plot of surface tension versus concentration and a single peak in chromatographs of high-performance liquid chromatography (HPLC) (Tosoh Co., Japan) with an ODS-120T column (MeOH/H2O ) 7/3). The solid sample of the cationic C14DMAOH‚Cl was obtained by mixing equimolar solutions of nonionic C14DMAO and HCl and then freeze-drying these solutions. Twice-distilled water was used. SNphS, SBzS, and Na2SO4 were purchased from Tokyo Kasei Kogyo and used without further purification. Tetradecyldimethylhydroxylammonium naphthalenesulfonate salt (C14DMAOH ‚NphS) was obtained by mixing equimolar solutions of C14DMAOH‚Cl and SNphS. The equimolar mixed solution was kept at 25 °C for a week, resulting in the precipitates of the solid crystalline C14DMAOH ‚NphS. The precipitate thus prepared was washed out by distilled water to remove NaCl and dried. Turbidity. Turbidity measurements were performed at 25 °C with a Jasco Ubest-50 UV/vis spectrophotometer, equipped with a thermostat cell holder and a magnetic stirring device, using quartz cells of 1 cm path length. Turbidity was measured at 400 nm and expressed in transmittance. Viscosity. The viscosity was measured at 25 ( 0.02 °C using an Ostwald viscometer (flow time of water, 91.7 ( 0.1 s). When fine crystals were produced by adding the aromatic salts into the surfactant solution, the viscosity measurements were performed for the clear solution after removing the solid by filtration. Dye Solubilization. The saturated solubilization of a waterinsoluble dye, Sudan III, by the micelle was examined for the aqueous solutions of C14DMAOH‚Cl in the presence of the aromatic counterions at 25 ( 1 °C. After removal of the excess undissolved dye by a Millipore filter of 0.2 µm, the absorbance of the solubilized dye in the solutions was measured with a Jasco Ubest-50 UV/vis spectrophotometer at 500 nm. Differential Scanning Calorimetry. The dissolution temperature of the C14DMAOH ‚NphS crystalline solid in the presence of water was determined with a Seiko Instrument DSC 120 at a heating rate of 1 °C/min. Polarizing Microscopy. The lyotropic lamellar phase was examined, using a Nikon polarization microscope E 600 POL, equipped with a LINKAM LTS-E 350 hot stage. Composition Analysis of the Precipitated Solid of the Mixture of C14DMAOH‚Cl/SNphS. The aqueous mixtures of (27) (a) Maeda, H.; Muroi, S.; Kakehashi, R. J. Phys. Chem. B 1997, 101, 7378. (b) Maeda, H.; Kanakubo, Y.; Miyahara, M.; Kakehashi, R.; Garamus, V.; Pedersen, J. S. J. Phys. Chem. B 2000, 104, 6174.

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Figure 1. Turbidity (expressed as transmittance at 400 nm) at 25 °C as a function of the molar ratios β [) X/C14DMAOH‚ Cl] (X ) SNphS, SBzS, and Na2SO4). Open circles, SNphS; closed circles, SBzS; broken line, Na2SO4. The surfactant concentration is fixed to be 20 mM. C14DMAOH‚Cl/SNphS at different mole ratios β ) (SNphS/ C14DMAOH‚Cl) (β ) 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) were maintained at 25 °C for a week, resulting in the precipitation of the solid crystals. The SNphS contents of the bulk solution in equilibrium with the precipitated solid were analyzed with the UV absorption of the SNphS at λ ) 320 nm. From the SNphS content of the total system and that of the bulk solution, we determined the composition of the crystalline solid.

Results Turbidity. Figure 1 shows a plot of the turbidity of C14DMAOH‚Cl aqueous solutions (a fixed surfactant concentration, 20 mM) at 25 °C as a function of the mole ratio β ) (X/C14DMAOH‚Cl) (X ) SNphS, SBzS, and Na2SO4). In the case of the SNphS, the turbidity increases above β ∼ 0.5, where the precipitates of solid crystals appeared. Hereafter, we define the β value where the precipitates appeared as β*. By further addition (β > 1), the precipitate was not solubilized by excess SNphS, in contrast to the CTAB/SHNC system in which the precipitate was solubilized by the excess SHNC.13 Qualitatively similar behavior was observed for the SBzS/C14DMAOH‚Cl system: the precipitate of solid crystals appears above a certain β* value. But the β* value is around 1.2, which is larger than that of the SNphS (β* ∼ 0.5). In the case of the Na2SO4, there is almost no change in the turbidity up to β ∼ 2 (a broken line). Viscosity. The addition of the aromatic counterions into the cationic surfactant solutions is known to induce an increase in the viscosity, which has been attributed to the formation of wormlike micelles. Such micelle growth was examined for the C14DMAOH‚Cl/aromatic counterion systems in this study. Figure 2 shows a plot of the specific viscosity (ηsp ) η/η0 - 1) of the C14DMAOH‚Cl aqueous solution (20 mM) at 25 °C as a function of the mole ratio β ) (X/C14DMAOH‚Cl) (X ) SNphS and SBzS). Here η and η0 are the viscosities of the solution and water, respectively. The viscosity increases sharply above a particular β value, suggesting the formation of the elongated micelles. The β value of SNphS required for the abrupt increases in the viscosity is lower than that of SBzS (β ∼ 0.2 for SNphS and β ∼ 0.5 for SBzS). It is also found that the increase in the viscosity with β is more remarkable for SNphS than for SBzS (the maximum specific viscosity is ∼20 for SNphS and ∼12 for SBzS), indicating the more pronounced micellar growth in the case of SNphS. Further increase in the β value induced gradual decrease in the viscosity above β ∼ 0.5 for SNphS and above β ∼

Figure 2. Absorbance of Sudan III (b) and the specific viscosity ηsp (O) in C14DMAOH‚Cl solutions at 25 °C as a function of the molar ratio β () X/C14DMAOH‚Cl) [X ) SNphS (a) and SBzS (b)]. The surfactant concentration is fixed to be 20 mM. The corresponding β* values where the precipitate occurs are denoted by arrows in the figure.

1 for SBzS due to the formation of the precipitates. A relatively high viscosity was observed even above the β* for both SNphS and SBzS. This suggests that the micelles coexist with the crystalline solid even above the β* value. In the case of the Na2SO4 system, on the other hand, the increase of the viscosity was small (ηsp ∼ 2 at β ∼ 1). Dye Solubilization. The color intensity is an easy indication of the solubilization capacity of a waterinsoluble dye into the micelle. To clarify the coexistence of the crystalline solid with the micelles above the β* value, suggested by the viscosity experiments, solubilization experiments of Sudan III were carried out for C14DMAOH‚Cl solutions in the presence of SNphS or SBzS at 25 °C. Figure 2 shows a plot of the absorbance of Sudan III in the C14DMAOH‚Cl aqueous solution (20 mM) as a function of the mole ratio β ) (X/C14DMAOH‚ Cl) (X ) SNphS and SBzS). The absorbance of Sudan III in the micelle solution strongly depends on the β value. Good correlation was found in both the SNphS and the SBzS between the change of the absorbance of Sudan III and the viscosity without Sudan III. This suggests that the micelle growth induced by adding the aromatic counterions increases the solubilization capacity of the dye into the micelles, which might reflect the larger solubilization capacity of the cylindrical part as compared to the hemispherical end cap of the rodlike micelle. Above the β*, on the other hand, the absorbance of Sudan III gradually decreases with increasing β due to the produced precipitates. The absorbance becomes almost zero above β ∼ 1 for SNphS and above β ∼ 2 for SBzS,

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lamellar phase for the C14DMAOH‚ NphS solution above 54 °C. The temperature-induced transition from the crystalline solid of C14DMAOH‚NphS to the lamellar phase can be detected by DSC, which showed a single endothermic peak at 54 °C (not shown) with an enthalpy change of 23.5 kJ/mol. The lamellar phase existed at temperatures as high as 98 °C without the transition to the isotropic micellar phase. The lamellar droplets of C14DMAOH‚NphS were observed even in the lower concentration of 1 mM at 60 °C. The stability of the lamellar droplets was, however, rather limited and depended on the surfactant concentration. The phase separation into the lamellar liquid crystalline phase and the clear isotropic solution was observed for a 1 mM concentration within several hours but it took longer than 4 days at 20 mM. Gentle shaking by hand, on the other hand, could disperse the lamellar phase into lamellar droplets at 60 °C. This suggests that the lamellar droplets are not a thermodynamically stable state. Various states at β ) 1 are schematically shown in Figure 4. In the case of the equimolar solutions of C14DMAOH‚ Cl and SBzS, on the other hand, no lamellar droplets (20 mM) were observed at elevated temperatures. Instead, a clear isotropic micellar solution was observed at 60 °C. Discussion

Figure 3. Light microscope micrograph of large spherical aggregates of C14DMAO‚NphS at 60 °C. The surfactant concentration is 20 mM. (a) Bright field; (b) crossed polars. The bar in the figure corresponds to 70 µm.

indicating no or negligible micelles. The existence of the absorbance of Sudan III above the β* value strongly suggests that the crystalline solid coexists with the surfactant aggregates (probably micelles) in the range of 0.5 < β < 1 for SNphS and 1.2 < β < 2 for SBzS. This is in conformity with the notion from the viscosity measurements. Specific Complexation. Compositions of the crystalline solids coexisting with the micelles were determined at different β values. The mole ratios of C14DMAOH+ to NphS- in the crystalline solid were nearly equal to 1 in the range of β values examined: the ratios were 1.1, 1.0, 1.1, 1.2, 1.1, and 1.0 for β ) 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0, respectively. This indicates that the crystalline solids coexisting with the micelles are a 1:1 complex between C14DMAOH+ and NphS-, irrespective of the β values. Formation of Lamellar Droplets from C14DMAOH‚ NphS without Salt. C14DMAOH ‚NphS was dissolved into water at 60 °C. It was found that the C14DMAOH ‚NphS (20 mM) without salt formed the lamellar droplets when the temperature was above 54 °C. The solutions of C14DMAOH ‚NphS became opaque due to the dispersed lamellar droplets above 54 °C. A typical example of the lamellar droplets at 60 °C is shown in Figure 3. Polydisperse spherical structures can be observed, and some have diameters as large as about 70 µm. The lamellar droplets are identified by the observation of a Maltese cross (Figure 3b), indicating a multilayer structure for the droplets. Polarization microscopy indicates the formation of a

In this study, the phase behaviors of C14DMAOH‚Cl in the presence of the aromatic counterions (SNphS or SBzS) as a function of the mixing ratio β were investigated at a fixed surfactant concentration of 20 mM at 25 °C. A schematic picture consistent with the observed phase behavior of the C14DMAOH‚Cl/SNphS and the C14DMAOH‚Cl/SBzS systems is shown in Figure 5. Qualitatively similar results were obtained for the SNphS and the SBzS. Adding the aromatic counterions to the C14DMAOH‚Cl solutions raised the viscosity (β > 0.2 for SNphS and β > 0.6 for SBzS), indicating the formation of the elongated micelles. Further addition, however, induced the crystalline solid above the critical β* value (β* ∼ 0.5 for SNphS and β* ∼ 1.2 for SBzS). A wide range for the coexisting region of the micelle with the crystalline solid was observed for both SNphS and SBzS (0.5 < β < 1 for SNphS and 1.2 < β < 2 for SBzS). The lower β* value of SNphS suggests that the NphS- ions associate more strongly with C14DMAOH+ micelles than the BzS- ions. This is expected from the increased hydrophobicity of the naphthalene ring compared to the benzene ring. The aromatic counterions, SNphS and SBzS, are considered to penetrate into the nonpolar parts of the amine oxide micelles, like C16TAB/SHNC and C16TAB/SS.7b,13 The penetrating aromatic counterions increase the surfactant packing parameter p ) v/Al due to the increase of v, in addition to a decrease of the area A induced by the screening of the electrostatic repulsion. The addition of SNphS is expected to increase the surfactant packing parameter more than the addition of SBzS because of the increased hydrophobic parts. It is considered, therefore, that the addition of SNphS brings about more pronounced micellar growth, finally leading to the liquid crystalline lamellar phase, but the addition of SBz does not. The difference is similar to that for SHNC and sodium salicylate in the case of C16TAB. It should be emphasized that the phase behavior of the present system of C14DMAOH‚Cl/SNphS is different from that of the other cationic surfactant/aromatic counterion systems at least in the following two respects. (i) To our knowledge, a large coexisting region of the crystalline solids and the micelles over a considerable

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Figure 4. A schematic picture showing various states of the C14DMAOH‚NphS solutions. Tc is the melting temperature of the solid.

modulus of Gaussian curvature, C1 and C2 are two principal curvatures, and C0 is the spontaneous curvature. In the case of symmetric bilayers composed of a single component, C0 ) 0. For spherical vesicles (C1 ) C2), we obtain the following equation for the bending energy of one spherical vesicle:28-30

Eves ) 4π(2kb + kG)

Figure 5. A schematic picture consistent with the observed phase behavior of the C14DMAOH‚Cl/SNphS and the C14DMAOH‚Cl/SBzS at 25 °C as a function of the mole ratio β ) (X/C14DMAOH‚Cl) (X ) SNphS and SBzS). The surfactant concentration is 20 mM.

range of β values was first observed in cationic surfactant/ aromatic counterion systems. (ii) The C14DMAOH‚Cl/SNphS system forms aggregates with a lower curvature than those seen for SHNC and SNphS in the case of C16TAB/aromatic salt systems, despite the shorter alkyl chain. C14DMAOH‚NphS did not form vesicles under the conditions beyond the micelle growth. Alternatively, C14DMAOH‚NphS produced solid crystals at 25 °C and above 54 °C the liquid crystalline lamellar phase even at the low surfactant concentration of 1 mM. This is very different from the cases reported for C16TAHNC, in which the lamellar phase was spontaneously dispersed into vesicles upon dilution.14 The C16TAHNC forms the vesicles (∼100 nm) in the concentration of 0.6 mM at 25 °C.17 Moreover, in equimolar mixtures of C16TAB with SNphS, only the formation of wormlike micelles has been reported at 25 °C (the overlap concentration, 6.6 mM).11 Here we consider why C14DMAOH‚NphS favors the formation of lower-curvature aggregates (the precipitate or the lamellar liquid crystalline phase) rather than vesicles, in contrast to the C16THNC. According to the Helfrich theory, the bending energy per unit area of fluid bilayer is given as28

E ) (1/2)kb(C1 + C2 - 2C0)2 + kGC1C2

(1)

where kb is the bending elastic modulus, kG is the elastic

(2)

The Helfrich model shows that the spherical vesicle can form spontaneously for soft bilayers where Eves is very small comparable to the thermal energy kT.31,32 In the case of negative kG values, Eves also becomes very small.30 For the flexible bilayers of the single-tailed surfactant, kb ∼ kBT.33 Such vesicles with a soft surface may be formed spontaneously by entropy stabilization.31,32 In the case of the rigid bilayers such as double-tailed phospholipid liposomes, kb ∼ 10-30 kBT.34 The vesicles with rigid surfaces are formed only in the kinetic process by the input of some sort of energy like sonication. From the viewpoint of the Helfrich theory, the rigidity of the bilayer surface plays an important role in the formation of spontaneous vesicles, and the rigid surfaces are expected to prefer a planar structure to curved bilayer structures. Among surfactant/aromatic counterion systems, C16TAHNC is well-known to form vesicles.14-17 In the case of the C16TAHNC vesicles, the penetrating aromatic counterion (HNC-) was suggested to act as a cosurfactant which reduces the bending rigidity of the bilayer by reducing the bilayer thickness,35 leading to the formation of vesicles spontaneously. The rough and tortuous surfaces reported for C16TAHNC vesicles may reflect the soft nature of the bilayer.14 In the present study, on the other hand, we found that C14DMAOH‚NphS prefers the planar bilayer structure (a precipitate or a lamellar liquid crystalline phase) to the curved bilayer structure (vesicles). (28) Helfrich, W. Z. Naturforsch. 1973, C28, 693. (29) Lasic, D. D. J. Colloid Interface Sci. 1990, 140, 302. (30) Helfrich, W. Prog. Colloid Polym. Sci. 1994, 95, 7. (31) Helfrich, W. Z. Naturforsch. 1978, A33, 305. (32) Lasic, D. D. Nature 1992, 355, 279. (33) Roux, D.; Nallet, F.; Freyssingeas, E.; Porte, G.; Bassereau, P.; Skouri, M.; Marignan, J. Europhys. Lett. 1992, 17, 575. (34) Evans, E.; Rawicz, W. Phys. Rev. Lett. 1990, 64, 2094. (35) Safinya, C. R.; Sirita, E. B.; Roux, D.; Smith, G. S. Phys. Rev. Lett. 1989, 62, 1134.

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Summary

Figure 6. An interference contrast microscope micrograph of vesicle-like aggregates induced by sonication at 25 °C in an equimolar mixture of C14DMAOH‚Cl and SNphS. The surfactant concentration is 20 mM. The bar in the figure corresponds to 20 µm.

This tendency may manifest the more rigid nature of the C14DMAOH‚NphS bilayer (at least, Eves > 2kT). Compared with C16TAHNC, the rigid nature of the C14DMAOH‚NphS bilayer is likely due to the proposed hydrogen bond formation between the two neighboring cationic headgroups,27 where their charge repulsion was suppressed by the counterion binding. The rigid surface properties of the C14DMAOH‚NphS bilayer should be clarified in the future. For such rigid bilayers, it is expected that vesicles are formed only in the kinetic process by the input of some kind of energy like sonication, as seen in double-tailed phospholipid liposomes. Actually, we found that solid crystals of C14DMAOH‚NphS were transformed into vesicle-like aggregates at 25 °C by sonication (Figure 6). The vesicle-like aggregates were stable at least for 3 months at 25 °C. Even for a large bending elastic modulus kb (kb . kT), spontaneous vesiculation is possible if the bilayers have a nonzero spontaneous curvature (C0 * 0).36 It was shown that the nonzero value of spontaneous curvature originates from different compositions of the inner and the outer monolayers in the case of the mixed surfactant bilayers. For C16TAHNC and C14DMAOH‚NphS composed of single-component ion pairs, it is unlikely that these bilayers have the spontaneous curvature since the nonzero spontaneous curvature requires that the bilayer is composed of at least two components. Thus, the different behaviors between C16TAHNC (vesicles) and C14DMAOH‚NphS (lamellae) are likely to be ascribed to the different rigidities of their bilayers. (36) Safran, S. A.; Oincus, P.; Andelman, D. Science 1990, 248, 354.

We investigated the phase behavior of cationic C14DMAOH‚Cl (a fixed surfactant concentration, 20 mM) at 25 °C in the presence of the counterions SNphS, SBzS, and Na2SO4 as a function of the molar ratios β () X/C14DMAOH‚Cl) (X ) SNphS, SBzS, and Na2SO4). Qualitatively similar results were obtained for the SNphS and the SBzS. Adding the aromatic counterion (SNphS or SBzS) to the C14DMAOH‚Cl solutions increased the viscosity due to the formation of the elongated micelles, similar to the other cationic surfactant systems. Good correlations between the solubilized amount of Sudan III and the viscosity without Sudan III were found. This suggests that the salt-induced micelle growth enhances the solubilization capacity of the dye into the micelles. Further addition of the aromatic counterions induced the precipitate of the solid crystals above the critical β* value (β* ∼ 0.5 for SNphS and β* ∼ 1.2 for SNphS). The lower β* value of SNphS suggests that NphS- ions associate more strongly with C14DMAOH+ micelles than BzS- ions do. This is expected from the increased hydrophobicity due to the naphthalene ring of SNphS. A large region of coexistence of the micelle and the crystalline solid was found: 0.5 < β < 1 for SNphS and 1.2 < β < 2 for SBzS. The crystalline solid coexisting with the micelle was a 1:1 complex between C14DMAOH+ and NphS-, irrespective of the β values. In the case of Na2SO4, there was no observation of the precipitate and a small increase in the viscosity up to β ∼ 2, suggesting that the hydrophobic interaction plays an important role in the strong binding of the aromatic counterion to the micelle. We found that the C14DMAOH‚NphS (20 mM) in water formed a lamellar liquid crystalline phase (lamellar droplets) above 54 °C (transition enthalpy, 23.5 kJ mol-1) but the lamellar phase was not spontaneously dispersed into vesicles upon dilution, in contrast to the stable vesicle formation of C16TAHNC. A phase separation into the lamellar liquid crystalline phase and the clear isotropic solution, rather than vesicle formation, took place for the C14DMAOH‚NphS. This tendency may reflect the rigid nature of the C14DMAOH‚NphS bilayer due to the proposed hydrogen bond formation between the two neighboring cationic headgroups, where their charge repulsion was weakened by the counterion binding. Acknowledgment. The authors thank Mr. A. Sasaki for carrying out the DSC and the polarizing microscopy. This work was supported, in part, by a Grant-in-Aid for Scientific Research (B) (No. 12440200) from The Ministry of Education, Culture, Sports, Science and Technology, Japan. LA020502L