Phase Behavior and Rheological Properties of Salt-Free Catanionic

Dec 30, 2010 - TTAOH/DA/H2O System in the Presence of Hydrophilic and ... (TTAOH)/decanoic acid (DA)/H2O, abundant phase behaviors were obtained in ...
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Phase Behavior and Rheological Properties of Salt-Free Catanionic TTAOH/DA/H2O System in the Presence of Hydrophilic and Hydrophobic Salts Wenjie Sun, Yuwen Shen, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, P. R. China Received October 18, 2010. Revised Manuscript Received December 6, 2010 In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H2O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical LR phases with the different compositions were characterized by the transmission electron microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The LR phase with the positive membrane charges can be finally transferred into an L1 phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed, and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.

Introduction The spontaneously formed stable vesicles were first observed from dilute single-tailed cationic and anionic (catanionic) surfactant mixtures in 1989.1 Since then, many catanionic surfactant mixed systems have been intensively investigated2-6 because they represent the typical models of surfactants in solutions. When the mixing composition and the ratio are varied, catanionic surfactant mixtures show fascinating phase behaviors such as micelle phases (L1 phases) with a variety of shapes, lamellar phases (LR phases) including vesicle phases and planar bilayer phases, and bicontinuous sponge phases (L3 phases) because of the transformation of aggregate geometric features. Two kinds of catanionic surfactant mixed systems can be constructed, i.e., with or without salts in solution.2 Vesicle phases with and without surface charges on the bilayer membranes have already been studied in salt-free surfactant mixed systems: (i) Salt-free vesicle-phases without surface charges, i.e., the bilayer membranes zero charges, can be obtained by adding equimolar water-insoluble fatty acids such as lauric acid (LA) into micellar solution of single-tailed cationic surfactant such as tetradecyltrimethylammonium hydroxide (TTAOH). In this case, the Hþ and OH- counterions can react to produce water (TTAOH þ LA f TTAL þ H2O) at equimolar ratio, leaving the cationic-anionic (catanionic) surfactant salt-free vesicle solutions.7 (ii) Salt-free vesicle phases with *Corresponding author. Fax: þ86-531-88564464; e-mail: [email protected]. cn. (1) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (2) Hao, J.; Hoffmann, H. Curr. Opin. Colloid Interface Sci. 2004, 9, 279. (3) Kaler, E. W.; Herrington, K. L.; Murthy, A. K. J. Phys. Chem. 1992, 96, 6698. (4) Gr€abner, D.; Zhai, L.; Talmon, Y.; Schmidt, J.; Freiberger, N.; Glatter, O.; Herzog, B.; Hoffmann, H. J. Phys. Chem. B 2008, 112, 2901. (5) Silva, B. F. B.; Marques, E. F.; Olsson, U.; Linse, P. J. Phys. Chem. B 2009, 113, 10230. (6) Michina, Y.; Carriere, D.; Charpentier, T.; Brito, R.; Marques, E. F.; Douliez, J. P.; Zemb, T. J. Phys. Chem. B 2010, 114, 1932. (7) Hao, J.; Liu, W.; Xu, G.; Zheng, L. Langmuir 2003, 19, 10635.

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surface charges of bilayer molecular membranes can be acquired by M2þ (Ca2þ, Ba2þ, and Zn2þ, etc.)-ligand coordination between anionic surfactants with M2þ and alkyldecyldimethylamine oxide such as the typical system of calcium dodecyl sulfate (Ca(DS)2) and single-chain zwitterionic tetradecyldimethylamine oxide (C14DMAO).8,9 In this case the NfO group of alkyldecyldimethylamine oxide can coordinate with anionic surfactant with M2þ counterions; i.e., at the appropriate mixing ratio regions M2þ-ligand coordination results in the formation of molecular bilayers, and M2þ ions are fixed on the bilayer membranes of the vesicles. There are nearly no free metal ions in the aqueous solutions. The vesicle phases with and without surface charges of bilayer membranes cannot be shielded by excess salts because of salt-free aqueous solutions. More than a century ago, Hofmeister10 proposed the particular order of inorganic salts based on their abilities to precipitate hen egg-white protein, which is now well-known as Hofmeister series.11-14 Homeister explained in his paper that the different effects of salts with the same ionic strength were a result of the differences of their water withdrawing power.13,14 In 1997, Collins et al. put forward the law of matching water affinities,15-17 indicating that oppositely charged ions with approximately equal free energy of hydration preferred forming ion pairs. In aqueous solutions of catanionic surfactant systems, charged aggregates such as micelles and vesicles resemble biological macromolecules and proteins in (8) Hoffmann, H.; Gr€abner, D.; Hornfeck, U.; Platz, G. J. Phys. Chem. B 1999, 103, 611–614. (9) Song, A.; Hao, J. Curr. Opin. Colloid Interface Sci. 2009, 14, 94. (10) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (11) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81. (12) Zhang, Y.; Cremer, P. S. Curr. Opin. Chem. Biol. 2006, 10, 658. (13) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 1. (14) Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 19. (15) Collins, K. D. Biophys. J. 1997, 72, 65. (16) Collins, K. D. Methods 2004, 34, 300. (17) Collins, K. D. Biophys. Chem. 2006, 119, 271.

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biological systems. To a large extent, the variation of aggregates in both biological systems and catanionic systems is dominated by the intra- and interaggregates electrostatic interactions among charged headgroups which means that the systems are highly sensitive to salts.18 Much work of salt effects on the catanionic systems has been done, for instance, from the Kunz group,19-21 and the order of ions was demonstrated to depend on the excessive surfactant and its headgroup feature. When the anionic surfactant was in excess, only the cation specificity could be found;19,20 on the contrary, the anion specificity was just reported in the catanionic surfactant system with excessive cationic surfactants.21 The cation order of the catanionic system with an excess of SDS,19 which followed the Hofmeister series, was completely converse to that with an excess of SL20 because their headgroup natures differed. In the present study, tetradecyltrimethylammonium hydroxide (TTAOH) and decanoic acid (DA) were chosen to build a catanionic salt-free biomimetic surfactant system, because a certain amount of carboxylic acid is usually consisted in cellular membrane and the trimethylammonium group is analogous to choline group also frequently present in membranes.22 The hydrophilic salts (NaBr, KBr, and LiBr) and hydrophobic salts [(CH3CH2CH2CH2)4NþBr-, (CH3CH2)4NþBr-, and C6H4(OH)COONa] were chosen to investigate their effects to phase behavior and rheological properties of the salt-free catanionic TTAD mixtures, which may provide the information on a classification of either the hydrophilic ions or the hydrophobic ions according to their capacity to affect aggregation and the understanding of interactions between ions and self-assembled structures in the aggregation of salt-free catanionic surfactant solution. Furthermore, these results may be useful for investigations of specific salt effects in biological systems23,24 and designs of particular surfactant mixed systems to meet the demands in academic research and practical applications.

Experimental Section Chemicals and Materials. TTAOH stock solution was prepared from tetradecyltrimethylammonium bromide (TTABr, high purity grade, Sigma-Aldrich) aqueous solution by anion exchanger (Ion exchanger III, Merck) at room temperature following the detailed procedures described previously.7 Bromide ions were not detected by AgNO3 in the TTAOH stock solution, so the ion exchange with hydroxide ion is >99%. Decanoic acid (DA, Alfa Aesar, 99%) was used without further purification. Sodium bromide, potassium bromide, monohydrate lithium bromide, tetrabutylammonium bromide (TBABr), tetraethylammonium bromide (TEABr), and sodium salicylate (SS) were all purchased from Sinopharm Chemical Reagent Co., Ltd., analytical grade. The water used in the experiments was prepared by a UPHW-III-90T-type apparatus, and the resistivity is 18.25 MΩ 3 cm. Phase Behavior Study. Different amounts of DA were added to 100 mmol L-1 TTAOH micellar solution. Since DA was immiscible with water at room temperature, heating was needed to ensure the neutralization reaction completely. After the samples were equilibrated at T = 25.0 ( 0.1 °C in a biochemical incubator for at least 4 weeks, three typical phase regions; i.e., the L1 phase, the L1/LR two-phase, and the LR phase could be observed by (18) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267. (19) Renoncourt, A.; Vlachy, N.; Bauduin, P.; Drechsler, M.; Touraud, D.; Verbavatz, J. M.; Dubois, M.; Kunz, W.; Ninham, B. W. Langmuir 2007, 23, 2376. (20) Vlachy, N.; Drechsler, M.; Verbavatz, J. M.; Touraud, D.; Kunz, W. J. Colloid Interface Sci. 2008, 319, 542. (21) Vlachy, N.; Drechsler, M.; Touraud, D.; Kunz, W. C. R. Chim. 2009, 12, 30. (22) Blusztajn, J. K. Science 1998, 281, 794. (23) Clarke, R.; L€upfert, C. Biophys. J. 1999, 76, 2614. (24) Sachs, J.; Nanda, H.; Pereache, H.; Woolf, T. Biophys. J. 2004, 86, 3772.

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Sun et al. visual inspection and optically with and without crossed polarizers. After equilibrium, different amounts of hydrophilic and hydrophobic salts were accurately weighed to the L1 phase of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O, the L1/LR two-phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O, and the LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O. These samples were also equilibrated at T=25.0 ( 0.1 °C for at least 4 weeks. The phase behaviors induced by adding different kinds of salts were also established by visual observation and optically with and without crossed polarizers. Rheological Measurements. Rheological measurements were performed on a Haake RS6000 rheometer with a coaxial cylinder system (Z41 Ti) and a double-gap sensor system for extremely dilute samples (DG43 Ti) at 25 ( 0.1 °C. In oscillatory measurements for viscoelastic solutions, an amplitude sweep at a fixed frequency of 1 Hz was carried out prior to the following frequency sweep in order to ensure the selected stress, in which the sample solution was in linear viscoelastic region. The viscoelastic properties were determined by oscillatory measurements from 0.01 to 100 Hz. In steady shear experiments, the shear rate was typically increased from 0.01 to 1000 s-1 in a stepwise mode within approximately 10-35 min. To achieve equilibrium as far as possible, the rheometer was set to ensure the gradient to be less than 0.5(Δτ/τ)/ Δt % at each shear rate step, and the maximum waiting time was 60 s. Polarized Optical Microscopy Observations. Polarized optical microscopy observations were performed on an AXIOSKOP 40/40 FL (Zeiss, Germany) polarized microscope at 25.0 ( 0.1 °C. Samples were dropped into a 1 mm thick trough, which were then covered by another glass slide to avoid solvent evaporation. Other methods to prepare polarizing samples can follow the detailed description in our previous work.25

Transmission Electron Microscopy (TEM) Observations.

A drop of sample (∼4 μL) was dropped on TEM grid (copper grid, 3.02 mm, 200 mesh, and coated with Formvar film) and then stained with ∼4 μL of 2 wt % phosphotungstic acid solution. After drying the solution in air, TEM images were performed on a JEOL’s JEM 100 cx II TEM (Japan) at an accelerating voltage of 100 kV.

Results and Discussion Phase Behavior of 100 mmol L-1 TTAOH Micellar Solution Mixed with Different Amounts of DA.7. The phase behavior photographs of 100 mmol L-1 TTAOH micellar solution mixed with different amounts of DA are shown in Figure S1 of the Supporting Information. With the increase of DA concentration, one can clearly see the phase transitions from an L1 phase to an L1/LR two-phase with a birefringent LR phase at the top of an isotropic L1 phase and finally a bluish birefringent LR phase. Although the phase transitions obtained in this catanionic TTAOH/ DA/H2O system are similar to the TTAOH/Lauric acid (LA)/ H2O system,7,25 the nonidentical phenomena can also be observed: (i) DA can dissolve in 100 mmol L-1 TTAOH micellar aqueous solution until its concentration reaches 190 mmol L-1. Above this concentration, DA cannot be completely dissolved. However, for the 100 mmol L-1 TTAOH/LA/H2O system, the insoluble concentration of LA is 105 mmol L-1.7 (ii) Equimolar mixtures of cationic and anionic surfactants in the TTAOH/LA/H2O and TTAOH/DA/H2O systems lie in different phase regions. The sample of 100 mmol L-1 TTAOH/100 mmol L-1 LA/H2O is a single birefringent LR phase while the sample of 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O is an L1/LR two-phase with a birefringent LR phase on the top of an isotropic L1 phase. However, these differences do not affect the investigations of birefringent LR phases; for instance, the rheological properties of the birefringent (25) Li, H.; Hao, J. J. Phys. Chem. B 2008, 112, 10497.

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Figure 1. Rheograms of the oscillatory shear as a function of the angular frequency at τ = 2 Pa (a) for the L1 phase of 100 mmol L-1 TTAOH/

90 mmol L-1 DA/H2O and the corresponding Cole-Cole plot (b) (the scatter represents the experimental data and the solid line represents fit to the Maxwell model). Plots of the yield stress (τ) and viscosity (η) against the shear rate for two L1 phases of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O (c) and 100 mmol L-1 TTAOH/90 mmol L-1 DA/500 mmol L-1 NaBr/H2O (d).

LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O can be measured to compare with the equimolar LR phase of the TTAOH/LA/H2O system. The different phase regions between TTAOH/LA/H2O and TTAOH/DA/H2O systems with the same composition could be ascribed to the shorter chain length of DA. In our study, we choose three typical samples;an L1 phase of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O, an L1/LR two-phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O, and an LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O;to perform further investigations of phase behavior induced by addition of hydrophilic or hydrophobic salts. Aqueous Two-Phase Transitions Induced by Addition of Hydrophilic Salts. The sample of 100 mmol L-1 TTAOH/ 90 mmol L-1 DA/H2O is a single viscoelastic L1 phase. With the addition of NaBr, the single L1 phase was transferred into an L1/ L1 two-phase which should be an aqueous two-phase solution26,27 and finally a highly viscous L1 phase again at much higher NaBr concentration. The typical photographs of the phase transitions with the addition of different amounts of NaBr, from L1, to L1/L1, and finally to L1, are shown in Figure S2 of the Supporting Information. By contrasting the rheological properties of both samples, we found out that the viscous L1 phase of 100 mmol L-1 TTAOH/ 90 mmol L-1 DA/H2O was totally different from the L1 phase of 100 mmol L-1 TTAOH/90 mmol L-1 DA/500 mmol L-1 NaBr/ H2O. The rheological properties of surfactant solutions which reflect the variation of aggregates of surfactants28,29 can be greatly influenced by addition of salts. Figure 1a shows the rheograms of the 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O system, which suggest the characteristic features of wormlike micelles. In low frequencies of oscillatory regions, the curves can be described by the Maxwell model, where both storage modulus G0 and loss (26) Zhao, G.; Xiao, J. J. Colloid Interface Sci. 1996, 177, 513. (27) Yin, H.; Mao, M.; Huang, J.; Fu, H. Langmuir 2002, 18, 9198. (28) Hoffmann, H.; Ulbricht, W. In Structure-Performance Relationships in Surfactants; Kunio, K., Ueno, M., Eds.; Marcel Dekker Inc.: New York, 1997; p 285. (29) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933.

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modulus G00 increase with frequency, and yet the slope of G0 is almost twice as large as G00 . However, in high frequencies, G0 values are higher than those G00 and then tend to be a constant value, while G00 reaches a maximum and then decreases continuously. Nevertheless, sometimes it is difficult to determine how well the experimental data fit the Maxwell model from rheograms of frequency sweep. A Cole-Cole plot (G00 as a function of G0 ) provides a better way of illustrating the Maxwellian behavior.30,31 The semicircle feature of a Maxwell fluid can be expressed as    2 G0 2 G0 2 ¼ G00 þ G0 2 2 The plateau modulus G0 is given by G0 at high frequencies. The Cole-Cole plot in Figure 1b shows that the experimental data closely follow the Maxwell model except at high frequencies. Generally, the viscoelastic wormlike micelles behave like a Maxwell fluid;30,31 therefore, it can be deduced that the L1 phase of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O consists of wormlike micelles. Figure 1c,d shows the contrast measurements of the plots of the yield stress (τ) and viscosity (η) against the shear rate to determine the different deformation of the two L1 phases of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O and 100 mmol L-1 TTAOH/ 90 mmol L-1 DA/500 mmol L-1 NaBr/H2O. Unlike the former L1 phase, the latter high salinity L1 phase has no obvious linear viscoelastic region in stress sweep. When the sample was determined by rotation measurements from low shear rate to high shear rate after being added to the cylinder and equilibrated for 0.5 h, it showed shear thickening behavior. On the contrary, it showed shear thinning behavior when the sample was swept from high shear rate to low shear rate. This behavior of the L1 phase has been explained;25,28,32 that is, in the limited measuring time the (30) Shrestha, R.; Shrestha, L.; Aramaki, K. J. Colloid Interface Sci. 2008, 322, 596. (31) Song, A.; Hao, J. J. Colloid Interface Sci. 2011, 353, 231. (32) Yamashita, Y.; Maeda, H.; Hoffmann, H. J. Colloid Interface Sci. 2006, 299, 388.

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Figure 2. Photographs of phase transitions from the L1/LR two-phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O induced by addition of NaBr. LR phases with 10-50 mmol L-1 NaBr, L1/LR two-phases with 100 mmol L-1 NaBr, L1/L1 two-phases with 200-500 mmol L-1 NaBr, and finally L1 phases above 500 mmol L-1 NaBr. Without (upper row) and with (lower row) crossed polarizers. T = 25 ( 0.1 °C.

Figure 3. Negative-staining TEM images of the LR phase for 100 mmol L-1 TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr with different magnifications.

sample with high viscosity, low elasticity, and long recovery time may not reach steady state and probably showed the pseudoshear thickening behavior in low shear rates. Probably due to high concentration of salts shielding the electrostatic repulsion, the sample possessed a low elasticity and a relatively high viscosity. We inferred that the system with salts contained rodlike micelles which were not yet overlapping and hence were highly sensitive to shear forces to behave shear thickening. These intermediate aqueous surfactant two-phase samples included two separate phases.26,27 The top L1 phase could be dyed to obvious red by the hydrophobic dye Sudan II whereas the lower phase could not. The suspected reason was that most of the micelles existed in the upper phase. The addition of salts screened the electrostatic repulsion and then promoted forming rodlike micelles along with the phase separation. When the amount of salts was sufficient, the phase was dominated by rodlike micelles, and then a new L1 phase emerged. In fact, the elongated micelles of surfactants in solutions have been made visible by cryogenic electron microscope (cryo-TEM) observations. These pictures clearly show the shape and the persistence length of the rodlike micelles.33-35 In general, globular micelle solutions of surfactants are Newtonian liquids with low viscosity (η). η increases linearly with the volume of fraction of the particles according to Einstein’s equation

If elongated micelles are formed, highly viscous surfactant solution samples are often obtained and also have elastic properties. In this case, the zero shear viscosity η0 is caused by a transient network of entangled rods that is characterized by a shear modulus G0 (= νkT, ν is the number density οf entanglement points) and a structural relaxation time (τ) according to the equation η 0 ¼ G0 τ

where ηs is the viscosity of the pure solvent and Φ is an effective volume fraction that takes into account the hydration of the surfactants.

Salt-Induced Phase Transitions from the L1/Lr TwoPhase. As shown in Figure 2, with increasing concentration of NaBr the phase behavior underwent L1/LR two-phase, LR phase, L1/LR two-phase again, L1/L1 two-phase, and finally L1 phase. The polarizing micrographs of the birefringent LR phase (Figure S3 in the Supporting Information) show lots of Maltese crosses which is a characteristic feature of lamellar structures.36 The lamellar structures were further confirmed by the TEM images to be vesicles, as shown in Figure 3. The rheological measurements of the LR phase of 100 mmol L-1TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr/H2O were performed, as shown in Figure 4a, which presents the properties of a Bingham fluid. One can see that both G0 and G00 remain unchanged with the increasing frequencies, the storage modulus G0 is almost 10 times larger than the loss modulus G00 , and the complex viscosity, |η*|, decreases over the whole frequency range from 0.01 to 10 Hz with a slope of -1, which is a typical feature for viscoelastic vesicle systems.7,37-39 The plots of the yield stress (τ) and viscosity (η) against the shear rate are shown in Figure 4b, which determines the deformation of the LR phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr/H2O.

(33) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (34) Bernherm-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000, 104, 4005. (35) Yuan, Z.; Dong, S.; Liu, W.; Hao, J. Langmuir 2009, 25, 8974.

(36) Regev, O.; Guillemet, F. Langmuir 1999, 15, 4357. (37) Hao, J.; Hoffmann, H.; Horbaschek, K. J. Phys. Chem. B 2000, 104, 10144. (38) Horbaschek, K.; Hoffmann, H.; Thunig, C. J. Colloid Interface Sci. 1998, 206, 439. (39) Shen, Y.; Hoffmann, H.; Hao, J. Langmuir 2009, 25, 10540.

η ¼ ηs ð1 þ 2:5ΦÞ

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Figure 4. Rheogram of the oscillatory shear as a function of the angular frequency (a) at τ = 0.05 Pa; plots of the yield stress (τ) and viscosity (η) against the shear rate (b) for the LR phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr/H2O.

Figure 5. Rheogram of the oscillatory shear as a function of the angular frequency (a) at τ = 8 Pa, the corresponding Cole-Cole plot (b) (the scatter represents the experimental data and the solid line represents fit to the Maxwell model), and plots of the yield stress (τ) and viscosity (η) against the shear rate (c) for the L1 phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/2000 mmol L-1 NaBr/H2O.

Figure 6. Photographs of phase transitions from the birefringent LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O induced by addition of NaBr. LR phases with 0-50 mmol L-1 NaBr, L1/precipitates two phases with 10, 20, 50, and 100 mmol L-1 NaBr, LR phases with 200-2000 mmol L-1 NaBr. Without (upper row) and with (lower row) crossed polarizers. T = 25 ( 0.1 °C.

Figure 5 demonstrates that the L1 phase with 2000 mmol L-1 NaBr has the main rheological characteristics of Maxwell fluids similar to the described L1 phase of 100 mmol L-1 TTAOH/90 mmol L-1 DA/H2O (Figure 1a-c), which suggests that the L1 phase is also a viscoelastic wormlike micelle phase. However, a larger deviation from the Maxwellian behavior can be seen in this Cole-Cole plot (Figure 5b) at high frequencies as a result of more changes in microstructures and viscoelasticity of the L1 phase sample. Generally, the shielding of electrostatic repulsion leads to the fusion and the formation of larger aggregates, whereas the results were completely contrary. The similar salt-induced phase transition from a viscoelastic LR phase to an L1 phase was first studied by Kaler et al. in the salt containing catanionic CTAB/ SOS/H2O system.18 It was explained in their published paper18 that the mixed surfactant system could automatically adjust the aggregate composition with the increasing addition of electrolyte Langmuir 2011, 27(5), 1675–1682

to reach the lowest possible free energy state and micelles were thermodynamically favorite microstructures. Catanionic Vesicles with Excess Anions without Salts and with High Salinity. The originally birefringent LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O, via an L1/precipitate two-phase, and finally a birefringent LR phase, was induced by increasing of NaBr concentration, which is shown in Figure 6. Compared to the highly viscous and bluish birefringent LR phase without salts, the LR phase with high salinity was slightly less viscous but still bluish birefringent. The polarizing images of the two birefringent LR phases are shown in Figure 7a-c, showing the cross or strip textures which indicate the existence of a lamellar phase. And the microstructures of birefringent LR phase with high salinity were determined by negative-staining TEM observations, as shown in Figure S4 of the Supporting Information; one can obviously see the vesicles. DOI: 10.1021/la104181b

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Figure 7. Polarized micrographs of the LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O with NaBr: 0 (a), 10 mmol L-1 (b), and

2000 mmol L-1 (c).

Figure 8. Rheograms of the oscillatory shear as a function of the angular frequency [a (at τ = 0.1 Pa) and c (at τ = 0.01 Pa)]; plots of the yield stress (τ) and viscosity (η) against the shear rate (b, d) for the two LR phases of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O and 100 mmol L-1 TTAOH/120 mmol L-1 DA/2000 mmol L-1 NaBr/H2O, respectively. T = 25.0 ( 0.1 °C.

The rheological properties of the two birefringent LR phases without salts and with high salinity were measured. The data are shown in Figure 8a-d. From Figure 8a,b, the characteristic features of viscoelastic vesicle systems can be obtained,7,37-39 where the storage modulus G0 and loss modulus G00 are almost independent of frequencies, the G0 is about an order of magnitude higher than G00 , and the complex viscosity, |η*|, decreases over the whole frequency range from 0.01 to 10 Hz with a slope of -1. For comparison, Figure 8c,d shows the rheological properties of the birefringent LR phase of high salinity, which are quite different from those of the birefringent LR phase for 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O. The storage modulus G0 , loss modulus G00 , and |η*| are much less than those of the birefringent LR phase without salts (Figure 8a,b). G0 and G00 are almost the same, and both increase slightly with frequencies; however, |η*| decreases and then increases with frequencies. We have investigated the reversible phase transitions between salt-free catanionic vesicles and high salinity catanionic vesicles in our previous work.40 It was demonstrated by FF-TEM that the salt-free catanionic LR phase mainly contained polydisperse unilamellar vesicles7 but multilamellar vesicles of TTAOH and oleic (40) Shen, Y.; Hao, J.; Hoffmann, H. Soft Matter 2007, 3, 1407.

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acid (OA) mixtures.41 However, after adding salts to the salt-free catanionic vesicle phase, a two-phase of precipitates/L1 phase could be observed at equimolar salts, e.g., 100 mmol L-1 TTAL aqueous solution added 100 mmol L-1 NaBr. The precipitates consisting of densely packed multilamellar vesicles (MLVs) located on the top of an L1 phase.40 Then MLVs swelled into a birefringent LR phase containing abundant irregular vesicles at higher salt concentration.40 The mechanism of this phase transition could be explained by DLVO theory. The double-layer electrostatic repulsion stabilizes vesicles while van der Waals attraction drives vesicles to associate. When salts were added to the LR phase, the interaggregates electrostatic repulsion was shielded by the ions, and hence unilamellar vesicles were transferred into aggregated MLVs. However, why an LR phase emerged again at high salinity? It was because the van der Waals attraction was progressively decreasing at high salt concentration, leading to the swelling of bilayers.42 Besides, it was inferred that on the microscopic scale the continuously added electrolytes could reduce the electrostatic attraction (41) Song, A.; Dong, S.; Jia, X.; Hao, J.; Liu, W.; Liu, T. Angew. Chem., Int. Ed. 2005, 44, 4018. (42) Petrache, H.; Zemb, T.; Belloni, L.; Parsegian, V. Proc. Natl Acad. Sci. U.S. A. 2006, 103, 7982.

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Figure 9. Photographs of 100 mmol L-1 TTAOH/100 mmol L-1 DA mixed with the hydrophobic salts, tetrabutylammonium bromide (TBABr), tetraethylammonium bromide (TEABr), and sodium salicylate (SS). Without (upper row) and with (lower row) polarizers, T = 25.0 ( 0.1 °C.

among oppositely charged headgroups,43,44 resulting in the dissolvation of precipitates and the formation of LR phase again. Explanation of the Different Phase Transitions of the Two Lr Phases. With increasing electrolyte concentration, a vesicle to micelle transition was observed in the LR phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr/H2O, while a reversible vesicle phase transition was acquired in the LR phase of 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O. These differences indicate that the original vesicles of the two LR phase have different composition and surface charge density. Therefore, we deduced that the bilayer vesicles of the LR phase of 100 mmol L-1 TTAOH and 100 mmol L-1 DA added a certain amount of NaBr should be positively charged because fewer D- anions were involved in the vesicle bilayers due to their stronger hydrophilicity. With salts being added continuously, the Gibbs free energy of the electrostatic interaction (GE) was reducing. Aggregates can adjust their composition by incorporating more anionic surfactant chains to lower the Gibbs free energy of the hydrophobic interaction (GH) to balance the drop of GE.18 Unlike this LR phase, the bilayer vesicles of the LR phase of 100 mmol L-1 TTAOH and 120 mmol L-1 DA contain more D- anions and hence are negatively charged. No more anionic hydrophobic chain could be incorporated into the aggregates to balance the reduction of GE. Consequently, densely packed multilamellar vesicles are the preferred microstructure of the LR phase at higher salt concentration when anionic surfactant was in excess. Phase Transitions Induced by Hydrophilic and Hydrophobic Salts. Phase transitions of the sample for 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O with other hydrophilic salts (43) Villeneuve, M.; Kaneshina, S.; Aratono, M. J. Colloid Interface Sci. 2003, 262, 227. (44) Hao, L.; Nan, Y. Colloids Surf., A 2008, 325, 186.

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(KBr and LiBr) are shown in Figure S5 of the Supporting Information. One can see that the sequences of phase behaviors induced by KBr and LiBr are consistent with that induced by NaBr. The phase transitions induced by adding hydrophobic slats (CH3CH2CH2CH2)4NþBr-, (CH3CH2)4NþBr-, and C6H4OHCOONa) were studied and shown in Figure 9. It can be seen that the sequences of phase behaviors induced by the three hydrophobic salts are also consistent with those induced by the three hydrophilic salts (LiBr, NaBr, and KBr). Comparing phase behavior transitions induced by LiBr, NaBr, KBr, (C4H9)4NþBr-, (C2H5)4NþBr-, and C6H4OHCOONa, we concluded that different amounts of salts were needed for the change of phase behaviors. Since all were bromide salts, different salt effects were caused by differences of cations. By comparison, we summarized that the tetrabutylammonium cation showed the largest effect and others following: Nþ(C4H9)4 > Kþ, Liþ > Naþ > Nþ (C2H5)4. Cation Effects on Phase Transitions. The law of matching water affinities proposed by Collins et al. explains how specific ion-ion interactions work. It indicates that oppositely charged kosmotropic ions (small ions of high charge density) bind each other and form inner sphere ion pairs, and it is similar for chaotropic ions (large ions of low charge density).15 But when a kosmotropic ion and a chaotropic counterion encounter, they will depart and both of them tend to be close to water molecules.15 Compared with sulfate headgroups, carboxylate headgroups behaved more like kosmotropic ions owing to its smaller size and relatively high charge density. And then according to Collins’ theory, SDS headgroups should come closer to Csþ, following the Hofmeister series with a chaotropic ion behavior, whereas dodecylcarboxylate headgroups should bind more Liþ, consistent with the reversed Hofmeister series, which have already been totally DOI: 10.1021/la104181b

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demonstrated by previous experiments.19,20 When anionic surfactant was in excess, only cation specificity could be observed because on average cations could be much closer to negatively charged aggregate surfaces than anions.21 Just like dodecylcarboxylate headgroups, decanoate headgroups were also supposed to be kosmotropic ions and according to Collins’ concept D- should bind with small kosmotropic ions of high charge density. However, the above order of cations on phase transitions for the sample of 100 mmol L-1 TTAOH/100 mmol L-1 DA/H2O was not entirely in consistent with the Hofmeister series or the reversed Hofmeister series. In catanionic surfactant systems, ion specificity can be obtained attributable to two main reasons: (i) different capacities to screen the electrostatic repulsion among charged headgroups as discussed in the matching water affinities model proposed by Collins; (ii) different influences to the van der Waals attraction caused by different ionic polarizability. The latter one can be explained by matching of refractive index of ionic aqueous solution to the refractive index of bilayers of the surfactant mixtures. For the present system, we deduced that it was the different weakening of van der Waals attraction that caused the inconsistent order of cations on phase transitions. As for the 100 mmol L-1 TTAOH/120 mmol L-1 DA/H2O systems, the phase transition sequences induced by other hydrophilic salts and hydrophobic salts but SS are also the same as NaBr, except that an L1 phase finally emerged at high hydrophobic salt concentration probably due to some the hydrophobic groups of the hydrophobic salts embedded into the palisade layer. Therefore, the area per surfactant headgroup was increased and (45) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (46) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (47) Israelachvili, J. N.; Marcelja, S.; Horn, R. G. Q. Rev. Biophys. 1980, 13, 121.

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the critical packing parameter p defined as v/a0lc was decreased. An L1 phase finally emerged which was consistent with the theory proposed by Israelachvili et al.45-47

Conclusions Abundant phase behaviors could be induced by hydrophilic and hydrophobic salts in the catanionic surfactant system TTAOH/ DA/H2O. Aqueous two-phase transitions induced by addition of hydrophilic salts could be observed when the cationic surfactant was in excess. Two vesicle phases could be obtained; the vesicle phase at lower salinity was viscoelastic whereas another one at high salinity was much less viscoelastic. The two different LR phases with different vesicle compositions and surface charge densities behaved entirely different phase behaviors with the increasing concentration of salts. In this catanionic surfactant system, ion specificity of hydrophilic and hydrophobic salts was discussed and the order of added electrolytes was concluded. The results could be significant for the further understanding of the Hofmeister effects on catanionic surfactant systems. Acknowledgment. This work was financially supported by the NSFC (Grants 20625307 and 21033005) and National Basic Research Program of China (973 Program, 2009CB930103), and NFS of Shandong Province (Z2008B01). Supporting Information Available: Phase behaviors of samples with different compositions, polarized micrographs of the LR phase of 100 mmol L-1 TTAOH/100 mmol L-1 DA/10 mmol L-1 NaBr and TEM images of the LR phases for 100 mmol L-1 TTAOH/120 mmol L-1 DA/2000 mmol L-1 NaBr/H2O. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2011, 27(5), 1675–1682