Phase Behavior and Rheological Properties of Salt-Free Catanionic

J. Phys. Chem. B 2010, 114, 9795–9804

9795

Phase Behavior and Rheological Properties of Salt-Free Catanionic Surfactant Mixtures in the Presence of Bile Acids Changcheng Liu,† Jingcheng Hao,*,† and Ziyu Wu‡ Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, and Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: June 24, 2010

The phase behavior, rheological properties, and structures of two salt-free catanionic surfactant systems, tetradecyltrimethylammonium hydroxide ((TTA)OH)/lauric acid (LA)/H2O and cetyltrimethylammonium hydroxide ((CTA)OH)/LA/H2O, in the presence of deoxycholic acid (DeCA) were investigated and compared with the results of cholic acid (CA). Small-angle X-ray scattering, deuterium nuclear magnetic resonance, and rheological measurements were employed to monitor the phase structure and transition. The surface tension was used to investigate the surface activities of the bile acid/(TTA)OH and bile acid/(CTA)OH mixtures in dilute solutions. The results show that they have a minimum surface tension in a solution with excess cationic surfactant, and the critical micelle concentration decreases with an increase of the cationic surfactant chain length and hydrophobicity of the bile acids. At equimolar mixtures of DeCA and cationic surfactants, or DeCA being in excess, phase separation occurs with a large diameter of droplets in the upper phase and a small volume of viscous liquid in the bottom phase. Compared with CA systems, in the salt-free catanionic surfactant systems containing DeCA, phase transition from the birefringent LR phase to the L1 phase occurs at a high molar fraction of DeCA, and the viscosity is higher at the same molar fraction of bile acid, indicating the significant influence of the molecule structures of bile acids despite only one hydroxyl group difference. Shear thickening is observed in the LR region, and a gradual evolvement of aggregates is predicted. Longer chain cationic surfactant can also increase the shear viscosity, which could be ascribed to the increase of the critical packing parameter, but with less influence on the phase transition. Introduction Bile acids and their salts are natural surfactants in mammals which display vital biological importance in many physiological processes such as in intestinal hydrolysis, dispersion and digestionoflipids,cholesterolsolubilization,anddrugabsorption.1-4 They are composed of a rigid steroidal backbone with one or more R-oriented hydroxyl groups and an appended short aliphatic chain with a terminal carboxylic acid group. The R-oriented hydroxyl groups on the concave side of the steroidal backbone display as a hydrophilic plane, and the steroidal backbone with methyl groups on the convex side acts as a hydrophobic face. Thus, the aggregation of bile salts in aqueous solution and interaction with other aggregates are unique and fascinating. The structures and shapes of bile salt micelles and their aggregation numbers and critical micellar concentration (cmc) have been scrutinized over decades.5-18 It has been reported that the aggregation numbers of micelles formed by bile salts are small at low concentration and the sizes of the micelles increase as the monomer concentration is raised.3,6,10,13,14 Due to their biocompatibility and fascinating aggregation behavior in aqueous solution, the research of bile acids and their derivatives has aroused the infinite passion of scientists. The aggregates formed by bile salts and their derivatives19-29 and the interaction between bile salts and phospholipids,30-43 vesicles,44-48 cyclodextrin,49-51 and other surfactants or * To whom correspondence should be addressed. E-mail: jhao@ sdu.edu.cn. Fax: +86-531-88564750. † Shandong University. ‡ Chinese Academy of Sciences.

additives52-60 have been widely investigated. Different from the extensive study of bile salts and their derivatives, the study of pure bile acids and their interaction with other aggregates, especially vesicles, is very scarce in the literatures due to their insolubility in water. The flow behavior and viscoelastic properties of the bile systems are considered to be of great importance for understanding the vital biological functions and practical applications. Thus, the rheological properties of such systems should be studied in more detail. We have studied the influence of CA on the phase transitions, rheological properties, and structures of the tetradecyltrimethylammonium hydroxide ((TTA)OH)/lauric acid (LA)/H2O system.61 We found that at equimolar mixtures of cationic ((TTA)OH) and anionic (cholic acid (CA) and LA) surfactants, with the variation of the molar fraction of CA (x ) nCA/(nLA + nCA)), the system underwent a phase transition from vesicles to micelles. These results induced by CA are quite different from the influence of the corresponding bile salt.44-48 Inspired by the interesting results, we proceeded to study the influence of different bile acids on the phase behavior and rheological properties of salt-free catanionic vesicles for obtaining theoretical concepts to understand physiological processes from the point of view of surfactants. We chose the familiar bile acid deoxycholic acid (DeCA) as a contrastive compound for which the hydroxyl group at 7-C of the steroidal backbone is absent compared with CA. Despite the small distinction in chemical structures, the properties, especially the hydrophobicity, vary significantly. The influence of the chain length of the cationic surfactants and the hydrophobicity of the bile acids was focused

10.1021/jp103916a  2010 American Chemical Society Published on Web 07/09/2010

9796

J. Phys. Chem. B, Vol. 114, No. 30, 2010

on. The effect of bile acids CA and DeCA on the phase behavior and rheological properties of (TTA)OH [or cetyltrimethylammonium hydroxide ((CTA)OH)]/LA/H2O systems was contrastively studied. The salt-free catanionic surfactant vesicles of TTAL or CTAL were prepared first. Two methods were employed to study the phase behavior: First, LA was gradually replaced by the bile acid CA or DeCA until LA was replaced completely, but the total molar amounts of bile acid and LA were fixed, i.e., an equimolar cationic and anionic surfactant system. With the variation of the molar fraction of bile acid, x (x ) nbile/(nbile + nLA)), the system underwent a phase transition from a viscoelastic LR phase to an L1/LR two-phase system to a viscous and dilute L1 phase. Second, different amounts of DeCA or CA were added into the TTAL (or CTAL) vesicle solutions to monitor the solubilization of bile acid in the vesicles. The surface tension measurements of the mixed solutions of bile acid and (TTA)OH (or (CTA)OH) at different molar ratios were also carried out to investigate the surface activities of the mixed systems and the interaction at the air/water interface. Experimental Section Chemicals and Materials. LA (high purity grade) was purchased from Tokyo Chemical Industry Co. Ltd.; (TTA)OH and (CTA)OH stock solutions were prepared from (TTA)Br (AMRESCO, high-purity grade) and (CTA)Br (Merck, analytical grade) aqueous solution, respectively, by anion exchange (Ion Exchanger III, Merck) following the procedures described previously.62 All reagents were used as received. The water used in the experiments was prepared by a UPHW-III-90T-type apparatus, and the resistivity is 18.25 MΩ · cm. For the preparation of (CTA)OH, the ion exchange column was held at 50 °C due to the low solubility of (CTA)Br at room temperature. CA (>99%) and DeCA (>98.5%) were purchased from Fluka and Acros Organics, respectively. Phase Behavior Study. The phase behavior was studied by visual inspection and optically with crossed polarizers. In the first method, solid LA and bile acid were added into empty test tubes with screw caps, and then the desired volume of (TTA)OH or (CTA)OH stock solution at fixed concentration was added into the tubes. In the second method, the desired amounts of solid bile acid were added into prepared TTAL vesicle solutions. Heating to about 60 °C and mixing by a vortex mixer were performed to accelerate the dissolution and equilibrium process. For samples with trapped air bubbles, centrifugation at 2000 rpm was performed to expel the bubbles. After that, the samples were kept at 25 °C in a biochemical incubator for at least 6 weeks before the phase behavior was inspected. Polarization Optical Microscopy (POM). Polarized microscopy observations were carried out on an AXIOSKOP 40/ 40 FL (ZEISS, Germany) microscope at room temperature. Samples were prepared by dropping several drops of solutions into a 1 mm thick trough, which was then covered by another glass slide to avoid solvent evaporation. Small-Angle X-ray Scattering (SAXS) Measurements. SAXS experiments were carried out at 298 K at Beamline 4B9A at the Beijing Synchrotron Radiation Facility, using an SAXS apparatus constructed at the station. The incident X-ray wavelength was 1.54 Å, and the imaging plate was a Mar 345 with a resolution factor of 3450 × 3450. The obtained twodimensional scattering pattern of SAXS consisted of concentric circles, and the intensity (I) versus scattering vector (q) profile was independent of the azimuthal angle. The range of the scattering vector was chosen from 0.05 to 4 nm-1 (q ) [4π sin(θ/2)]/λ, where θ and λ are the scattering angle and the

Liu et al. wavelength, respectively). The distance from the sample to the detector was 1730 mm, and the data accumulation time was 1800 s for each sample. Deuterium Nuclear Magnetic Resonance (2H NMR). The samples for 2H NMR measurements were prepared in H2O as stated above first and then freeze-dried at about -50 °C in a vacuum of 1 Pa. After that, a suitable amount of such prepared solid was weighed into a vial, and 1 mL of D2O was added. The samples were heated and subjected to ultrasonic treatment to dissolve and then were transferred into 5 mm NMR tubes which were held at 25.0 ( 0.1 °C for 2 weeks. The 2H NMR spectrum was recorded on a Bruker Avance 400 spectrometer equipped with a pulsed field gradient module (z-axis). The experiments were operated at 25.0 ( 0.1 °C. The 2H NMR technique probes the motionally averaged electric quadrupole couplings between the deuterium nuclei (spin I ) 1) and the electric field gradients at the sites of the observed nuclei.63 The D2O molecules in the solution have two species: free in the isotropic environment and bound to the hydrophobic/ hydrophilic interface of the aggregates. The two species have a fast exchange in the two sites. When D2O molecules experience a macroscopically anisotropic environment, the quadrupolar interaction has a nonzero average, leading to a splitting of the resonance.63 Surface Tension Measurements. The surface tension measurements were carried out on a thermostated Kru¨ss K100 processor tensiometer (the precise degree of measurement is 0.01 mN · m-1) using the Wilhelmy plate at 25.0 ( 0.1 °C. The plate was cleaned with distilled water and flamed before each measurement. The surface tension of deionized water (the resistivity is 18.25 MΩ · cm) was measured to calibrate the tensiometer and to check the cleanliness of the sample pool. In the experiments, 50 mL of deionized water was added to the sample pool, and then the desired volume of bile acid/ (TTA)OH (or (CTA)OH) stock solution was injected into the water by a microsyringe. After this, stirring for 2 min was performed. Then the solution was held in the sample pool at 25 °C for 5 min before the measurements. Rheological Measurements. Rheological experiments were carried out on a Haake RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). The diameters of the rotor and the shear cell are 41.420 and 43.400 mm, respectively. In steady shear experiments, the shear rate was typically increased from 0.001 to 1000 s-1 in a stepwise mode. In oscillatory measurements for viscoelastic samples, an amplitude sweep at a fixed frequency of 1 Hz was performed prior to the following frequency sweep to ensure the selected stress was in the linear viscoelastic region. The volume of each sample used for the measurements was about 12 mL. The experiments were performed at 25 °C. All samples for rheological measurements were prepared at least 6 weeks before the measurements. Results and Discussion Surface Tension of (TTA)OH or (CTA)OH Solutions in the Presence of Bile Acids. The isotherms of surface tension vs concentration for CA/(TTA)OH, CA/(CTA)OH, and DeCA/ (TTA)OH solutions at different molar ratios are presented in Figure 1. In each system of Figure 1a, the surface tension decreases to a minimum value with an increase of the concentration, which is followed by an increase before it achieves a stable value. The minimum surface tension increases with an increase of the molar ratio of CA to (TTA)OH. In an equimolar CA and (TTA)OH solution, the surface tension curve

Salt-Free Catanionic Surfactant Mixtures

J. Phys. Chem. B, Vol. 114, No. 30, 2010 9797

Figure 1. Isotherms of surface tension vs concentration of CA/(TTA)OH (a), CA/(CTA)OH (b), and DeCA/(TTA)OH (c) in mixed solutions at different molar ratios. T ) 25.0 ( 0.1 °C.

is monotonously decreased before it achieves a stable value. Thus, it can be concluded that the minimum surface tension is caused by the excess cationic surfactant in the system. When the molar ratio of CA to (TTA)OH is lower than 1, the sample is a mixed micelle solution composed of CA-(TTA)OH ionic pair micelles and (TTA)OH monomers (the concentration of (TTA)OH in the solution is lower than its cmc of 1.8 mmol · L-1).64 Because of the dissimilar molecule structures and huge difference in surface activity between CA and (TTA)OH, the binding between excess highly surface active (TTA)OH and CA-(TTA)OH ionic pairs can further decrease the surface tension to a minimum value. The cmc values of these systems are small (with a magnitude of 10-4 mol · L-1). Similar results are observed in CA/(CTA)OH and DeCA/(TTA)OH systems. In Figure 1b, with a longer alkyl chain of the cationic surfactant, the cmc values of mixed systems can be further decreased (about 10-5 mol · L-1), but the surface tension after the cmc is almost the same as that of the CA/(TTA)OH system. In the more hydrophobic bile acid DeCA/(TTA)OH systems (Figure 1c), the cmc values also become smaller, and the surface tension after the cmc is lower than that of the CA/(TTA)OH system. The cmc values of these mixed systems are much smaller than those of (TTA)OH, (CTA)OH, and the corresponding bile salts10,17,18 and also smaller than that of the corresponding (CTA)Br-bile salt system in 50 mmol · L-1 NaCl solution at a low molar ratio.65 From the facially amphiphilic character of bile acids, at low concentrations, the bile acids lie flat at the air/water interface with the hydrophilic R-face contacting the water, which occupies a large area of the interface. Hence, the cmc values of the mixed systems are much smaller. Compared with the salt-containing (CTA)Br-bile system,65 the unscreened electrostatic interaction between the oppositely charged headgroups makes the alignment of ionic pairs at the air/water interface not so dense, which leads to a smaller cmc value. Optical Microscope Observations of Spherical Aggregates. It is interesting that, in the DeCA/(TTA)OH and DeCA/ (CTA)OH systems, when the molar ratio of (TTA)OH to DeCA is 1 or slightly lower than 1, such as 0.95, the solution separates into two phases with a small volume of viscous liquid in the

bottom phase. The aggregates formed in the upper phase were observed by optical microscopy, and the images are presented in Figure 2. Spherical droplets are observed in both DeCA/ (TTA)OH and DeCA/(CTA)OH systems, the majority of which are several micrometers in diameter. The diameter of these aggregates formed by the longer chain cationic surfactant (CTA)OH (Figure 2b) is larger than that of (TTA)OH (Figure 2a). For the sample with a molar ratio of (TTA)OH to DeCA of 0.95 (Figure 2c), the diameter and the polydispersity both become larger. The spherical aggregates are very stable, and there is no obvious variation even after several months. When additional (TTA)OH or (CTA)OH was added into the above solutions, the samples became homogeneous and no aggregates could be found under the optical microscope. According to the critical packing parameter theory,66 P ) υ/a0l0, due to the large steroidal skeleton of the DeCA molecule, the P value of the DeCA-(TTA)OH ionic pair is larger than 1; thus, it is inclined to form a reverse structure, but the aqueous environment hinders the tendency. Thus, most of the aggregates separated into a condensed viscous phase, and the residual surfactants in the upper phase formed droplets in the emulsificationofsurfactants.Duetothedifferenceinhydrophilic-hydrophobic properties between CA and DeCA, at the same conditions, the (TTA)OH (or (CTA)OH)/CA system did not separate into two phases, but a precipitate was formed when CA was in excess. Phase Behavior of (TTA)OH Mixed with Equimolar DeCA and LA at Different Molar Fractions of DeCA (x). The evolution of the phase behavior of the (TTA)OH/(DeCA + LA)/H2O system as a function of x (x ) nDeCA/(nDeCA + nLA)) in a 100 mmol · L-1 (TTA)OH solution is shown in Figure 3. One can see the phase transition with a decrease of x: dilute or viscous micelles to an L1/LR two-phase system to the LR phase. Similar phase behavior was observed at a low concentration of 50 mmol · L-1. The phase transition from the L1 phase to the LR phase occurs at about 0.35 for both (TTA)OH concentrations. In the LR region, different birefringence textures with variation of x are observed, indicating different types of aggregates. H. Hoffmann et al.67 investigated the effect of the charge density on the phase transition of the tetradecyldimethylamine

9798


Liu et al.

Figure 2. Images of aggregates in the upper phase of 50 mmol · L-1 (TTA)OH/50 mmol · L-1 DeCA (a), 50 mmol · L-1 (CTA)OH/50 mmol · L-1 DeCA (b), and 47.5 mmol · L-1 (TTA)OH/50 mmol · L-1 DeCA (c). The bar in the images represents 50 µm.

Figure 3. Phase behavior of 100 mmol · L-1 (TTA)OH mixed with equimolar DeCA + LA with the variation of x: top, without polarizers; bottom, with polarizers.

oxide (C14DMAO)/(TTA)Br/1-hexanol/water system. They found that when the ratio of (TTA)Br to C14DMAO varied, the system underwent a planar lamellar phase to planar lamellar/vesicle coexistence to vesicle structure transition. The birefringence pattern between crossed polarizers also varied corresponding to the structure transition. In the present system of 100 mmol · L-1 (TTA)OH micelle solution mixed with equimolar DeCA + LA, similar birefringence patterns are observed with the variation of x. At x between 0.35 and 0.25, the samples exhibit a typical domainlike birefringence pattern between crossed polarizers. It has been stated by H. Hoffmann et al.67,68 that the domainlike birefringent pattern is due to the presence of lamellar structures, and its intensity corresponds to the amount of lamellae present. Thus, at x between 0.35 and 0.25, the planar lamellar structure is dominant in the solution, which also can be proved by the 2H NMR results and rheological properties in the latter. At x ) 0.2, the birefringence texture is changed, and at x < 0.2, the birefringence is weak, which indicates the vesicles are the main form in the solutions.67,68 The SAXS results in Figure 4 prove the lamellar structure of the aggregates. At x )

0.2 (Figure 4a), q1:q2 ) 1:2, which accords with the characteristic of lamellar structures. The distance of interlayers calculated from d ) 2π/q1 is about 43.0 nm, which indicates the hyperswollen structure. However, at x ) 0.3 (Figure 4b), only one obvious peak is obtained, and the position of the peak is identical with the first-order peak of x ) 0.2. The formation of the planar lamellar phase can be proved by 2 H NMR spectroscopy. Some 2H NMR spectra are shown in Figure 5. For the samples with a low concentration (Figure 5a,b), the spectra exhibit a single sharp peak; while at a higher concentration (Figure 5c,d), splitting of the peak is observed. At the same molar fraction of DeCA, the samples exhibit similar polarization textures at low and high concentration; thus, the aggregate structures are the same at different concentrations. In the long-range-ordered, macroscopically anisotropic planar lamellar phase solutions, D2O molecules in the anisotropic site experience a nonzero average of the quadrupole interaction; thus, doublet quadrupole splitting is seen in the phase. In vesicle solutions, a single peak is usually observed, because D2O molecules in the interface of closed bilayers exhibit local anisotropy rather than macroscopic anisotropy, but the single peak does not always indicate the existence of vesicles. In dilute solutions, such as 100 mmol · L-1 in the current case, due to the small amount of surfactants in the solution, the thickness of the hydrophilic layer is larger (see the SAXS results), which causes a less anisotropic environment of the individual molecules.69 In addition, a disordered lamellar phase consisting of many domains of extended flat layers whose orientations are isotropically distributed in space also can result in a less anisotropic environment. Therefore, a single peak is seen in the 2 H NMR spectra at low concentration. The formation of the planar lamellar phase and vesicles also can be predicted from the theoretical side. After the complete reaction of (TTA)OH with DeCA and LA, (TTA)OH-LA and (TTA)OH-DeCA ionic pairs were formed, and the pH value of the solution was about neutral. According to the critical



Figure 4. SAXS curves of the 100 mmol · L-1 (TTA)OH micelle solution mixed with equimolar DeCA + LA at x ) 0.2 (a) and 0.3 (b).

Figure 5. 2H NMR spectra of samples at different molar fractions of DeCA and different concentrations: (a) x ) 0.25, C ) 100 mmol · L-1; (b) x ) 0.3, C ) 100 mmol · L-1; (c) x ) 0.25, C ) 970 mmol · L-1; (d) x ) 0.3, C ) 990 mmol · L-1.

packing parameter theory,66 similar to the CA system we have explained,61 the P value of the (TTA)OH-DeCA ionic pair is larger than 1, while it is in the range of 0.5-1 for the (TTA)OH-LA ionic pair; thus, with a decrease of the DeCA molar fraction and increase of the LA content, the P value of the mixed system adjusts. In a range of DeCA molar fractions, the P value of the mixed system is about 1, which favors formation of the planar lamellar structure. A further decrease of the DeCA molar fraction causes the P value to be smaller than 1. Then the bilayer membranes close and vesicles form. Typical polarization optical microscopy images for samples of x ) 0.3, 0.25, and 0.1 at 100 mmol · L-1 (TTA)OH (Figure S1a-c, Supporting Information) show a typical striped texture, demonstrating the formation of a lamellar structure.70 The influence of CA on the (TTA)OH/LA/H2O system at 100 mmol · L-1 (TTA)OH is also examined. The phase transition from the L1 phase to the LR phase occurs at xCA ) 0.2, which is smaller than that of the DeCA system at the same concentration. These results indicate different influences of the two bile acids on the salt-free catanionic surfactant system, despite only one hydroxyl group distinction in structure. Moreover, at different concentrations of (TTA)OH, the phase transition almost occurred at the same molar fraction of bile acid. Hence, the structure of bile acid, i.e., hydrophobicity, is the primary factor for the difference in the phase transition, while the influence of the concentration is less. The absence of a hydroxyl group at 7-C on the sterol backbone of the DeCA molecule makes it

more hydrophobic, and the volume of the hydroxyl groups occupied also becomes smaller. When these molecules replace LA, they can pack more densely in the bilayers than CA molecules, so the spontaneous curvature of the membrane varies slowly with an increase of the DeCA molar fraction. This leads to a wider LR region and a gradual transition from the LR phase to the L1 phase. The solubilization of DeCA in the TTAL vesicle solution is also examined. At concentrations of DeCA lower than 15.3 mmol · L-1, the solution is still in the single LR phase with a somewhat turbid appearance. When it reaches 19.1 mmol · L-1, phase separation occurs with the precipitate in the upper phase. The Schlieren birefringence texture between crossed polarizers is observed, indicating the vesicle structure can be maintained in the presence of DeCA but the fluidity varies. Typical polarization photos of 100 mmol · L-1 TTAL with different concentrations of DeCA are shown in Figure S1 of the Supporting Information. Image d shows a crossed pattern texture which is a feature of lamellar structures. Images e and f show Malthesian crosses, which are features of multilamellar vesicles, but the diameters change with the concentration of DeCA, indicating different diameters of close-packed spherulites.71 It can be speculated that, with an increase of DeCA in the TTAL vesicle solution, the structure of the vesicles modulates slightly. Rheological Properties of (TTA)OH/(DeCA + LA) Systems as a function of x at Different Concentrations. The rheological properties at different x values can reflect the

9800


Liu et al.

Figure 6. Shear viscosity vs shear rate curves (a) and the variation of yield stress (b) as a function of x for 100 mmol · L-1 (TTA)OH mixed with equimolar DeCA + LA.

characteristics of different aggregates. As shown in Figures 3, the phase transition can be induced by an increase of x from 0 to 0.5, which also can be monitored by rheological data. For 50 mmol · L-1 (TTA)OH mixed with equimolar DeCA + LA (Figure S2, Supporting Information), at x > 0.35, the solutions behave as wormlike micelles: Newtonian fluid behavior at a wide shear rate range followed by shear thinning at a high shear rate.72-75 The zero-shear viscosity (viscosity at the Newtonian fluid region) increases first and then decreases with a decrease of x. At x < 0.3, shear thinning behavior is observed in the whole shear rate range. Compared with the results we have obtained for the (TTA)OH/(CA + LA) system,61 at the same molar fraction of bile acid, the zero-shear viscosity is higher and the shear rate range of the Newtonian fluid is larger. When the concentration is increased to 100 mmol · L-1 (Figure 6a), in the wormlike micelle region, the zero-shear viscosity is higher, and the shear rate at which the shear thinning behavior is initiated moves to a higher value. The formation of wormlike micelles we speculate is caused by the following: due to the steric hindrance of the large steroidal skeleton, at a high molar fraction of DeCA, the mixed micelles are small with a low aggregation number and may adopt a helical structure or polymer-like aggregates by the intermolecular hydrogen bonding between the hydroxyl and the carboxyl group of the other molecule together with partial hydrophobic interaction (backto-back), which have been determined in sodium deoxycholate solution.5,7,11 With a decrease of the DeCA content, the (TTA)OH-LA ionic pairs suppress the disorderly dispersion of the mixed micelles; thus, the aggregates favor onedimensional micellar growth, and the viscosity increases. In fact, the formation of threadlike cylindrical micelles in alkyltrimethylammonium halide (CnTAX)-bile salt systems at low concentration has been observed;53-55 thus, in current salt-free catanionic surfactant mixtures of a condensed solution, the wormlike micelles and more profound micellar growth could be predicted, which should lead to the formation of a more strengthened network. After the maximum value of zero-shear viscosity is approached, a further decrease of the DeCA molar fraction leads to a decrease of the viscosity due to formation of micellar joints in the network structure. At x between 0.35 and 0.2, shear thinning including a shear thickening behavior at an intermediate shear rate range is observed. We have shown that the planar lamellar phase is formed at this range. Evidence given in the literature indicates that vesicles can be prepared from the classical stacked lamellar phase by shear,67,68,76-79 resulting in a higher viscosity. It has been pointed out that80 the lamellar phase usually contains many defects especially in the vicinity of phase transition boundaries81 in the form of dislocation loops82 or thermodynamically stable defects where the lamellar bilayer is highly curved,83 and the

most common defect in lyotropic lamellar systems is edge dislocations.84,85 As an effect of shear, the lamellar phases with defects tend to reorient and form vesicles, which result in an increase of viscosity. At x < 0.15, shear thinning behavior is observed with a gentle variation in the intermediate shear rate range. Yield-stress values are relative to different kinds of interactions between the molecules which confer a three-dimensional internal structural network.86 The yield stress in the experiments is obtained by the turning points of the stress-deformation curves in the double-logarithm mode. The yield stress for samples of 100 mmol · L-1 (TTA)OH solution mixed with equimolar DeCA + LA as a function of x in the LR region (x e 0.35) is shown in Figure 6b. At x ) 0.35 and 0.3, the values are very low, which accords with the characteristic of the classical lamellar phase. At x < 0.15, the values are at the same level as those of 100 mmol · L-1 TTAL, which indicates highly viscoelastic vesicles exist in the solution. At x between 0.25 and 0.2, the values are at a moderate level, which can be considered as the coexistence of the classical lamellar phase and vesicles. The evolvement of yield stress is consistent with the variation of the phase behavior and shear viscosity, which can prove the gradual transition of aggregate structures with the variation of x. The viscoelasticity of the systems is characterized, and the variation of the elastic modulus G′ is shown in Figure S3 of the Supporting Information. At both concentrations, for samples of x > 0.25, they are low viscoelastic solutions. G′ increases with the frequency in the measured range. The low values of G′ accord with the features of the classical lamellar phase and are consistent with the results of the phase behavior and viscosity characteristic. For samples of x < 0.2, high viscoelastic features are observed, and G′ is more than 1 order of magnitude larger than that of x > 0.25 at low frequency. The relationship between G′ and G′′ shows the typical feature of vesicles.87-90 Rheological Properties of (CTA)OH/(DeCA + LA) Systems. To investigate the influence of the chain length of the cationic surfactant, the phase behavior and rheological properties of the salt-free catanionic (CTA)OH/(DeCA + LA)/H2O system at equimolar (CTA)OH and DeCA + LA are examined. The evolution of the phase behavior with the molar fraction of DeCA is similar to that of the (TTA)OH/(DeCA + LA)/H2O system. The phase transition from the LR phase to the L1 phase occurs at about x ) 0.3, which is smaller than that of the (TTA)OH system. The variation of viscosity as a function of x is shown in Figure 7. In the wormlike micelle region (Figure 7a), the zero-shear viscosity was increased distinctly at the same DeCA molar fraction. In the LR region (Figure 7b), rheological behavior similar to that of the (TTA)OH/(DeCA + LA)/H2O system is observed (Figure 6a) and the viscosity is also increased.



Figure 7. Curves of shear viscosity vs shear rate for the 100 mmol · L-1 (CTA)OH solution mixed with equimolar DeCA + LA as a function of x.

Figure 8. Curves of shear viscosity vs shear rates for the 100 mmol · L-1 (TTA)OH solution mixed with equimolar CA + LA as a function of x.

The only difference between the two systems is the chain length of the cationic surfactant; thus, the increase of the zeroshear viscosity is caused by the chain length. According to the critical packing parameter theory66 and the equations of υ and l091

υ (nm3) ) (27.4 + 26.9n) × 10-3

(1)

l0 (nm) ) (0.15 + 0.1265n)

(2)

where n is the number of carbon atoms in the alkyl chain, the P value for (CTA)OH is a little larger than that of (TTA)OH; thus, the tendency to form threadlike cylindrical micelles is larger for the (CTA)OH/(DeCA + LA)/H2O system than for the (TTA)OH/(DeCA + LA)/H2O system, indicating that, at the same molar fraction of DeCA, a more strengthened network of aggregates is formed, which leads to an increase of the viscosity. Influence of the Molecule Structures of Bile Acids. The effect of the molecule structures, i.e., hydrophobicity, of bile acids is examined. As has been referred to above, due to the difference in the hydrophobicity, the phase transition from the LR phase to the L1 phase occurs at different x values for the two bile acids. For the system containing CA, the phase transition occurs at a lower x value (around x ) 0.2) with a narrow LR region. The different influences also can be reflected by rheology. The shear viscosity curves as a function of the molar fraction of CA for equimolar (TTA)OH and CA + LA systems are shown in Figure 8. In the wormlike micelle region, the zero-shear viscosity increases first and then decreases with x from 0.5 to 0.25, similar to the case of DeCA and other wormlike micelles,72-75 but the values of zero-shear viscosity are much lower in the current case at the same molar fraction of bile acids. In the LR region, shear thinning including shear thickening behavior for some samples is observed.

Compared with the results for the system containing DeCA in Figure 6, the viscosity is lower at the same molar fraction of bile acids. This is ascribed to the decrease of the hydrophobicity of CA compared to DeCA. When LA molecules are replaced by bile acid in the bilayer membrane, the contact of the hydrophilic R-face of the bile acid with aliphatic chains of (TTA)OH or LA is unavoidable due to the formation of ionic pairs by reaction between (TTA)OH and bile acid. In addition, the insertion of the large (TTA)OH-bile acid ionic pairs decreased the curvature of the bilayer membranes. Compared with the case of the two-hydroxyl DeCA, the interaction between aliphatic chains of (TTA)OH and the three-hydroxyl CA is weaker, which results in the packing of the surfactants in the bilayers being not as dense as in the case of DeCA. Thus, the interfacial curvature has a larger variation at the same molar fraction of bile acids, which results in a narrow region of the LR phase and a lower viscosity. In the wormlike region, at the same molar fraction of bile acids, the more hydrophobic DeCA system prefers one-dimensional growth and can form more elongated wormlike micelles, and a more strengthened network is formed. Hence, a distinct increase of zero-shear viscosity is observed. It has been referred to above that the existence of bile acids in the bilayers leads to the alignment of the surfactants being looser than that of TTAL vesicle membranes. As a result, the viscoelastic properties of the system will decrease, which is proved by the oscillatory experiments. With an increase of x, decreases of G′, G′′, and the complex viscosity are observed as shown in Figures 9 and 10. It is interesting that, at x ) 0.1, G′ < G′′ at low frequency while G′ > G′′ at high frequency for both the (TTA)OH/(CA + LA)/H2O and (CTA)OH/(CA + LA)/ H2O systems. At x < 0.1, the sample exhibits the typical character of vesicles,87-90 while at x > 0.1, it is a low viscoelastic solution with G′ a little larger than G′′, and G′ is about 1 order of magnitude smaller than that of vesicles. In the DeCA-containing system, at x < 0.2, the samples exhibit the typical character of vesicles and higher viscoelastic

9802


Liu et al.

Figure 9. Oscillatory rheograms of the 100 mmol · L-1 (TTA)OH solution mixed with equimolar CA + LA at x ) 0 (a), 0.1 (b), 0.15 (c), and 0.2 (d).

Figure 10. Oscillatory rheograms of the 100 mmol · L-1 (TTA)OH solution mixed with equimolar DeCA + LA at x ) 0.3 (a), 0.25 (b), 0.2 (c), and 0.15 (d).

properties. At x > 0.2, the solutions also exhibit viscoelasticity, but it is lower than that of the samples at x < 0.2; G′ increases a little with frequency, and G′′ is independent of the frequency at low frequency while it increases steeply at higher frequency, which is in accord with the feature of the planar lamellar phase. Influence of DeCA on the TTAL Vesicle Solutions. Figure 11 shows plots of viscosity and shear stress vs shear rate for TTAL vesicles in the presence of different concentrations of DeCA. The shear viscosity generally decreases first and then increases, but not monotonously. The solutions that solubilize 2.5-7.6 mmol · L-1 DeCA show typical shear thinning behavior in the double-logarithmic plots. Such a shear thinning is already observed in the collapsed lamellar phase86 or a relatively densely packed dispersion of hard spheres.92 The slopes in the doublelogarithmic plots are 0.8-0.9 (Table 1), which are very similar to the values obtained for other lamellar structure systems.67,86,93

However, at higher concentrations of DeCA (10.2 and 12.7 mmol · L-1), a viscosity plateau is observed in the intermediate shear rate range, corresponding to the stress plateau in Figure 11b, and the slopes in the double-logarithmic plots show a small variation in the low and high shear rate ranges (Table 1). We believe this is induced by the structure modulation of the vesicles. Due to its large volume, the solubilization of DeCA at higher concentration in the vesicular hydrophobic bilayers or palisade layers decreased the interfacial curvature of the membranes,61 which makes the vesicles sensitive to shear. Hence, the outer shells of the multilamellar vesicles are stripped off under shear at a certain shear rate, and the number density of aggregates increases due to the fact that more vesicles are formed in the solution, which results in an increase of viscosity. The yield



Figure 11. Shear viscosity and shear stress versus shear rate for 100 mmol · L-1 TTAL with different concentrations of DeCA at 25 °C.

TABLE 1: Slopes in the Double-Logarithmic Curves of Shear Viscosity versus Shear Rate at Different Concentrations of DeCA at 25 °C c (mmol · L-1) n

2.5 0.8074

5.1 0.9072

7.6 0.8956

10.2a 0.9273 0.8288

12.7a 0.8132 0.8343

a The two values of n represent the slopes at lower and higher shear rates, respectively.

stress also increases with an increase of the DeCA concentration, which can be seen from the shear stress curves (Figure 11b). The viscoelastic properties are also changed (Figure S4, Supporting Information). G′ increases with an increase of the DeCA concentration. At a low concentration of DeCA, G′ is lower; at a high concentration, G′ is comparable to that of 100 mmol · L-1 TTAL. A decrease of G′ also has been observed in the system of 50 mmol · L-1 TTAL with different concentrations of CA.61 The reason, we speculate, is that the solubilization of bile acids at a low concentration resulted in the formation of large oligolamellar vesicles, for which the alignment of the molecules in the membranes is looser and the rigidity of the bilayer is small. Macroscopically it was reflected as a decrease of the elastic modulus. With an increase of the concentration of bile acids, the original vesicles were destroyed, new multilamellar vesicles were formed, and the rigidity of the membrane increased, which resulted in an increase of the elastic modulus. Conclusion The influence of two bile acidsscholic acid and deoxycholic acidson salt-free catanionic surfactant systems of (TTA)OH/ LA/H2O and (CTA)OH/LA/H2O is contrastively studied. Due to one hydroxyl group less on the steroidal skeleton of DeCA compared to CA, the hydrophobicities of the two bile acids are different; thus, they have different influences on the systems. Surface tension results show that at a molar ratio of bile acid to (TTA)OH or (CTA)OH lower than 1, the surface tension decreases to a minimum value and then increases before it achieves a stable value. With an increase of the chain length of the cationic surfactant, or hydrophobicity of bile acids, the cmc’s of the systems decrease. For the DeCA-containing system, the phase transition from the LR phase to the L1 phase occurs at a higher molar fraction of bile acid, and the shear viscosity is higher at the same molar fraction of bile acid both in wormlike micelles and in viscoelastic LR regions. The phase transition is induced by the large volume of steroidal skeleton in the hydrophobic area of the bilayers and the unfavorable interaction between aliphatic chains and the hydrophilic R-face of bile acids, which make the alignment of the molecules in the bilayers not very dense. Due to DeCA being more hydrophobic than CA, the interfacial curvature varies slowly, so the phase transition occurs at a higher molar fraction. In addition, the system

containing more hydrophobic DeCA prefers one-dimensional growth of wormlike micelles, which results in a higher shear viscosity. The longer chain (CTA)OH system also has a higher shear viscosity and a larger critical packing parameter. The study is of help not only for fundamental understanding of structural transition induced by bile acids, but also for practical applications of the mixed surfactant/bile acid systems in pharmaceutical applications and biocompatible materials. Shear-induced aggregate transition is observed in the LR region especially in the DeCA system, which needs further investigation. Acknowledgment. We thank the NSFC (Grant No. 20624307) and National Basic Research Program of China (973 Program, Grant 2009CB930103) for financial support. Supporting Information Available: Some typical polarized images, shear viscosity vs shear rate at 50 mmol · L-1 as a function of x, and variation of G′ for samples at 100 mmol · L-1 as a function of x and for a 100 mmol · L-1 TTAL vesicle solution with different concentrations of DeCA. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Luca, D.; Minucci, A.; Zecca, E.; Piastra, M; Pietrini, D.; Carnielli, V. P.; Zuppi, C.; Tridente, A.; Conti, G.; Capoluongo, E. D. IntensiVe Care Med. 2009, 35, 321. (2) Wiedmann, T. S.; Liang, W.; Herrington, H. Lipids 2004, 39, 51. (3) Small, D. M. In The Bile Acids; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971. (4) Carey, M. C.; Small, D. M. Am. J. Med. 1970, 49, 590. (5) Blow, D. M.; Rich, A. J. Am. Chem. Soc. 1960, 82, 3566. (6) Mazer, N. A.; Carey, M. C.; Kwasnick, R. F.; Benedek, G. B. Biochemistry 1979, 18, 3064. (7) Murata, Y.; Sugihara, G.; Fukushima, K.; Tanaka, M.; Matsushita, K. J. Phys. Chem. 1982, 86, 4690. (8) Kratohvil, J. P.; Hsu, W. P.; Kwok, D. I. Langmuir 1986, 2, 256. (9) Giglio, E.; Loreti, S.; Pavel, N. V. J. Phys. Chem. 1988, 92, 2858. (10) Li, G.; McGown, L. B. J. Phys. Chem. 1994, 98, 13711. (11) Briganti, G.; D’Archivio, A. A.; Galantini, L.; Giglio, E. Langmuir 1996, 12, 1180. (12) D’Archivio, A. A.; Galantini, L.; Giglio, E.; Jover, A. Langmuir 1998, 14, 4776. (13) Bottari, E.; D’Archivio, A. A.; Festa, M. R.; Galantini, L.; Giglio, E. Langmuir 1999, 15, 2996. (14) D’Archivio, A. A.; Galantini, L.; Tettamanti, E. J. Phys. Chem. B 2000, 104, 9255. (15) Bonincontro, A.; D’Archivio, A. A.; Galantini, L.; Giglio, E.; Punzo, F. J. Phys. Chem. B 1999, 103, 4986. (16) Bonincontro, A.; D’Archivio, A. A.; Galantini, L.; Giglio, E; Punzo, F. Langmuir 2000, 16, 10436. (17) Posˇa, M.; Kevresˇan, S.; Mikov, M.; Círin-Novta, V.; Saˆrbu, C.; Kuhajda, K. Colloids Surf., B 2007, 59, 179. (18) Posˇa, M.; Kevresˇan, S.; Mikov, M.; Círin-Novta, V.; Kuhajda, K. Colloids Surf., B 2008, 64, 151. (19) Soto Tellini, V. H.; Jover, A.; Meijide, F.; Va´zquez Tato, J.; Galantini, L.; Pavel, N. V. AdV. Mater. 2007, 19, 1752. (20) Willemen, H. M.; de Smet, L.; Koudijs, A.; Stuart, M. C. A.; Heikamp-de Jong, I.; Marcelis, A. T. M.; Sudholter, E. J. R. Angew. Chem., Int. Ed. 2002, 41, 22.

9804


(21) Terech, P.; Sangeetha, N. M.; Deme´, B.; Maitra, U. J. Phys. Chem. B 2005, 109, 12270. (22) Terech, P.; Dourdain, S.; Bhat, S.; Maitra, U. J. Phys. Chem. B 2009, 113, 8252. (23) Soto Tellini, V. H.; Jover, A.; Galantini, L.; Pavel, N. V.; Meijide, F.; Va´zquez Tato, J. J. Phys. Chem. B 2006, 110, 13679. (24) Zhong, Z.; Yan, J.; Zhao, Y. Langmuir 2005, 21, 6235. (25) Alcalde, M. A.; Jover, A.; Meijide, F.; Galantini, L.; Pavel, N. V.; Antelo, A.; Va´zquez Tato, J. Langmuir 2009, 25, 9037. (26) Luo, J.; Chen, Y.; Zhu, X. X. Langmuir 2009, 25, 10913. (27) Galantini, L.; Leggio, C.; Jover, A.; Meijide, F.; Pavel, N. V.; Soto Tellini, V. H.; Va´zquez Tato, J.; Di Leonardo, R.; Ruocco, G. Soft Matter 2009, 5, 3018. (28) Marques, E. F.; Edlund, H.; La Mesa, C.; Khan, A. Langmuir 2000, 16, 5178. (29) Willemen, H. M.; Vermonden, T.; Koudijs, A.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Colloids Surf., A 2003, 218, 59. (30) Thurmond, R. L.; Lindblom, G.; Brown, M. F. Biophys. J. 1991, 60, 728. (31) Walter, A.; Vinson, P. K.; Kaplon, A.; Talmon, Y. Biophys. J. 1991, 60, 1315. (32) Edwards, K.; Almgren, M. Langmuir 1992, 8, 824. (33) Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. J. Phys. Chem. 1995, 99, 1299. (34) Li, C. Y.; Wiedmann, T. S. J. Phys. Chem. 1996, 100, 18464. (35) Luk, A. S.; Kaler, E. W.; Lee, S. P. Biochemistry 1997, 36, 5633. (36) Cohen, D. E.; Thurston, G. M.; Chamberlin, R. A.; Benedek, G. B.; Carey, M. C. Biochemistry 1998, 37, 14798. (37) Egelhaaf, S. U.; Schurtenberger, P. Phys. ReV. Lett. 1999, 82, 2804. (38) Ulmius, J.; Lindblom, G.; Wennerstroem, H.; Johansson, L. B. A.; Fontell, K.; Soederman, O.; Arvidson, G. Biochemistry 1982, 21, 1553. (39) Sun, C.; Sano, Y.; Kashiwagi, H.; Ueno, M. Colloid Polym. Sci. 2002, 280, 900. (40) Leng, J.; Egelhaaf, S. U.; Cates, M. E. Biophys. J. 2003, 85, 1624. (41) Subuddhi, U.; Mishra, A. K. J. Chem. Sci. 2007, 119, 169. (42) Singh, J.; Unlu, Z.; Ranganathan, R. J. Phys. Chem. B 2008, 112, 3997. (43) Arleth, L.; Bauer, R.; Øgendal, L. H.; Egelhaaf, S. U.; Schurtenberger, P.; Pedersen, J. S. Langmuir 2003, 19, 4096. (44) Jiang, L.; Wang, K.; Deng, M.; Wang, Y.; Huang, J. Langmuir 2008, 24, 4600. (45) Alves, F. R.; Feitosa, E. Thermochim. Acta 2006, 450, 76. (46) Marques, E. F.; Regev, O.; Edlund, H.; Khan, A. Langmuir 2000, 16, 8255. (47) Jiang, L.; Wang, K.; Ke, F.; Liang, D.; Huang, J. Soft Matter 2009, 5, 599. (48) Youssry, M.; Coppola, L.; Marques, E. F.; Nicotera, I. J. Colloid Interface Sci. 2008, 324, 192. (49) Cabrer, P. R.; Alvarez-Parrilla, E.; Meijide, F.; Seijas, J. A.; Nunez, E. R.; Tato, J. V. Langmuir 1999, 15, 5489. (50) Liu, Y.; Zhao, Y.; Zhang, H. Langmuir 2006, 22, 3434. (51) Holm, R.; Shi, W.; Hartvig, R. A.; Askjær, S.; Madsen, J. C.; Westh, P. Phys. Chem. Chem. Phys. 2009, 11, 5070. (52) Sva¨rd, M.; Schurtenberger, P.; Fontell, K.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1988, 92, 2261. (53) Vethamuthu, M. S.; Almgren, M.; Brown, W.; Mukhtar, E. J. Colloid Interface Sci. 1995, 174, 461. (54) Vethamuthu, M. S.; Almgren, M.; Bergenståhl, B.; Mukhtar, E. J. Colloid Interface Sci. 1996, 178, 538. (55) Vethamuthu, M. S.; Almgren, M.; Karlsson, G. Langmuir 1996, 12, 2173. (56) Amundson, L. L.; Li, R.; Bohne, R. Langmuir 2008, 24, 8491. (57) Wang, Y.; Zhang, J.; Zhu, X.; Yu, A. Polymer 2007, 48, 5565.

Liu et al. (58) Nichifor, M.; Zhu, X.; Cristea, D.; Carpov, A. J. Phys. Chem. B 2001, 105, 2314. (59) Carpov, S. H.; Huang, Y. E.; Raghavan, S. R. Soft Matter 2008, 4, 1086. (60) Pal, A.; Basit, H.; Sen, S.; Aswa, V. K.; Bhattacharya, S. J. Mater. Chem. 2009, 19, 4325. (61) Liu, C.; Hao, J. J. Phys. Chem. B 2010, 114, 4477. (62) Hao, J.; Liu, W.; Xu, G.; Zheng, L. Langmuir 2003, 19, 10635. (63) Medronho, B.; Shafaei, S.; Szopko, R.; Miguel, M. G.; Olsson, U.; Schmidt, C. Langmuir 2008, 24, 6480. (64) Li, H.; Hao, J. J. Phys. Chem. B 2008, 112, 10497. (65) Vethamuthu, M. S.; Almgren, M.; Hansson, P.; Zhao, J. Langmuir 1996, 12, 2186. (66) Israelachvili, J. N.; Mitchell, D. J.; Ninhem, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (67) Bergmeier, M.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. J. Phys. Chem. B 1999, 103, 1605. (68) Bergmeier, M.; Hoffmann, H.; Thunig, C. J. Phys. Chem. B 1997, 101, 5767. (69) Lukaschek, M.; Mu¨ller, S.; Hansenhindl, A.; Grabowski, D. A.; Schmidt, C. Colloid Polym. Sci. 1996, 274, 1. (70) Shen, Y.; Hao, J.; Hoffmann, H.; Wu, Z. Soft Matter 2008, 4, 805. (71) Mu¨ller, S.; Bo¨rschig, C.; Gronski, W.; Schmidt, C. Langmuir 1999, 15, 7558. (72) Sharma, S. C.; Shrestha, R. G.; Shrestha, L. K.; Aramaki, K. Colloid Polym. Sci. 2008, 286, 1613. (73) Sharma, S. C.; Shrestha, L. K.; Tsuchiya, K.; Sakai, K.; Sakai, H.; Abe, M. J. Phys. Chem. B 2009, 113, 3043. (74) Acharya, D. P.; Hattori, K.; Sakai, T.; Kunieda, H. Langmuir 2003, 19, 9173. (75) Varade, D.; Rodrı´guez-Abreu, C.; Shrestha, L. K.; Aramaki, K. J. Phys. Chem. B 2007, 111, 10438. (76) Laüger, J.; Weigel, R.; Berger, K.; Hiltrop, K.; Richtering, W. J. Colloid Interface Sci. 1996, 181, 521. (77) Montalvo, G.; Rodenas, E.; Valiente, M. J. Colloid Interface Sci. 1998, 202, 232. (78) Panizza, P.; Roux, D.; Vuillaume, V.; Lu, C.; Cates, M. Langmuir 1996, 12, 248. (79) Hao, J.; Hoffmann, H.; Horbascheck, K. J. Phys. Chem. B 2000, 104, 10144. (80) Berni, M. G.; Lawrence, C. J.; Machin, D. AdV. Colloid Interface Sci. 2002, 98, 217. (81) Kekicheff, P.; Cabane, B.; Rawiso, M. J. Phys., Lett. 1984, 45, L813. (82) Allain, M. Europhys. Lett. 1986, 2, 597. (83) Oswald, P.; Allain, M. J. Colloid Interface Sci. 1988, 126, 45. (84) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Clarendon Press: Oxford, U.K., 1993. (85) Lejcek, L.; Oswald, P. J. Phys. II 1991, 1, 931. (86) Montalvo, G.; Valiente, M.; Khan, A. Langmuir 2007, 23, 10518. (87) Hao, J.; Hoffmann, H.; Horbaschek, K. Langmuir 2001, 17, 4151. (88) Abdel-Rahem, R.; Gradzielski, M.; Hoffmann, H. J. Colloid Interface Sci. 2005, 288, 570. (89) Hoffmann, H.; Munkert, U.; Thuning, C.; Valiente, M. J. Colloid Interface Sci. 1994, 16, 217. (90) Hoffmann, H.; Thuning, C.; Schmeidel, P.; Munkert, V. Langmuir 1994, 10, 3972. (91) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (92) van der Werff, J. C.; de Kruif, J. C. J. Rheol. 1989, 33, 421. (93) Bergenholtz, J.; Wagner, N. J. Langmuir 1996, 12, 3122.

JP103916A

Phase Behavior and Rheological Properties of Salt-Free Catanionic

Recommend Documents