Re-entrant Phase Behavior of a Concentrated Anionic Surfactant

Mar 20, 2009 - Department of Physics, Indian Institute of Science, Bangalore 560012, India. Langmuir , 2009, 25 (15), pp 8497–8506. DOI: 10.1021/la8...
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Re-entrant Phase Behavior of a Concentrated Anionic Surfactant System with Strongly Binding Counterions† Sajal Kumar Ghosh,‡ Vikram Rathee,§ Rema Krishnaswamy,§ V. A. Raghunathan,‡ and A. K. Sood*,§,^ ‡

Raman Research Institute, Bangalore 560080, India, §Department of Physics, Indian Institute of Science, Bangalore 560012, India, ^ CSIR Bhatnagar Fellow, Department of Physics, Indian Institute of Science, Bangalore 560012, India Received December 31, 2008. Revised Manuscript Received February 12, 2009

The phase behavior of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the strongly binding counterion p-toluidine hydrochloride (PTHC) has been examined using small-angle X-ray diffraction and polarizing microscopy. A hexagonal-to-lamellar transition on varying the PTHC to SDS molar ratio (R) occurs through a nematic phase of rodlike micelles (NC) f isotropic (I) f nematic of disklike micelles (ND) at a fixed surfactant concentration (φ). The lamellar phase is found to coexist with an isotropic phase (I0 ) over a large region of the phase diagram. Deuterium nuclear magnetic resonance investigations of the phase behavior at φ = 0.4 confirm the transition from NC to ND on varying R. The viscoelastic and flow behaviors of the different phases were examined. A decrease in the steady shear viscosity across the different phases with increasing R suggests a decrease in the aspect ratio of the micellar aggregates. From the transient shear stress response of the NC and ND nematic phases in step shear experiments, they were characterized to be tumbling and flow aligning, respectively. Our studies reveal that by tuning the morphology of the surfactant micelles strongly binding counterions modify the phase behavior and rheological properties of concentrated surfactant solutions.

1.

Introduction

Investigations over the past few decades on the phase behavior of different lyotropic systems such as surfactants, lipids, or block copolymers have revealed the rich structural polymorphism of the “intermediate” phases between the hexagonal-to-lamellar transition in these systems.1 A common sequence of transition in a binary lyotropic surfactant system from the hexagonal phase where the aggregates have a positive interfacial curvature to a lamellar phase with zero interfacial curvature is through a bicontinuous cubic phase where the interface is a triply periodic minimal surface characterized by vanishing mean curvature everywhere on the surface.2 Intermediate phases with nonuniform interfacial curvature also occur in binary systems, albeit over a narrow region of the phase diagram; an excellent example of this being the SDS-water system.3,4 However it was subsequently seen that the range of intermediate phase can be extended in mixed surfactant systems by tuning the interfacial curvature of the aggregates, with counterions, inorganic salts, alcohol, or the chain length.2,5-7 The general consensus that has emerged from these experimental studies is that the role of these additives is to impart a nonuniform interfacial curvature to the micellar aggregates † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. E-mail: [email protected].

(1) Neto, F.; Antonio, M.; Salinas, R. A. Physics of Lyotropic Liquid Crystals: Phase Transitions and Structural Properties; Oxford University Press: New York, 2005. (2) Holmes, M. C.; Leaver, M. S. In Bicontinuous Liquid Crystal, Intermediate Phases; Lynch, M. L., Spicer, P. T., Eds.; Taylor and Francis: London, 2005; pp 15-39. (3) Kekicheff, P.; Cabane, B. Acta Crystallogr. 1988, 44, 395. (4) Luzzati, V. Biol. Membr. 1968, 1, 71. (5) Hendrikx, Y.; Charvolin, J. Liq. Cryst. 1992, 11, 677. (6) Hendrikx, Y.; Charvolin, J. J. Phys. (Paris) 1981, 42, 1427. (7) Funari, S. S.; Rapp, G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 7756.

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through microphase separation;a prerequisite for the formation of many intermediate phases, as revealed from theoretical calculations.8 A distinct class of mixed surfactant systems are those formed by oppositely charged surfactants, where the interfacial curvature of the micellar aggregates is tuned by varying the ratio of the two amphiphiles. In dilute systems, the morphology of the aggregates thus transforms from cylinders f wormlike micelles9 f vesicles.10,11 A similar behavior is also reported for ternary systems consisting of surfactants with oppositely charged strongly binding hydrophobic counterions (hydrotopes).12 The role of the counterion is again to modify the morphology of the aggregate as the salt-to-amphiphile molar ratio is increased. However, there are very few reports on the phase behavior of such systems at low water content. Recently, we have shown that in the concentrated phases of a mixed surfactant system consisting of a cationic surfactant and a strongly binding anionic counterion the hexagonal-to-lamellar transition occurs through a rhombohedral intermediate mesh phase (R3m).13 A lamellar phase with curvature defects was also observed in these systems which transforms to a regular lamellar phase through an intermediate mesh phase on varying the surfactant concentration at a fixed salt-to-amphiphile molar ratio.14,15 (8) Schrder, G. E.; Hyde, T. S. Curr. Opin. Colloid Interface Sci. 2003, 8, 5. (9) Dreiss, C. A. Soft Matter 2007, 3, 956. (10) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Shivkumar, C. J. Phys. Chem. 1993, 97, 13792. (11) Koehler, R. D.; Raghavan, S. R.; Kaler, E. W. J. Phys. Chem. B 2000, 104, 11305. (12) Mishra, B. K.; Samant, S. D.; Pradhan, P.; Mishra, S. B.; Manohar, C. Langmuir 1993, 9, 894. (13) Ghosh, S. K.; Ganapathy, R.; Krishnaswamy, R.; Bellare, J.; Raghunathan, V. A.; Sood, A. K. Langmuir 2007, 23, 3606. (14) Krishshnaswamy, R.; Ghosh, S. K.; Lakshmanan, S.; Raghunathan, V. A.; Sood, A. K. Langmuir 2005, 21, 10439. (15) Ghosh, S. K.; Raghunathan, V. A. Langmuir, in press.

Published on Web 03/20/2009

DOI: 10.1021/la804330x

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A detailed phase diagram of the SDS-water system was proposed by Kekicheff et al.3 A hexagonal phase was observed over a large range of surfactant concentration, followed by a number of intermediate phases over a narrow range of surfactant concentration, before the lamellar phase is obtained at a low water content. The role of oppositely charged amphiphiles dodecyltrimethylammonium bromide (DTAB)16 and octyltrimethylammonium bromode (OTAB)17 on the phase behavior of SDS in the concentrated regime has also been investigated. In these systems, as the molar ratio of the two amphiphiles approaches one, the surfactant concentration at which the lamellar phase was observed also shifted to lower values. However, a more extensive study on ternary systems consisting of SDS has been carried out in the SDS-decanol-water systems. At low surfactant concentrations, on increasing the decanol to SDS molar ratio, the hexagonal to lamellar transition occurs through a nematic phase of rodlike and disklike micelles. Besides small-angle X-ray investigations which revealed the low aspect ratio (3-4) of the micelles in the nematic phases,18 deuterium NMR spectra were used to distinguish between the morphology of the aggregates.19 The present study deals with the phase behavior of an anionic surfactant system in the presence of a strongly binding cationic counterion;a class of systems whose phase behavior in the concentrated regime has not been reported as yet. This class of systems was introduced by Kaler and co-workers where a rodlike micelle to a wormlike micelle transition was reported in dilute solutions of the anionic surfactant sodium dodecyl sulfate (SDS) and p-toluene hydrochloride (PTHC)20 on varying the PTHC-to-SDS molar ratio. Subsequently, this has been debated, and it was proposed from rheological measurements that an increase in chain length or the hydrophobicity of the counterion was crucial for the formation of wormlike micelles in these systems.21 Finally, our studies are also motivated by the tremendous interest in understanding the complex flow behavior of the surfactants with strongly binding counterions,9,22 where the long cylindrical micelles entangle to form a viscoelastic gel. Many of these studies are reported until now in cationic surfactant systems. In these systems, under steady shear, above a critical stress, formation of shear bands (where bands of different shear rates coexist over the region that is sheared) is accompanied by a complex spatiotemporal dynamics.23-25 Presently, we have investigated the role of the organic salt PTHC on the phase behavior of SDS-water system using different techniques like polarizing microscopy, smallangle X-ray diffraction, and deuterium NMR spectroscopy. At R = [PTHC]/[SDS] < 1, the coexistence of lamellar and isotropic phase is observed over a large range of surfactant (16) Chen, D. H.; Hall, D. G. Kolloid Z. Z. Polym. 1973, 262, 41. (17) Barker, C. A.; Saul, D.; Tiddy, G. J.; Wheeler, B. A.; Willis, E. J. Chem. Soc., Faraday Trans. 1 1974, 70, 154. (18) Hendrikx, Y.; Charvolin, J.; Rawiso, M.; Liebert, L.; Holmes, M. C. J. Phys. Chem. 1983, 87, 3991. (19) Quist, P.; Halle, B.; Furo, I. J. Chem. Phys. 1992, 96, 3875. (20) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (21) Nakamura, K.; Shikata, T. Langmuir 2006, 22, 9853. (22) Berret, J. F. In Rheophysics of Wormlike Micelles, Molecular Gels; Terech, P., Weiss, R., Eds.; Elsevier: Amsterdam, 2005. (23) Ganapathy, R.; Sood, A. K. Phys. Rev. Lett. 2006, 96, 108301. (24) Bandhyopadhyay, R.; Basappa, G.; Sood, A. K. Phys. Rev. Lett. 2000, 84, 2022. (25) Decruppe, J. P.; Lerouge, S.; Berret, J. F. Phys. Rev. E 2001, 63, 022501.

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concentrations. At a surfactant concentration of φ = 0.3, a transition from isotropic phase of rodlike micelles to lamellar phase occurs through a nematic phase of disklike micelles on varying R, which indicates the active role of the counterion in modifying the morphology of the micelles in the present system. This is further confirmed at a higher surfactant concentration (φ = 0.4) where a hexagonal f nematic (NC) f isotropic f nematic (ND) f lamellar transition is observed with increase of R. Here NC and ND correspond to a nematic phase of rodlike and disklike micelles, respectively, identified from the quadrupole splitting of the deuterium NMR spectra. The route of the hexagonal f lamellar transition in the present system is thus clearly distinct from their cationic counterparts where the transition occurs through an intermediate phase.13,15 At present there are very few investigations on the linear and nonlinear rheological properties of concentrated micellar solutions of isotropic and nematic phases. A strong analogy has been observed between the flow behavior of nematic phases of wormlike micelles and liquid crystalline polymers,26 with the director dynamics always exhibiting a tumbling behavior. However, rheo-NMR studies of the nonlinear flow behavior of the two nematic phases27 in the SDS-decanolwater system revealed them to be a textured flow aligning nematic. In this context, it is worthwhile to investigate the director dynamics as well as the flow-texture coupling of the nematic phases observed in the SDS-PTHC-water system. A distinct viscoelastic behavior was observed for the two nematic phases, with the nematic phase of disklike micelles exhibiting two relaxation modes. Moreover, the steady shear viscosity is found to decrease with increase in R, indicating a decrease in the aspect ratio of the micelles. Transient steady shear response of the two nematic phases revealed a tumbling behavior of the director for the rodlike nematic and a flow aligning behavior for the disklike nematic phases.

2.

Materials and Methods

Sodium dodecyl sulfate (SDS) (99% purity) and p-toluidine hydrocloride (PTHC) (98% purity) were purchased from Aldrich and were used without further purification. To prepare the ternary solutions, deionized water (Millipore) was added to appropriate amounts of PTHC and SDS weighed into sample tubes to obtain the desired concentration. For each molar ratio (R = [PTHC]/[SDS]) of the two components, the total surfactant weight fraction (φ = (SDS + PTHC)/(SDS + PTHC + water)) was varied from 0.1 to 0.8. The sample tubes were then sealed and allowed to homogenize at 40 C typically for 2 weeks. For microscopy observations, samples taken between a glass slide and a coverslip and placed in a temperature-controlled chamber (heating or cooling rate was 1 C/min) were observed between crossed polarizers. The temperature of each sample was varied from 20 to 80 C. 2.1. Small-Angle X-ray Diffraction. For small-angle X-ray diffraction studies the samples were taken in glass capillaries (Hampton Research, 0.7-1 mm diameter) and flamesealed. Diffraction patterns were obtained using Cu KR (0.154 nm) radiation from a rotating anode X-ray generator (Rigaku, UltraX 18) operating at 50 kV and 80 mA. Data were collected using 2-D image plate detector (Marresearch). Typical exposures lasted for an hour, and the instrumental resolution (fwhm) was 0.18 nm-1. (26) Berret, J. F.; Roux, D. C. J. Rheol. 1995, 39, 725. (27) Thiele, T.; Berret, J. F. J. Rheol. 2001, 45, 29.

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2.2. NMR Spectroscopy. The quadrupole splitting of H-labeled water in the mesophases is characteristic of the phase, where the quadrupole splitting is given by 2

3 Δν ¼ δð3 cos2 θ - 1Þ 4

ð1Þ

Here δ = e2qQ/h is the quadrupole coupling constant with e being the electronic charge, eq the electric field gradient, eQ the electric quadrupole moment of the deuterium nucleus, and θ the angle between the external magnetic field and the director. Hence, the splitting depends on the molecular motion within the sample as well as the orientation of the director with the external field. Thus, in an isotropic phase with a randomly oriented director distribution, the splitting is zero, and only a single broad spectrum is obtained. A maximum splitting of 3δ/2 will be obtained when the director is aligned parallel to the field, and the splitting reduces to half of this value when the director is aligned perpendicular to the field. Hence, a spectrum consisting of a doublet may be expected for a liquid crystalline phase. However, it should be noted that for a powder sample with very small domain size of the liquid crystalline phase where the D2O molecules can diffuse easily, the splitting may not be observable. 2 H NMR spectra were obtained with Bruker DRX-500 NMR spectrometer operating in Fourier transform mode equipped with Bruker BVT 3000 temperature controller. The spectra were taken at resonance frequency of 76.757 MHz with a typical spectral width of 3063.726 Hz, over a single scan, at a pulse width of 50 μs and an acquisition time of 1.337 s. 2.3. Rheological Measurements. A stress-controlled rheometer (MCR 300, Anton Paar) was used for rheological measurements. Frequency sweep and flow curve measurements were made in the Couette geometry (diameter of inner cylinder is 27 mm and a gap of 1 mm). For solutions of low viscosity, a measuring system of coaxial cylinders with a double gap of outer radii 13.8 and 12.3 mm were used (gap thickness 0.47 and 0.42 mm). Stress relaxation measurements were carried out in a cone-plate (cone diameter 50 mm, cone angle 2) geometry. Rheo-optical measurements were made in a home-built optically transparent parallel plate geometry (plate diameter 43 mm) placed between crossed polarizers. The images were recorded on an 8-bit color CCD camera (Lumenera, 0.75C, 640  480 pixels) fitted with a microscope objective (Nikon, 0.15 NA 10).

3.

Results and Discussion

3.1. Phase Behavior of SDS-PTHC-Water System. The phase diagram for SDS-PTHC-water system for PTHC-to-SDS molar ratio (R = [PTHC]/[SDS]) e1 was constructed using polarizing microscopy and X-ray diffraction. 3.1.1. Influence of Surfactant Concentration on the Phase Behavior. The phase diagrams at R = 0.25 and 0.5 are shown in Figure 1. At R = 0.25, when the surfactant concentration (φ) is varied, the transition from isotropic to lamellar phase, identified by the oily streak texture, occurs through a nematic phase (around φ = 0.35). The characteristic Schlieren textures of the nematic phase (Figure 1A, inset), however, vanish over time or coexist with large regions of dark patches. The presence of the dark regions implies that the director is aligned homeotropically (perpendicular to the substrate) along the optic axis. The diffraction pattern of the nematic phase at R = 0.25, φ = 0.35 at 30 C reveals Langmuir 2009, 25(15), 8497–8506

Figure 1. Phase diagram of SDS-PTHC-water system at R = (A) 0.25 and (B) 0.5. The texture of the nematic phase under crossed polarizers is shown in the inset of (A). The scale bar corresponds to 50 μm. The lamellar periodicities at different surfactant concentrations (φ) for R = 0.75 (open triangles) and R = 1 (open circles) are shown in the inset of (B). The solid line corresponds to the ideal swelling behavior of the lamellar phase described in the text. φs is the weight fraction of the surfactant (φs = φ X 100). a broad peak, with the peak position corresponding to a d-spacing of 4.9 nm, shifting to larger values of the scattering wave vector q with increase in surfactant concentration. The transition to the lamellar phase at higher surfactant concentration is always accompanied by a coexistence of the isotropic phase (I0 ). The diffraction pattern (Figure 2A) of the partially aligned lamellar phase at R = 0.25, φ = 0.4 at 30 C reveals sharp reflections with the corresponding d-spacings in the ratio 1:1/2:1/3. The lamellar periodicity is 4.04 nm. However, a diffuse peak at low q values in a direction perpedicular to the lamellar stacking is not observed. On heating, a phase transition from lamellar (LR) f nematic f isotropic (I) phases is obtained at 45 C (Figure 2B) and 60 C (Figure 2C). At R = 0.5, the nematic phase is found to be absent, and the isotropic-to-lamellar transition occurs at φ = 0.1 (Figure 1B). For R > 0.7, the coexistence of isotropic and lamellar phase occurs over the entire range of surfactant concentrations. At equimolar ratios, a complex which exhibits the oily streak texture of lamellar phase under crossed polarizers phase separates out of the solution, leaving behind an isotropic phase. X-ray diffraction patterns at R = 0.7 and 1, however, reveal a decrease in d-spacing of the lamellar phase with increase in surfactant concentration (Figure 1B, inset). A homeotropic alignment of the director in the nematic phase at R = 0.25 indicates that the morphology of the micelles in the phase is disklike with the disks aligning parallel to the glass substrate. This is supported further by the transition from lamellar to nematic phase observed on heating (Figure 3) at R = 0.25, which suggests a change in the morphology of the aggregates from bilayers to disks. A similar phase transition has been reported in the cesium pentadecafluorooctanoate (CsPFO)-water system where the disklike morphology of the nematic phases has been well DOI: 10.1021/la804330x

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Figure 2. Diffraction pattern of the partially aligned (A) lamellar phase at 30 C, (B) nematic phase at 45 C, and (C) isotropic phase at 60 C. All the patterns correspond to sample with φ = 0.4 and R = 0.25. established.28,29 The low values of the d-spacings in the lamellar phases are consistent with the coexistence of the isotropic phase. The bilayers in the lamellar phase thus show a marked deviation from the expected swelling behavior for a regular lamellar phase (Figure 1B, inset) where the lamellar periodicity d is related to the bilayer thickness δ and the volume fraction of water φw (= 1 - φ) as d = δ/(1 - φw). The swelling behavior may be understood in terms of the screening of the electrostatic interaction between the bilayers. At any value of R, increasing φ increases the concentration of dissociated Na+ and Cl- ions in the solution. The electrostatic potential of the bilayer is given by32 ψx ¼ ψ0 e -kx

ð2Þ

where ψ0 is the potential at the bilayer surface and 1/k, the Debye length, is the characteristic decay length of the potential. At 25 C the Debye length of aqueous solutions can be written as 1 0:304 ¼ pffiffiffiffi k C

ð3Þ

for 1:1 electrolytes. C is the molar concentration of salt. The presence of large amounts of dissociated salt in the present system makes the Debye length extremly short, in the range 0.3-0.13 nm when the surfactant concentration φ is varied between 0.3 and 0.7 at R = 1 at 30 C. This can qualitatively explain the low values of the lamellar spacing observed in this system. A noteworthy aspect of the diffraction pattern of the lamellar phases is also the absence of the diffused peak at low q values. This indicates that bilayers consisting of waterfilled regions due to curvature defects, reported in the lamellar phases of mixed surfactant systems,5,13 do not occur in the present system. 3.1.2. Variation of R at Fixed Surfactant Concentration. To study the influence of PTHC on the morphology of the aggregates in the concentrated phases, R was varied at a fixed surfactant concentration. Phase Behavior at φ = 0.3: At φ = 0.3, a transition from isotropic to lamellar phase occurs through a nematic phase (28) Leaver, M. S.; Holmes, M. C. J. Phys. II 1993, 3, 1357. (29) Boden, N.; Corne, S. A.; Jolley, K. W. J. Phys. Chem. 1987, 91, 4092.

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Figure 3. Phase diagram of SDS-PTHC-water system at (A)

φ = 0.3 and (B) φ = 0.4. The diffraction patterns obtained at 30 C for the isotropic phases at φ = 0.3 at (a) R = 0.125 and (b) R = 0.256 are shown in the inset of (A). The variation of the lamellar periodicity with R at φ = 0.3 (filled triangles) and φ = 0.4 (filled circles) are shown in the inset of (B). The texture of the nematic phase obtained at R = 0.1 at 30 C is also shown in the inset of (B).

(Figure 3A). The scattering intensity profile (Figure 3A, inset) at 30 C in the isotropic phase for R = 0.13 (curve a) with a broad peak centered at 4.79 nm is clearly distinct from that obtained at R = 0.26 (curve b) where the average peak position shifts to 4.18 nm. In the nematic phase at R = 0.32, the broad peak is centered around 3.8 nm. The distinct scattering profiles observed in the isotropic phase indicates that the morphologies of the micellar aggregates in the two phases (Figure 3A, inset) are different. Moreover, the shift in the peak position also suggests that the intermicellar separation decreases with R. In concentrated micellar solutions, the scattered intensity I(q) has contributions from the form factor P(q) and the structure factor S(q) which cannot be separated out easily. This poses the main problem for any detailed analysis of the SAXS data to obtain the microstructure of the micellar aggregates in the different phases. Hence, it is difficult to separate out the two effects in the diffraction pattern. Phase Behavior at φ = 0.4: At φ = 0.4, an unusual phase sequence from hexagonal f nematic f isotropic f nematic f lamellar is observed with the increase in R (Figure 3B). A strongly birefringent nematic phase (NC) with a striped texture is observed for R > 0.05 (Figure 3B, inset). On heating above 40 C, the texture is similar to the focal conic texture of hexagonal phase. With further increase in R to 0.15 an isotropic phase (I) is observed, which on heating transforms back to a nematic phase (NC) at 55 C. The Schlieren texture of the nematic phase (ND) at R = 0.2 at 30 C (Figure 1A, inset) is distinct from that of the nematic phase (NC) at lower values of R. On heating, an isotropic phase is observed at 60 C. At higher PTHC concentrations Langmuir 2009, 25(15), 8497–8506

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Figure 4. Powder diffraction pattern of the different phases at φ = 0.4 at different values of R: (a) 0, (b) 0.1, (c) 0.15, (d) 0.2, and (e) 0.4 at 30 C. (R = 0.25) a lamellar phase coexisting with another isotropic phase (I0 ), discussed earlier, is obtained. The diffraction patterns from powder samples in different phases at φ = 0.4 at 30 C for different values of R are shown in Figure 4. The diffraction patterns observed at R = 0.1 (curve b) and R = 0.2 (curve d), with a broad peak, are distinct from that observed for the hexagonal phase of SDSwater system at φ = 0.4 (curve a). A very broad peak is also observed for the isotropic phase (curve c) in between the two nematics though the width of the peak has been found to be higher. The diffraction pattern observed at R = 0.4 at 30 C (curve e) is characteristic of a lamellar phase. An increase in the lamellar periodicity with R is observed (Figure 3B, inset). The correlation length L in the different phases can be calculated from the peak width at half-maxima Δq using Scherrer’s formula given by L = 4πK/Δq, where the constant K is 0.89. The correlation length is minimum in the isotropic phase at 10.6 nm and increases in the nematic phases to ∼14.1 and 22.8 nm at R = 0.1 and 0.2, respectively. A nematic phase of disklike or rodlike micelles can be distinguished from X-ray diffraction pattern of samples under alignment in a magnetic field. In the present system, SDS-PTHC micelles orient with their long axis parallel to the external magnetic field due to the negative anisotropy of the magnetic susceptibility of the alkyl chains. Hence, in the columnar nematic phase, the rods will orient with their long axis along the magnetic field whereas in the disklike nematic phase, the disks will orient with the disk normal perpendicular to the field. The absence of any preferred orientation of the director in the plane perpendicular to the magnetic field will give rise to a polydomain sample, with the director field constant in each domain, for the disklike nematic phase. Hence, the diffraction pattern is a ring in a plane perpendicular to the field for both the disklike and rodlike nematic phases. A technique used to distinguish between the two Langmuir 2009, 25(15), 8497–8506

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nematic phases from the diffraction pattern is by rotating the sample about an axis perpendicular to the magnetic field.30 For a disklike nematic phase, the sample rotation about an axis perpendicular to the field direction gives rise to a monodomain sample with the ring condensing into two spots that lie on the axis of rotation of the capillary. For a rodlike nematic phase, however, the diffraction pattern continues to be a ring in a plane perpendicular to the resultant magnetic field. Hence, rotating the sample in an external magnetic field will result in an increase in the intensity of the peaks for the disklike nematic phases unlike the rodlike nematic phase, where the peak intensity does not change with sample rotation. It should be noted that applying a magnetic field along two perpendicular directions will also give similar results. Since neither of these could be achieved in our experimental geometry, the present X-ray diffraction studies did not yield sufficient information on the morphology of the micelles in the two nematic phases. It will be worthwhile to pursue these studies in these experimental geometries. The transition from a nematic to hexagonal phase at higher temperatures can arise when the unbinding of the PTH+ counterions from the cylindrical micelles leads to an increase in the aspect ratio of the micelles. An increase in R also progressively decreases the charge, thus increasing the flexibility of the micelles. This is evident from the increase in the lamellar periodicity on varying R at a fixed φ. With increase in R at a fixed φ, the ionic strength of the solution increases due to the release of the counterions, which should have decreased the electrostatic repulsion and hence the separation between the bilayers. However, the increase in R also decreases the bilayer charge density, which is likely to increase the flexibility of the bilayers. Hence, the increase in lamellar periodicity with R arises from a steric repulsion between the bilayers due to their thermal undulation.31 3.1.3. Hexagonal-to-Lamellar Transition Probed Using Deuterium NMR Spectroscopy. The proximity to the lamellar phase suggests that the morphology of the micellar aggregates in the nematic phase at R = 0.2 cannot be very different from that in the lamellar phase. Thus, the nematic phase at R = 0.2 is expected to consist of disklike micelles. This is further confirmed through NMR measurements of the quadrupole splitting of the 2H nucleus which can be used to distinguish between the nematic phases of rodlike and disklike micelles. For the SDS-PTHC solutions prepared in D2O for R in the range 0.1-0.25, keeping the surfactant concentration fixed at φ = 0.4, the phase diagram was constructed from polarizing microscopy (data not shown). A slight shift in the upper phase boundaries for the transition temperatures was observed in the presence of D2O. At φ = 0.4, NMR spectra were obtained for different values of R by varying the temperature in the range 20-70 C (Figure 5). At R = 0.1, the spectrum shows a simple doublet structure (Figure 5, curve a). As the temperature is varied from 20 to 60 C, the splitting increases from Δν = 685 to 968 Hz (Figure 5, inset). No further increase in splitting is observed at higher temperatures. With increase in R to 0.15 only a singlet was observed over this range of temperature (Figure 5, (30) Neto, A. M. F.; Levelut, A. M.; Liebert, L.; Galerne, Y. J. Phys., Lett. 1985, 46, L-499. (31) Israelachvili, J. Intermolecular and Surfaces Forces, 2nd ed.; Academic Press: London, 1991.

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Figure 5. Deuterium NMR spectra of the nematic phase at R = 0.1

at 30 C (curve a), isotropic phase at R = 0.15 at 50 C (curve b), and the nematic phase at R = 0.2 at 50 C (curve c). The variation of the quadrupole splitting with temperature in the nematic phase at R = 0.1 (open triangles), hexagonal phase at R = 0.1 above 40 C (filled triangles), lamellar (filled circles), and nematic (open circles) phase at R = 0.2 are shown in the inset.

curve b). At R = 0.2, a singlet is observed at 20 C. A doublet appears along with a singlet at 25 C. Between 40 and 55 C, only a doublet is observed (Figure 5, curve c), beyond which a coexistence of doublet and singlet is found up to 65 C. At higher temperatures, a singlet is obtained. The quadrupole splitting decreases with temperature during the transition from the lamellar to nematic phase (Figure 5, inset). At R = 0.25, a singlet was observed in the temperature range 20-70 C. The NMR spectra may be interpreted as follows. SDSPTHC micelles orient with their long axis parallel to the external magnetic field due to the negative anisotropy of the magnetic susceptibility of the alkyl chains. The doublet spectra observed at R = 0.1 are consistent with a unique orientation of the director with respect to the field. In the nematic phase of rodlike micelles, the rods will orient with their long axis along the magnetic field. The increase in splitting with temperature is consistent with a transition to the hexagonal phase and is possibly due to the increase in the micellar length. The singlet observed at R = 0.15 is consistent with the presence of the isotropic phase at these sample compositions. At R = 0.2, the single peak observed at 20 C is characteristic of a dispersion of multilamellar vesicles. The presence of a doublet and a singlet at higher temperatures (25-40 C) indicates the coexistence of isotropic and lamellar phases (confirmed from polarizing microscopy). In the lamellar phase, the director is expected to orient perpendicular to the magnetic field. The vanishing of the singlet and the presence of the doublet in the range 40-55 C is consistent with transition to a nematic phase at these temperatures. A change in splitting during the transition can arise from the change in the aggregate size or in the orientation of the director with respect to the field. However, the alignment of the director along the field expected for a nematic phase of cylindrical micelles would have given rise to an increase in splitting. Hence, the decrease in splitting observed with the transition suggests that the nematic phase is made of disklike micelles with their director continuing to remain perpendicular to the field. The decrease in the splitting possibly arises from the change in aggregate size as 8502 DOI: 10.1021/la804330x

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bilayers break up into disks, similar to that seen in the CsPFO-water system.29 3.2. Rheology of Concentrated Solutions of SDSPTHC-Water System. The change in the morphology of the micelles during the hexagonal to lamellar transition that is observed in the present system on varying R can be expected to influence the viscoelastic and flow behavior of the different mesophases. Moreover, the transient response of the shear stress under steady shear can reveal the director dynamics in the two nematic phases. The rheological properties of the concentrated micellar solutions at a surfactant concentration φ = 0.4, with R varying in the range 0-0.25 was investigated under oscillatory and steady shear. 3.2.1. Linear Rheology. Frequency sweep measurements were carried out on the surfactant solutions, keeping the strain amplitude fixed at 0.1% corresponding to the linear viscoelastic regime (Figures 6 and 7). The linear viscoelastic spectra of the storage (G0 ) and loss (G00 ) moduli at R = 0.05 and R = 0.1 corresponding to the nematic phases at 20 C are shown in Figure 6. In the viscoelastic spectra at R = 0.05, G0 approaches a plateau at high frequencies (Figure 6A), where it crosses above G00 . For R = 0.1, at low frequencies, the slopes of the curves of G0 and G00 are nearly in the ratio 2:1. At high frequencies the two curves merge with their slope ∼ 0.5 (Figure 6B). The viscoelastic spectra of the isotropic phase at R = 0.15 (Figure 7A) exhibits two distinct crossover frequencies with G0 > G00 . The high-frequency crossover occurs at around 100 rad/s. Moreover, a power law dependence of G0 and G00 on ω is observed for ω > 5 rad/s. At lower frequencies, however, G0 approaches a plateau value of ∼3 Pa. At R = 0.2 corresponding to the nematic phase of disklike micelles (Figure 7B), the high-frequency crossover shifts to 500 rad/s. The viscoelastic spectra obtained are similar to that of the isotropic phase, though the plateau in G0 is observed for frequencies G00 over the range (32) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: New York, 1999. (33) Watanabe, H.; Kotaka, T. Macromolecules 1983, 16, 769.

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Figure 6. Frequency sweep measurements on the different mesophases at φ = 0.4 at (A) R = 0.05 and (B) R = 0.1 at 30 C are shown. The strain amplitude was fixed at 0.1%. of angular frequencies probed suggest a glassy behavior of the dispersion of multilamellar vesicles. The high-frequency behavior can arise from a slip between the vesicles which increases the viscous dissipation.34 3.2.2. Nonlinear Rheology. Steady Shear. The flow behavior of the different phases were examined under steady : shear in a Couette geometry by varying shear rate (γ) in -1 the range 0.001-1000 s (Figure 8). Under steady shear, at R = 0.05 (Figure 8, curve a) and 0.1 (Figure 8, curve b) corresponding to the nematic phase, a finite yield stress is observed in the flow curve at low shear rates which is followed by a Newtonian plateau over a narrow range of shear rates. At higher shear rates >1 s-1, a shear thinning behavior is observed, where the viscosity decreases with shear rate. The shear thinning exponent is -0.6 at R = 0.05, which increases to -0.5 at R = 0.1. In the isotropic phase (Figure 8, curve c), the viscosity remains independent of shear rate up to 30 s-1 beyond which shear thinning occurs. A distinct flow behavior is observed for the disklike nematic phase at R = 0.2 (Figure 8, curve d), where at low shear rates, a shear thinning behavior is observed with a shear thinning exponent of -0.55 up to a shear rate of 0.3 s-1. A Newtonian plateau is observed at higher shear rates, followed by a shear thinning behavior with an exponent of -0.3, at shear rates >100 s-1. Moreover, with increase in R, the steady shear viscosity at a shear rate of 1 s-1 at 30 C decreases from the hexagonal to the lamellar phase (Figure 8, inset). The yield stress at low shear rates in the nematic phases of rodlike micelles arises from the tightly packed polydomain textures which results in a solidlike behavior at low shear rates.32 It is likely that the prominent shear thinning behavior observed at high shear rates arises when the micelles orient with their long axis along the flow direction.32 Consistent with this, the shear thinning is observed for the different (34) Liu, A. J.; Ramaswamy, S.; Mason, T. G.; Gang, H.; Weitz, D. A. Phys. Rev. Lett. 1996, 76, 3017.

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Figure 7. Frequency sweep measurements on the different mesophases at φ = 0.4 at R = (A) 0.15, (B) 0.2, and (C) 0.25 at 30 C are shown. The strain amplitude was fixed at 0.1%.

Figure 8. Viscosity vs shear rate curves of the different phases at φ = 0.4 at different values of R = (a) 0.05, (b) 0.1, (c) 0.15, and (d) 0.2 at 30 C. The dependence of the steady shear viscosity at a shear rate of 1 s-1 on R is shown in the inset. The solid line is drawn as a guide to the eye. : phases for γτ > 1 where τ is the average structural relaxation time obtained from frequency sweep measurements. This further implies that the fast relaxation observed in the frequency sweep measurements may correspond to the orientational relaxation of the micelles. Moreover, the shift in the shear thinning behavior to higher shear rates on DOI: 10.1021/la804330x

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increasing R reflects the decrease in the aspect ratio of the micelles on varying R. To describe the flow behavior of nematic phases with polydomain textures, a mesoscopic domain theory has been proposed.36 In this model, over a micron-sized domain size a, the orientation of the director is constant, and a mesoscopic order parameter is used to describe the distribution of domain directors which couples to the flow field. Under shear, the domain size is determined by the balance between : viscous stress (ηγ) and the elastic stress (K/a2) required to distort the domain, where η and K are the viscosity of the nematic fluid and the Frank elastic constant, respectively. Such a model predicts the linear dependence of the shear stress on shear rate and can thus explain the plateau region in the flow curve for nematic phases. However, the model fails to hold at high shear rates where shear distorts significantly the orientational distribution of nematic director. Very few data exist at present on the flow behavior of nematic phases of disklike micelles.27 Hence, a notable feature of the present study is also the low shear rate shear thinning behavior known as region I behavior, observed for the disklike nematic phase at R = 0.2 (Figure 8, curve d). Region I is believed to arise from a tightly packed, defect saturated, texture elasticity of a polydomain nematic seen in highly viscous liquid crystalline polymers.37 Under shear, the nematic fluid flows around the defects which act as surfaces with strong homeotropic anchoring conditions. The viscos: ity η scales as γ-0.5 under these boundary conditions.32 Region I behavior is observed in liquid crystalline polymers when the length scale of the tightly packed defects is determined by the presence of coexisting hexagonal domains or a cholesteric phase.38 Understanding the origin of the region I behavior in a relatively low viscosity nematic phase of disklike micelles as seen presently will require further studies. Transient Response under Steady Shear. The transient response of the shear stress under steady shear may be used to classify the director dynamics in the nematic phases. The measurements were carried out in a cone-plate geometry to ensure a uniform shear rate. The procedures followed for the measurements are similar to those outlined in the similar studies on liquid crystalline37 and nematic living polymerse.26 To prepare samples independent of shear history, : they were presheared at γ1 for a shearing period corresponding to 400 strain units. The shear rate was abruptly increased : : to γ2. After shearing for a time t = 400/γ2, the shear rate was : increased once again abruptly to γ3, and the same procedure : was also followed for γ4. It was also verified that a stationary state of flow is reached within 400 strain units at each of these shear rates. After sample loading, the shear rate was increased from 0.125 to 0.25 to 0.5 and finally to 1 s-1. The reverse sequence was also followed after the sample was allowed to rest briefly for 100 s. In these two sequences, the first shearing period was not considered. Hence, the : : : : : : parameter Γ (= γ2/γ1 = γ3/γ2 = γ4/γ3) is 2 for the forward sequence and 0.5 for the reverse sequence. The hexagonal phase at R = 0 reveals a single overshoot in shear stress (data not shown), and a steady state is reached within 20 strain units. In the nematic phase at R = 0.1, damped periodic oscillations of the shear stress, which (35) Winter, H. H.; Chambon J. Rheol. 1986, 30, 367. (36) Larson, R. G.; Doi, M. J. Rheol. 1991, 35, 539. (37) Walker, L. M.; Wagner, N. J.; Larson, R. G.; Mirau, P. A. J. Rheol. 1995, 39, 925. (38) Hongladarom, K.; Burghardt, W. R. Rheol. Acta 1998, 37, 46.

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Figure 9. Normalized shear stress vs strain curves at R = 0.1 for

Γ = 0.5. The symbols are described in the text. Shear stress response obtained at a shear rate of 0.5 s-1 after a preshear at 1 s-1 is shown in the inset. The texture observed 100 s after the cessation of shear is also shown in the inset. The sample was sheared at a shear rate of 1 s-1 for 300 s. The cross in the inset indicates the direction of the cross polarizers with respect to flow, where the flow direction is shown by an arrow.

determines the transient regime, are followed by a steady state (Figure 9, inset). The time period of oscillation as well as the time required to reach a steady state was found to vary inversely with shear rate (data not shown). This indicates that the relevant quantity which determines the transient behavior is the deformation or strain applied to the sample. The shear stress normalized with respect to the steady state value is plotted against the strain for Γ= 0.5 (Figure 9). A remarkable scaling is observed and indicates that around 250 strain units is required to reach a steady state. The period of : oscillation is given by P = γT, where T is the time period of : oscillation of the shear stress at a shear rate γ. The scaling behavior also indicates that both the amplitude and the period of deformation required to reach the steady state are identical at a given value of the parameter Γ. However, it should be noted that the scaling holds good only for shear rates lying in the plateau region and is not valid in the shear thinning region. At R = 0.15 and 0.2, no periodic oscillations in shear stress were observed under step shear carried out according to the procedure discussed above (Figure 10). To examine the textures of the different phases under steady shear, the samples were also sheared in a transparent, rotating, parallel plate device placed between crossed polarizers, with the gap between the plates fixed at 300 μm. Upon cessation of shear, a banded texture, with birefringent bands oriented perpendicular to the shear direction, is observed for the nematic phases at R = 0.05 and 0.1 (Figure 9, inset). No characteristic pattern is observed in the texture of the nematic phase of disklike micelles at R = 0.2 (Figure 10, inset). The single overshoot, followed by a steady state, for the hexagonal phase at R = 0 indicates the alignment of the director along the flow direction. The birefringent pattern of banded texture arises from the undulation of the directors aligned along the flow direction. The periodic oscillations are observed in the nematic phase when the director of the mesoscopic domain rotates under flow when the viscous stress is balanced by the elastic stress.36 Hence, it is classified as a tumbling nematic where the tumbling parameter λ is related to the period of deformation P as P = 4π/[(1 - λ2)1/2]. For the present system, λ is 0.92, very close to that predicted by the Doi theory of rodlike molecules39 (λ = 0.91) where a (39) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: New York, 1986.

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Figure 11. Schematic of the proposed change in the morphology of the micelles from cylinders to bilayers during the hexagonal-tolamellar transition on varying PTHC to SDS molar ratio (R) at fixed surfactant concentration (φ).

Figure 10. Normalized shear stress vs strain curves at (A) R = 0.15 and (B) R = 0.2 for Γ = 0.5. The symbols are described in the text. The texture observed at a shear rate of 1 s-1 after shearing for 300 s is shown in the inset. The cross in the inset indicates the direction of the cross polarizers with respect to flow, where the flow direction is shown by an arrow.

value of λ > 1 corresponds to a flow aligning nematic. The scaling behavior observed for the nematic phase is also predicted by the mesoscopic domain theory according to which the normalized shear stress evolves in time depending on the strain applied to the sample and the parameter Γ. The absence of scaling in the high shear rate shear thinning regime is consistent with similar observations in liquid crystalline polymers and nematic living polymers. The absence of periodic oscillations in shear stress in the nematic phase of disklike micelles at R = 0.2 also reveals that the nematic is flow aligning. Moreover, disklike micelles with high aspect ratio are expected to always align with their director perpendicular to the flow, along the gradient or vorticity direction.27,32,40 In conclusion, the rheological studies on the present system highlight, in particular, the distinct viscoelastic and flow behavior of the two nematic phases. 3.3. Route from Hexagonal-to-Lamellar Transition: A Comparison with SDS-Decanol-Water System. The present investigations have revealed a new route for the phase transition from hexagonal to lamellar in a mixed surfactant system: 2D-hexagonal f nematic f isotropic f nematic f 1D-lamellar. NMR studies indicate that the two nematic phases are made up of rodlike and disklike micelles, respectively. The linear rheology of the present system at φ = 0.4 at different R values is reminiscent of the viscoelastic behavior of cross-linked polymer gels32,35 near the gelation point. In polymer gels, a liquidlike behavior with G0 < G00 is observed before cross-linking and a solidlike behavior (G0 > G00 ) with G0 flattening at low frequencies after cross-linking, above the gelation point. At the gelation point where the cross-linking starts to occur, G0 and G00 obey power laws over the entire frequency range. A similar trend is observed for the linear visoelastic spectra of the present system at different R values. At R = 0.1, G0 and G00 merge at high frequencies (Figure 6b), and for R > 0.1, a solidlike behavior is observed with G0 > G00 and G0 approaching a constant value at low frequencies (Figure 7B). However, in the present system, by increasing R at a fixed surfactant concentration, a transition (40) Hammouda, B.; Mang, J.; Kumar, S. Phys. Rev. E 1995, 51, 6282.

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from a hexagonal to lamellar phase is observed, which occurs through a nematic phase of rodlike and disklike micelles. The change in the morphology of the micelle from a rod to disk is expected to occur through a decrease in the aspect ratio of rods as shown schematically in Figure 11. Hence, the additional evidence obtained from polarizing microscopy, smallangle X-ray diffraction, and deuterium NMR spectroscopy, regarding the phase behavior of the SDS-PTHC-water system, rules out a gelation phenomena in the present system. Rheological measurements also reveal a drastic decrease in steady shear viscosity by an order of magnitude with the addition of PTHC (at R = 0.05) as compared to the hexagonal phase of SDS-water system at φ = 0.4 (Figure 8, inset). The viscosity is lowest in the nematic phase of disklike micelles at R = 0.2. In concentrated micellar solutions, the decrease in steady shear viscosity with increasing salt concentration can arise from a decrease in the aspect ratio of the aggregates.11 However, a similar effect may also occur in micellar solutions due to an increase in the flexibility of the micelles in the presence of salt, analogous to that seen in polyelectrolyte solutions.41 Another possibility is the intermicellar branching, often reported for wormlike micellar systems at high salt concentrations giving rise to a decrease in the steady shear viscosity.24,32 In the present system, the transition from a rod to disk suggests that the decrease in the steady shear viscosity with increasing R is likely to arise from the decrease in the aspect ratio of rods due to the addition of salt. The sequence of transition also suggests that the isotropic phase in between the two nematic phases may consist of spherical micelles (Figure 11). It is likely that the decrease in the aspect ratio of the rods with increase in R drives the hexagonal f nematic and the nematic f isotropic transition at a fixed surfactant concentration. However, it is surprising that the addition of a counterion like PTHC decreases the length of the rodlike micelles. In general, addition of a hydrophobic counterion is known to increase the length of the rod in order to decrease the end-cap energy.31 Understanding the opposite trend observed in the present system, compared to its cationic counterparts,12,13,15 will require further studies on such systems. The present study on SDS-PTHC-water system indicates similarities with the SDS-decanol-water systems, with some marked differences. Though the hexagonal-tolamellar transition occurs through the presence of rodlike and disklike nematic phases, an isotropic phase is present between the two nematic phases in the SDS-PTHC-water system. There are no known reports on the viscoelastic behavior of the nematic phases of SDS-decanol-water systems. However, in the present system, a distinct viscoelastic behavior is observed for the two nematic phases. The low-frequency behavior of the concentrated isotropic phase with a slow relaxation mode is also found to be distinct from the concentrated isotropic micellar solutions of the SDS(41) Majid, L. J. J. Phys. Chem. B 1998, 102, 4064.

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water system (data not shown) which will be a subject of our future investigations. Another interesting aspect is the director dynamics of the two nematic phases which shows a tumbling and flow aligning behavior. This is clearly distinct from that observed for the nematic phases of the SDSdecanol-water system, where both are found to be flow aligning.

4.

Conclusions

In conclusion, the influence of the strongly bound cationic counterion PTH+ on the phase behavior of concentrated aqueous solutions of the anionic surfactant SDS was studied using X-ray diffraction and polarizing optical microscopy. On increasing the concentration of the counterion the hexagonal phase of SDS is found to show the following sequence of transformations: hexagonal f nematic f isotropic f nematic f lamellar. Deuterium NMR measurements identified the nematic phase near the hexagonal phase to be formed by rodlike micelles and that near the lamellar phase to be made up of disklike micelles. This sequence of phases suggests a gradual prolate to oblate change in the aggregate morphology with increasing counterion concentration. Such a morphological change seems to prevent the formation of other

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intermediate phases usually seen between the hexagonal and lamellar phases. The change in morphology significantly modifies the viscoelastic behavior besides decreasing the steady shear viscosity. Transient shear stress response under steady shear classified the rodlike and disklike nematic phases to be tumbling and flow aligning, respectively. Our study hence highlights the significant role of strongly binding counterions in modifying the phase behavior as well as viscoelastic and flow properties of concentrated surfactant solutions. We hope that this will motivate further experiments and theoretical work to understand the phase transitions and flow behavior of these class of systems formed by oppositely charged surfactants. Acknowledgment. A.K.S. thanks the Council of Scientific and Industrial Research (CSIR) for the prestigious Bhatnagar Fellowship. R.K. thanks the Indian Institute of Science for the IISc Centenary Fellowship. V.R. thanks CSIR for the research fellowship. We thank Prof. K. V. Ramanathanand Sankeerth Hebbar for their invaluable help in NMR measurements. Additional SAXS and rheology measurements by Antara Pal and Ajay Singh Negi, respectively, are gratefully acknowledged.

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