Langmuir 1998, 14, 6025-6029
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Rheology of Wormlike Micelles with Varying Hydrophobicity of the Counterion P. A. Hassan,† S. J. Candau,‡ F. Kern,‡ and C. Manohar*,† Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India, and Laboratoire de Dynamique des Fluides Complexes, 4 rue Blaise Pascal, Universite Louis Pasteur, 67070, Strasbourg Cedex, France Received March 25, 1998. In Final Form: July 9, 1998
Dynamic viscoelastic experiments were performed on mixtures of cetyl trimethylammonium tosylate (CTAT) and cetyl trimethylammonium 3-hydroxy naphthalene 2-carboxylate (CTAHNC) to understand the effect of hydrophobicity of the counterions on the structure of aggregates formed. The total surfactant concentration is fixed at 100 mM, and the ratio of the two counterions HNC- to tosylate- (x2) is varied from 0 to 0.7. The zero shear viscosity (η) and the terminal time of stress relaxation (τR) initially increases slowly with x2 and then decreases drastically for x2 > 0.5. The initial increase of both η and τR can be explained by an increase of the micellar length owing to the more hydrophobicity of HNC-, which reduces the scission energy of the micelles due to the decrease of the electrostatic contribution. A decrease of the η and τR above x2 ) 0.5 is attributed to the formation of intermicellar connections, which is favored at very high salt content. The presence of intermicellar branching speeds up the reptation process and decreases the average micellar lengths thereby reducing η and τR. Analysis of the data by the theory of Cates suggests that increasing the fraction of HNC- increases the ratio of entanglement length to the contour length of the micelles. This is consistent with the temperature dependence of the rheological data. The present investigation supports the earlier observation that branched micelles constitute the intermediate structures between linear micelles and bilayers.
Introduction The rheological properties of aqueous solutions of cationic surfactants such as alkyl trimethylammonium surfactants with different binding organic counterions have been studied extensively.1-15 Elongated flexible micelles have been observed in similar systems and were investigated in detail by techniques such as light scattering,6-9 NMR,10-12 flow birefringence,13 and rheology.14,15 Cetyl trimethylammonium (CTA+) forms elongated micelles in the presence of counterions such as tosylate (T-), and the linear viscoelastic properties of the CTAT-water system have been examined in detail by rheology,14,15 fluorescence recovery after fringe pattern photobleaching,16 etc. Rheological studies have revealed that this system forms elongated micelles at low and * To whom correspondence should be addressed. † Bhabha Atomic Research Centre. ‡ Universite Louis Pasteur. (1) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1989, 93, 5888. (2) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (3) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1989, 5, 398. (4) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (5) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474. (6) Porte, G.; Appell, J.; Poggi, Y. J. Phys. Chem. 1980, 84, 3105. (7) Appell, J.; Porte, G.; Khatory, A.; Kern, F.; Candau, S. J. J. Phys II 1992, 2, 1045. (8) Rahege, H.; Hoffmann, H.; Wunderlich, I. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1071. (9) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 744. (10) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (11) Ulmius, J.; Wennerstrom, H.; Johannson, L.B.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (12) Anet, F.A. L. J. Am. Chem. Soc. 1986, 108, 7102. (13) Hoffmann, H.; Kalus, J.; Thurn, H.; Ibel, K. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1120. (14) Soltero, J. F. A.; Puig, J. E.; Manero, O.; Schulz, P. C. Langmuir 1995, 11, 3337. (15) Soltero, J. F. A.; Puig, J. E.; Manero, O. Langmuir 1996, 12, 2654.
intermediate concentrations and it yields a hexagonal phase above 27% (w/w) at 25 °C.15 Recent studies have shown the existence of vesicles in aqueous solutions of CTAHNC17,18 up to a concentration of 3% (w/w), and above 3% the vesicles form bilayer structures. The vesicular phase is obtained at room temperature, and this undergoes a phase transition to form a viscoelastic liquid either by heating or by adding surfactants.19 The physical origin of such vesicle-micelle transition could be viewed in terms of two contributing factors: the geometric packing considerations,20 which impose limit on the possible values of the headgroup area in a single component micellar aggregate, and the curvature energy21,22 of the aggregate, which imposes restrictions on the bending rigidity modulus of a bilayer or monolayer surface. It suggests that the hydrophobicity of the counterion plays an important role in deciding the structure of the supramolecular assemblies such as vesicles, micelles, etc. Thus by changing the relative ratio of HNC-/T- in a fixed concentration of CTA+, one can probe the intermediate structures formed as it transforms from cylindrical micelles (as in pure CTAT) to vesicles or bilayers (as in pure CTAHNC). With a view to understand this effect of the hydrophobicity of the counterions on the morphology of the aggregates formed, rheological experiments were performed on mixtures of (16) Narayanan, J.; Manohar, C.; Langevin, D.; Urbach, W. Langmuir 1997, 13, 398. (17) Hassan, P. A.; Valaulikar, B. S.; Manohar, C.; Kern, F.; Bourdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350. (18) Hassan, P. A.; Narayanan, J.; Menon, S. V. G.; Salkar, R. A.; Samant, S. D.; Manohar C. Colloids Surf., A 1996, 117, 89. (19) Salkar, R. A.; Hassan, P. A.; Samant, S. D.; Valaulikar, B. S.; Kumar, V. V.; Kern, F.; Candau, S. J.; Manohar, C. Chem. Commun. 1996, 1223. (20) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (21) Helfrisch, W. Z. Naturforsch. 1978, 33a, 305. (22) Helfrich, W. J. Phys. (Paris) 1985, 46, 1263.
S0743-7463(98)00335-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/13/1998
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CTAHNC and CTAT, keeping the total surfactant concentration the same. The present paper is an attempt to study the effect of hydrophobicity of the counterions on the structure of the aggregates formed. Here we report the rheological investigations on a mixture of CTAHNC and CTAT at various temperatures and mole fractions. In all the measurements the total surfactant concentration is kept as 100 mM and the mole fraction of CTAHNC (x2) is varied from 0 to 0.7. For x2 > 0.7, the system begins to form vesicles or bilayers as indicated from the turbidity of the solution at room temperature. Thus all measurements were restricted to those mixtures for which x2 e 0.7, where one has a clear homogeneous solution. The rheological results were analyzed in terms of the theory of Cates for “living” polymer solutions. Materials and Methods CTAB and CTAT were purchased from Sigma Chemicals and SHNC was from Atul Products, Bombay. CTAHNC was prepared from CTAB and SHNC by removing the counterions (Na+ and Br-).17-19 The product was purified by recrystallization. Samples were prepared by weighing CTAHNC, CTAT, and water directly in the sample vial and keeping the mixture at 70 °C for sufficient time to dissolve the substance and to obtain a homogeneous solution. After thorough mixing the samples were kept in an oven at 40 °C for 1 day to reach the equilibrium. The rheological measurements were done on a Rheometrics RFS II fluid spectrometer using Couette geometry and parallel plate geometry. The parallel plate geometry consisted of a titanium upper plate and an aluminum-coated lower plate, both of 50 mm diameter. Proper care was taken to avoid evaporation of the sample. The Couette geometry consisted of a 34 mm diameter aluminum-coated cup and a titanium bob of diameter 32 mm and height 33.3 mm. The frequency range investigated was from 0.1 to 100 rad/s. Experiments were carried out with imposed strain equipment with appropriate correction for inertia effects in the high-frequency range. The temperature range investigated was from 30 to 55 °C.
Theoretical Considerations In the semidilute regime the viscoelastic behavior of the entangled micelles is reminiscent of that of polymeric networks. One important difference in the transient character of a micellar solution from a “dead” polymeric network is that the micelles can break and recombine on a time scale that is characteristic of the system under investigation. The dynamics of such a “living” polymer solution was studied in detail by Cates and co-workers.23,24 The model of Cates is based on the reptation model of polymer dynamics but including the effect of reversible scission kinetics on the viscoelastic behavior. This model involves two relevant time scales that are the reptation time, τrep, and the breaking time, τb. The reptation time corresponds to the curvilinear diffusion of a chain of the mean length L h along a tube that is constrained by the entanglements from other chains and the breaking time corresponds to the meantime required for a chain of length L h to break into two pieces. It is assumed that the chemical relaxation process is the reversible unimolecular scission characterized by a temperature-dependent rate constant k1 per unit arc length per unit time, which is same for all elongated micelles and is independent of time and of volume fraction. This assumption results in
τb ) (k1L h )-1
(1)
For τb . τrep, the dominant stress relaxation mechanism (23) Cates, M. E. Macromolecules 1987, 20, 2289. (24) Cates, M. E.; Turner, M. Europhys. Lett. 1990, 11, 68.
is reptation. Then the stress relaxation function indeed obeys the equation23
µ(t) ∼ exp(-t/τrep)1/4
(2)
as expected for a system of chains with exponential polydispersity. Thus in this regime the terminal relaxation time, τR ) τrep. The zero shear viscosity η0 is related to the terminal time τR and the plateau modulus Go by the relation
τR ) ηo/Go
(3)
When τb , τrep, an interesting new regime occurs. In this regime, chain breakage and recombination will both occur often, for a typical chain, before it reptates out of the tube segment. The stress relaxation is then characterized by a new intermediate time scale, τR ) (τbτrep)1/2, associated with a process whereby the chain breaks at a point close enough to a given segment of tube for reptation relaxation of that segment to occur before a new chain end is lost by recombination. The stress relaxation function thus becomes more like a single exponential because before a given tube segment relaxes, the chain occupying it typically undergoes many scission and recombination reactions, so that there is no memory of either the initial length of the chain or the position on the chain initially corresponding to the tube segment. Thus at low frequencies the behavior of the liquid is Maxwellian and is described by the equations
G′(ω) )
G0(ωτ)2 2
1 + (ωτ)
and
G′′(ω) )
G0ωτ 1 + (ωτ)2 (4)
where G0 is the plateau modulus and τ ) η0/G0 is the relaxation time. The Cole-Cole representation, in which the imaginary part G′′(ω) of the frequency-dependent shear modulus is plotted against the real part G′(ω), can be used to get an estimate of the relaxation time, τ. The Maxwellian behavior is ascertained by a semicircular shape of the Cole-Cole plot G′′(G′), but deviations from the half circle occur at a circular frequency, ω, of the order of the inverse of the breaking time of the micelles. Thus at low frequencies the behavior of the liquid is Maxwellian and is ascertained by a semicircular shape of the ColeCole plot G′′(G′), but deviations from the half circle occur at a circular frequency, ω, of the order of the inverse of the breaking time of the micelles. The model of Granek and Cates25 can also be applied to study the regimes involving small time scales where the dominant polymer motion is not reptation but either breathing (which arises from the tube length fluctuations) or the local Rouse-like motion (arising from stretches of chain shorter than the entanglement length, le). This regime is characterized by an apparent turn up of both G′(ω) and G′′(ω) at high frequencies. This results in a minimum in the G′′(G′) in the Cole-Cole representation. This picture applies when the entanglement length, le, is much larger than the persistence length, lp, and the breaking time is much larger than the Rouse time, τe. It was found that, provided τb . τe, the value of G′′ at the dip obeys the relation
h G′′min/G′∞ ) le/L
(5)
From the values of G′′min, G′∞, and le one can thus obtain (25) Granek, R.; Cates, M. E. J. Chem. Phys. 1992, 96, 4758.
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Langmuir, Vol. 14, No. 21, 1998 6027
Figure 1. Variation of the storage modulus (G′), loss modulus (G′′), and real part of the complex viscosity (η′) as a function of frequency (ω) at 35 °C for a CTAHNC-CTAT mixture. The mole fraction of CTAHNC (x2) is 0.1, and the total surfactant concentration is 100 mM.
Figure 2. Variation of the storage modulus (G′), loss modulus (G′′), and real part of the complex viscosity (η′) as a function of frequency (ω) at 35 °C for a CTAHNC-CTAT mixture. The mole fraction of CTAHNC (x2) is 0.7 and the total surfactant concentration is 100 mM.
an estimate of micellar length, L h . For flexible micelles the entanglement length, le, can be estimated from the relation26
G′∞ )
kBT ξ
3
)
kBT 9/5 6/5 le lp
(6)
where ξ is the correlation length. Results and Discussion Figures 1 and 2 show the variation of G′, G′′, and η′ as a function of frequency ω, for CTAHNC-CTAT mixtures at 303 K. The figures are the representative curves for two mole fractions of CTAHNC, say x2 ) 0.1 and x2 ) 0.7. It can be seen that, in both the figures, the behavior of G′(ω) and G′′(ω) is characteristic of that of a viscoelastic surfactant solution. This result in quantitative agreement with the theory of Cates et al.,23,24 which predicts that the behavior is close to Maxwellian at low frequencies (i.e., at ωτ < 1). A notable difference between the two spectra is that the frequency range at which a close to Maxwellian behavior is observed is shifted to higher frequencies as x2 (26) Berret, J. F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851.
Figure 3. Variation of the terminal time of stress relaxation (τR) as a function of mole fraction of CTAHNC (x2) at two temperatures 30 and 50 °C.
increases. The ω at which G′ and G′′ crosses each other (ωR) is approximately equal to the reciprocal of the stress relaxation time (τR), and this frequency ωR is higher for higher values of x2. This indicates that the stress relaxation time decreases as one replaces some of the tosylate ions in CTAT with more hydrophobic HNC- ions. A decrease of τR and consequent shift of ωR to higher frequencies have been observed in other viscoelastic surfactant solutions with an increase of temperature, and this has been attributed to the decrease of the average length of the micelles. Figure 3 shows the variation of τR (obtained from ηo and Go using eq 3) as a function of x2 at two different temperatures 35 and 50 °C. As is seen from the figure, at a given mole fraction, the stress relaxation time (τR) decreases with an increase of temperature. It seems reasonable to attribute this change to a decrease of the average length of the micelles. At a given temperature, with an increase of x2, τR increases initially and then decreases above x2 ∼ 0.5. This suggests that an increase in the ratio of HNC- to tosylate tends to increase the average length of the micelles initially and then decreases at high mole fractions of HNC-. The initial increase of the length of the micelles arises from a decrease of the electrostatic interaction due to the high lipophilicity of HNC- relative to tosylate. The more hydrophobic HNCforms a 1:1 complex with CTA+ thereby giving a neutral species, whereas in CTAT the degree of dissociation is expected to be more than that observed in CTAHNC. Thus replacing some of the tosylate ions with HNC- leads to a decrease in the charge density of the aggregate surface thereby decreasing the electrostatic effect. It has been observed in other ionic micellar systems that the addition of moderate amounts of electrolyte leads to an increase in the average micellar length whereas a decrease in length is observed at very high salt content.27,28 It is generally admitted that increasing the salt concentration amounts to increase the curvature energy of the surfactant molecules in the end-cap. This leads to an increase in the average micellar length. At very high salt content, it has been proposed that any change of conditions that increases the end-cap energy should decrease the energy required to form a cross link, and hence intermicellar connections could be a favored aggregate structure. The presence of such intermicellar connections, first proposed by Porte et (27) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (28) Candau, S. J.; Khatory, A.; Lequeux, F.; Kern, F. J. Phys. IV 1993, 3, 197.
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Figure 4. Variation of the zero shear viscosity as a function of mole fraction of CTAHNC (x2) at 35 °C.
al.,29 then suggested from rheological studies,7 is now well established from cryogenic temperature transmission electron microscopy experiments.30 According to Lequeux,31 the reptation process is speeded up by the formation of intermicellar connections and hence the terminal time is increasingly reduced. Thus one can think of the formation of branched micelles in CTAT-CTAHNC mixtures at x2 > 0.5, which are the precursors for bilayers or vesicles. This can also be understood from a change in the zero shear viscosity of the solution as function of x2. Figure 4 shows the variation of zero shear viscosity (ηo) of CTAT-CTAHNC mixtures as a function of x2 at 35 °C. It can be seen that the viscosity initially increases and then decreases drastically when the mole fraction of CTAHNC in the mixture becomes greater than 0.5. This variation in ηo reflects the variations observed in τR. This observed variation in the viscosity of the solutions is in accordance with the earlier observations on the effect of salt content on the rheological behavior of the micelles.27-28,32 For semidilute solutions of flexible polymer chain, the plateau modulus, Go depends only on the mesh size, which depends on the concentration. Since the total surfactant concentration in all the mixtures is kept constant as 100 mM, we expect the plateau modulus to remain the same. It is found that the Go value remains nearly constant for all mixtures up to x2 ) 0.5, its value being approximately 50 Pa (see Figure 1). For x2 ) 0.6, the Go increases rapidly to a value of 90 Pa. In the case of a mixture having x2 ) 0.7, in the frequency range under investigation, the viscoelastic spectra is not complete enough to measure the value of Go. But, according to the Maxwell model, one can approximate the value of Go as twice the value of the modulus where G′ and G′′ cross each other. From Figure 2 one can see that G′ and G′′ cross each other at ∼100 Pa, and hence the value of Go can be approximated to 200 Pa, which is much higher than the value of 50 Pa observed for mixtures having x2 e 0.5. This increase in Go suggests a decrease in the mesh size of the network possibly due to an increase in the persistence length of the micelles. Temperature dependence of the rheological spectra were measured in order to understand the effect of temperature on various rheological properties and hence to get an (29) Porte, G.; Gomati, R.; El Haitamy, O.; Appell, J.; Marignan, J. J. Phys. Chem. 1986, 90, 5746. (30) Danino, D.; Talmon, Y.; Levy, H.; Beinert, G.; Zana, R. Science 1995, 269, 1420. (31) Lequeux, F. Europhys. Lett. 1992, 19 (8), 675. (32) Narayanan, J.; Manohar, C.; Kern, F.; Lequeux, F.; Candau, S. J. Langmuir 1997, 13, 5235.
Hassan et al.
Figure 5. Semilogarithmic representation of the variation of zero shear viscosity (η) versus the inverse temperature for different values of x2.
Figure 6. Variation of the activation energy for viscosity versus CTAHNC mole fraction (x2).
insight into the flexibility of the micelles. Figure 5 show the plots of logarithm of viscosity versus reciprocal of the absolute temperature for different mole fractions of CTAHNC. It is found that the plots are linear at all x2, indicating an Arrhenian type behavior. From the slope of the plots the activation energies have been evaluated and are depicted in Figure 6. As is clear from the figure, the activation energy remains the same for mole fractions up to x2 ) 0.5, being approximately equal to 75kT, but decreases drastically for x2 >0.5. This relatively high value of activation energy has been observed in other long flexible micellar solutions. For typical polymer solutions, the viscosity is a weak function of temperature (and hence low value of activation energy) in contrast to the living polymer solutions. This strong dependence of viscosity on temperature has been interpreted to occur as a consequence of the decrease in the average length upon increasing the temperature. Thus the low value of activation energy observed upon increasing x2 indicates that the micelles formed are rigid and/or more stable than those observed in pure CTAT solutions. The increased rigidity of the micelles can also be understood from an increase of the parameter ζ (τb/τrep) and the ratio h as is observed from the Cole-Cole plots. Figure 7 le/L shows the normalized Cole-Cole representation of G′′(G′) of CTAHNC-CTAT mixtures at three different mole fractions of CTAHNC. It is found that the data follow a Maxwellian behavior at low frequencies, and deviation from the Maxwellian behavior occurs at higher frequencies
Rheology of Wormlike Micelles
Figure 7. Normalized Cole-Cole plots of the CTAHNC-CTAT mixtures at three different mole fractions of CTAHNC (x2) at 30 °C.
due to the involvement of the breaking time of the micelles24 and/or by the overlapping of reptation and Rouse processes. In such a situation where the Rouse mode and reptation processes overlap, one can clearly see a minimum in the Cole-Cole plot and the ratio of G′′min to Go decides h . For mole fractions less than 0.5 the the parameter le/L h behavior is almost similar indicating that the ratio le/L remains the same. As is seen from the figure, the highfrequency part of the normalized Cole-Cole plots lift up and up as the mole fraction increases from 0.5, showing h . This shift of the dip in an increase in the parameter le/L the Cole-Cole plot occurs both by a decrease in L h and by an increase in le. If the plateau modulus Go remains the h can be accounted same, then the increase in the ratio le/L for solely by a decrease in L h . But, in the present case the plateau modulus Go is constant only up to x2 ) 0.5, above which it increases sharply (see the difference in Go for x2 ) 0.1 and x2 ) 0.7 in Figures 1 and 2, respectively). Thus it seems that the variation in le/L h possibly comes from a combined effect of increase in the entanglement length and a decrease in the contour length of the micelles. Time-temperature superposition master curves33 can be obtained for the dynamic moduli at all mole fractions of CTAHNC. Figure 8 shows one representative superposed master curve for a mole fraction of 0.1 The shift factors used are those of WLF equation and the reference temperature chosen was at 45 °C. It can be seen that the superposition is achieved remarkably well except at very high frequencies where the dominant polymer motion is not reptation but breathing or local Rouse-like motions. The WLF shifting factors (ln aT) obtained are tabulated in Table 1 for different mole fractions and at different temperatures. It is clear from the table that for x2 ) 0, ln aT varies from 4.5 at 30 °C to -2.8 at 55 °C whereas for x2 ) 0.7, it varies from 0.3 to -0.4. The very high spread in the value of the shift factor at small x2 is an indication of the increased flexibility and/or less stability of the micelles. Thus all this evidence leads to the conclusion that as one increases the hydrophobicity of the counterions, that is to say on changing from tosylate to HNC-, the micelles become more stable toward temperature. (33) Ferry, J. D Viscoelastic Properties of Polymers; Wiley: New York, 1980.
Langmuir, Vol. 14, No. 21, 1998 6029
Figure 8. Time-temperature superposition master curves for both G′ and G′′ of CTAHNC-CTAT mixture at x2 ) 0.1. Table 1. Variation of the WLF Shifting Parameter, ln aT, for Different Mole Fractions of CTAHNC ln aT x2
T) 30 °C
T) 35 °C
T) 40 °C
T) 45 °C
T) 50 °C
T) 55 °C
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
4.5 4.0 4.0 3.5 2.3 2.3 1.2 0.3
3.3 2.7 2.7 2.3 1.6 1.6 0.9 0.2
1.5 1.5 1.5 1.2 0.9 0.9 0.6 0.15
0 0 0 0 0 0 0 0
-1.3 -1.2 -1.2 -1.3 -0.9 -0.9 -0.5 -0.15
-2.8 -2.4 -2.4 -2.4 -1.9 -1.9 -1.0 -0.4
Conclusion In conclusion, we have studied the dynamics of flexible polymer-like micelles formed in mixtures of CTAT and CTAHNC at various temperatures. By changing the relative ratio of HNC- to tosylate in the mixture, one can probe the effect of hydrophobicity of the counterions on the structure of aggregates formed. CTAT forms elongated micelles whereas CTAHNC, having a more hydrophobic counterion, forms bilayers at 100 mM concentration. It is observed that an increase of the hydrophobicity of the counterions, will have the same effect as that of addition of salt to a solution of charged micelles. This can be viewed as a consequence of the ion-pairing of the surfactant and counterions which effectively reduces the charge density on the aggregate. The mean micellar length tends to follow the same pattern as that of screening of the electrostatic interactions by the addition of salt. At small mole fractions of x2, the micelles grow because of the increase in the end-cap energy, whereas at high x2 intermicellar connections become favorable. Thus it seems that branched micelles constitute the intermediate structures formed between wormlike micelles and bilayers. This is consistent with the earlier conclusions made from the rheological studies on addition of salt to viscoelastic solutions.31 Acknowledgment. This work was performed under the Indo-French collaboration project (Project No. 1007-1 sanctioned by IFCPAR). LA980335I
