Langmuir 2006, 22, 9853-9859
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Threadlike Micelle Formation of Anionic Surfactants in Aqueous Solution Kenji Nakamura and Toshiyuki Shikata* Department of Macromolecular Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed April 15, 2006. In Final Form: September 3, 2006 We have investigated the formation of threadlike micelles consisting of anionic surfactants and certain additives in aqueous solution. Threadlike micelles long enough to be entangled with each other were formed in a clear aqueous solution of two anionic surfactants, sodium hexadecyl sulfate and sodium tetradecyl sulfate. These solutions also contained pentylammonium bromides or p-toluidine halides and exhibited remarkable viscoelasticity. Because the molar ratio of surfactants to cationic additives in these micelles seemed close to unity, they formed 1:1 stoichiometric complexes between surfactant anions and additive cations, as previously found in systems of cationic surfactants such as hexadecyltrimethylammonium bromide and sodium salicylate. The viscoelastic behavior of these anionic threadlike micellar systems was adequately described by a simple Maxwell element with a single relaxation time and strength, as in many similar cationic systems.
Introduction Surfactant molecules dissolved in aqueous solution form micelles of many shapes depending on their molecular structure and also on the solution conditions such as temperature, concentration, and the presence of additives.1,2 Threadlike micelles are enormously long, sufficient to entangle each other and to cause pronounced viscoelasticity in solution. These are among the largest molecular assemblies and can be thought of as onedimensional supramolecular polymers constructed in aqueous solution via intermolecular interactions such as hydrophobic, electrostatic, and cation-π interactions. Most of the threadlike micelles reported in the literature3,4 have been constructed from cationic surfactants and added salts, for example, from cetyltrimethylammonium (or hexadecyltrimethylammonium) bromide (CTAB) in aqueous solution containing simple salts such as KBr at concentrations higher than 0.25 M.5 The addition of a sufficient amount of simple salts to spherical micellar systems effectively screens strong electrostatic repulsions between surfactant headgroups, trimethylammonium cations, on the micellar surface, so the effective size of the headgroup is reduced, inducing a micellar shape transition from a spherical to a threadlike shape. Highly entangled long threadlike micelles have also been constructed in aqueous CTAB using hydrophobic aromatic salts such as sodium salicylate (NaSal)6,7 or sodium p-toluenesulfonate (NapTS),7-9 classified as hydrotropes, in the form of 1:1 stoichiometric saltlike complexes between CTA+ and Sal- (or pTS-). Because these anionic hydrotropes10,11 preferentially intercalate between headgroups * To whom correspondence should be addressed. E-mail: shikata@ chem.sci.osaka-u.ac.jp. (1) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (2) Israelachivili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: New York, 1991. (3) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (4) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (5) Debye, P.; Anacker, E. W. J. Phys. Colloid Chem. 1951, 55, 644. (6) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (7) Imai, S.; Shikata, T. J. Colloid Interface Sci. 2001, 244, 399. (8) Imai, S.; Kunimoto, E.; Shikata, T. Nihon Reoroji Gakkaishi 2000, 28, 61. (9) Soltero, J. F. A.; Puig, J. E. Langmuir 1996, 12, 2654. (10) Balasubramanian, D.; Srinivas, V.; Gaiker, V. G.; Sharma, M. M. J. Phys. Chem. 1989, 93, 3865. (11) Bhat, M.; Gaikar, V. G. Langmuir 1999, 15, 4740.
of CTA+ on the micellar surface and effectively reduce the electrostatic repulsive forces between positively charged surfactant headgroups, they are more efficient in inducing the transition from spherical to threadlike micelles, which show remarkable viscoelasticity at a lower concentration than that of simple salts. These results have revealed that the essential mechanism of formation of threadlike micelles in the CTAB and NaSal (or NapTS) system is slightly different from that in the CTAB and KBr system. Only a few studies have been reported on the construction of such micelles from anionic surfactants.12-16 Anionic threadlike micelles have been formed by the addition of a sufficient amount of simple salts such as NaCl to aqueous solutions of anionic surfactants such as sodium dodecyl sulfate (NaC12S), as in the cationic case, whereas the anionic system behaves as a slightly viscous liquid even at high surfactant concentrations.12 The length of these anionic micelles is not as great as in the cationic systems. Recently, Kaler et al.14 have reported the formation of anionic threadlike micelles in a system consisting of NaC12S with a relatively small amount of a cationic hydrotrope, p-toluidine hydrochloride (H3tolCl). They have shown that the hydrotrope induced the growth of threadlike micelles by screening the electrostatic repulsive forces between the anionic surfactant headgroups, as in other anionic surfactant systems. However, the micellar solution was of low viscosity and became turbid when NaC12S and H3tolCl were mixed in a 1:1 molar ratio in aqueous solution, indicating that the electrostatic interaction between C12S- and H3tol+ was stoichiometric and too strong for the formation of stable anionic threadlike micelles. Clearly, it is important in the construction of anionic threadlike micelles to control the magnitude of the electrostatic interaction not only between surfactants (repulsive interaction) but also between surfactants and additive salts (attractive interaction). It is well-known that two factors are essential for the construction of threadlike micelles: one is the control of the two electrostatic interactions just mentioned, and the other is control (12) Magid, L. J.; Li, Z.; Butler, P. D. Langmuir 2000, 16, 10028. (13) Mu, J. H.; Li, G. Z.; Jia, X. L.; Wang, H. X.; Zhang, G.. Y. J. Phys. Chem. B 2002, 106, 11685. (14) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543. (15) Raghavan, S. R.; Gerhard, F.; Kaler, E. W. Langmuir 2002, 18, 3797. (16) Kalur, G. C.; Raghavan, S. R. J. Phys. Chem. B 2005, 109, 8599.
10.1021/la061031w CCC: $33.50 © 2006 American Chemical Society Published on Web 10/26/2006
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over the physicochemical properties of the hydrocarbon chains of the surfactants.1 Surfactants bearing hydrocarbon chains longer than tetradecyl (C14) groups are sometimes able to form threadlike micelles, while surfactants with shorter chains are not. When surfactants carry linear hydrocarbon chains longer than octadecyl (C18), their solubility is too low to form micelles at room temperature. However, surfactants such as oleyldimethylamine oxide, bearing an oleyl (Ole) group, which includes a double bond in the middle of its alkyl chain and has much lower crystallinity than the octadecyl group with the same number of carbons, do form stable threadlike micelles at room temperature. It has been widely accepted that cetyl (or hexadecyl, C16) groups are the best linear alkyl chains for threadlike micelle formation at room temperature because the corresponding cationic surfactant, CTAB, forms long threadlike micelles with so many kinds of hydrotropes due to their highly controlled 1:1 stoichiometric interaction with CTAB. If one intends to construct anionic threadlike micelles using commonly available short alkyl chain surfactants such as NaC12S, one must precisely tune the electrostatic interaction between NaC12S and carefully chosen cationic hydrotropes. However, employing sulfate-type surfactants bearing alkyl chains longer and more hydrophobic than C12 can be expected to slightly increase the possibility of anionic threadlike micelle formation in aqueous solution. When one chooses a combination of an alkyl sulfate-type surfactant with longer alkyl chains and an ammonium-type cationic hydrotrope for the formation of such micelles, exchanging some or all of the hydrogen atoms attached to the ammonium nitrogen of the cationic hydrotrope for slightly bulkier alkyl groups such as methyl and/or ethyl groups permits the precise control of charge separation and therefore the electrostatic interaction between charge centers of the anionic (sulfate) and cationic (ammonium) groups in the micelles. In this paper we describe an investigation of anionic threadlike micelle construction from anionic surfactants and cationic hydrotropes in aqueous solution, systematically taking into account the two essential points described above. The pronounced viscoelastic behavior of these solutions will be discussed. Certain sodium alkyl sulfates, such as NaC12S and its analogues with longer C14 (NaC14S) and C16 (NaC16S) linear alkyl chains, were employed as anionic surfactants. For the cationic hydrotropes, a series of N,N,N-trialkyl-p-toluidine halides (R3tolX; R ) H, Me, and Et, X ) Cl-, Br-, and I-) and also of n-pentyl-N,N,Ntrialkylammonium bromides (C5R3NBr; R ) H, Me, and Et) possessing ammonium headgroups of various sizes, were used (Scheme 1). Experimental Section Materials. Sodium alkyl sulfates NaC12S, NaC14S, and NaC16S were purchased from Wako Pure Chemicals (Osaka) and used without further purification. p-Toluidine, N,N-dimethyl-p-toluidine, and
Nakamura and Shikata 1-bromopentane for the synthesis of cationic hydrotropes were purchased from Sigma-Aldrich Japan (Tokyo), Nacarai Tesque (Kyoto), and Tokyo Kasei (Tokyo), respectively. Iodomethane and bromoethane were purchased from Sigma-Aldrich Japan and Wako Pure Chemicals, respectively. Triethylamine and 30% trimethylamine aqueous solution were purchased from Wako Pure Chemicals. Dimethylethylamine and diethylmethylamine were purchased from Tokyo Kasei and Aldrich (Milwaukee), respectively. Deuterium oxide (D2O) was purchased from Aldrich and used as the solvent for NMR measurements. Highly deionized water with specific resistance higher than 16 MΩ cm, obtained using a Milli-Q system (Japan Millipore, Tokyo), was used as the solvent for sample preparation. Synthesis. R3tolX compounds were synthesized by the quarternization of p-toluidine derivatives with various alkyl halides or hydrohalogenic acids. p-Toluidine hydrochloride (H3tolCl) and N,Ndimethyl-p-toluidine hydrobromide (Me2HtolBr) were prepared by the neutralization of p-toluidine and N,N-dimethyl-p-toluidine with aqueous HCl and HBr, respectively. N,N,N-Trimethyl-p-toluidine iodide (Me3tolI) and N,N-dimethyl-N-ethyl-p-toluidine bromide (Me2EttolBr) were synthesized by refluxing a methanol solution of N,Ndimethyl-p-toluidine with excess iodomethane and bromoethane, respectively, at 60 °C for 12 h. The species of halogen ion present in the cationic hydrotropes did not significantly affect the results. A series of pentyl-N,N,N-trialkylammonium bromides (C5R3NBr) were synthesized by the quarternization of N,N,N-trialkylamines with 1-bromopentane as follows: a methanol solution of 1-bromopentane containing excess amine (R3N) was refluxed at 60 °C for 12 h. After the reaction, the solvents and unreacted amines were evaporated completely, and the residues were washed with ether to obtain the desired products in powder form. The purity of the synthesized cationic hydrotropes was confirmed by 1H NMR, proving that unreacted reagents were completely eliminated. Measurements. Dynamic viscoelastic measurements were performed using a stress-controlled rheometer (DynAlyser 100, ReoLogica, Lund) having a cone-plate geometry with a plate diameter and cone angle of 40 mm and 4°, respectively. Storage and loss moduli, G′ and G′′, in the linear viscoelastic regime were determined as functions of the angular frequency (ω) ranging from 2.49 × 10-3 to 1.00 × 102 rad s-1, generally at 25 °C. 1H NMR spectra were recorded on JEOL EX-270 spectrometers in the deuterium-locked mode at 30 °C. Electrical conductivity measurements were performed using an RF LCR meter (4287A, Agilent Technologies) equipped with a homemade electrode cell at 25 °C and a frequency of 1.0 × 107 rad s-1.
Results Formation of Anionic Threadlike Micelles. Conditions for the formation of threadlike micelles were first investigated in mixtures of the above-mentioned anionic surfactants and cationic hydrotropes. In these aqueous solutions, the concentration of anionic surfactants or detergents (cD) was fixed at 50 mM while that of cationic hydrotrope salts (cS) ranged from 0 to 75 mM (cScD-1 ) 1.5). Table 1 summarizes the phase behavior and the maximum values (ηmax) of zero shear viscosities (η0 ) limωf0[(G′2 + G′′2)1/2/ω] ) limωf0(G′′/ω)) of such solutions at 25 and 30 °C. NaC12S was mixed with the R3tolX series, whereas NaC14S and NaC16S were mixed with both the R3tolX and C5R3NBr series. The viscosity of an aqueous system of NaC12S and H3tolCl (NaC12S:H3tolCl/W) was highest at cScD-1 ) 0.7, with ηmax close to that reported by Kaler et al.,14 and as they found, this system became turbid at cScD-1 g 0.9. As pointed out in the Introduction, electrostatic interactions between C12S- and H3tol+ in the NaC12S: H3tolCl/W system are too strong to allow construction of stable anionic threadlike micelles with a 1:1 composition. Threadlike micelle formation was also observed in aqueous solutions of NaC12S and Me2HtolBr. The NaC12S:Me2HtolBr/W system was transparent over the range of cScD-1 examined, although the
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Table 1. Phase Behavior and Maximum Values of the Zero Shear Viscosity ηmax for Aqueous Micellar Systems of Various Sulfate-Type Surfactants Bearing Chains of Different Lengths and Cationic Hydrotropes with Different Charged Groups at cD ) 50 mM and a cScD-1 Ratio from 0 to 1.5a
a
The superscript # indicates 30 °C.
system was less viscous, with ηmax observed at cScD-1 ) 1.5. In the NaC14S:H3tolCl/W system containing the longer surfactant NaC14S, the viscosity showed a maximum at cScD-1 ) 0.5, the system became turbid just above this threshold ratio, and ηmax at this value of cScD-1 was 104 times higher than that of the NaC12S:H3tolCl/W system at cScD-1 ) 0.7. This revealed that the hydrophobicity of the surfactant alkyl chain affected the phase behavior, stability, and length of the threadlike micelles formed and controlled the viscosity of such micellar systems, because the only difference between the two foregoing systems was the alkyl chain length. In contrast, the NaC14S:Me3tolI/W system was clear over the entire range of cScD-1 examined. Although the measured viscosity was the lowest of systems containing NaC14S, it increased with increasing concentration of Me3tolI and showed ηmax at the highest cScD-1 value of 1.5 (cf. the NaC12S:Me2HtolBr/W system). This indicated that Me3tolI, which bore the largest cationic group (Me3N+), formed threadlike micelles with NaC14S in 1:1 composition only with difficulty, owing to weak electrostatic interactions caused by the large separation between the charge centers of sulfate and ammonium groups. The effects of the Me3tolI concentration in the NaC14S:Me3tolI/W system were similar to those caused by simple salts such as NaBr in ordinary cationic threadlike micelles. Me2HtolBr, which has an intermediate-sized cationic group, Me2HN+, is possibly the most appropriate hydrotrope for the construction of 1:1 anionic threadlike micelles with NaC14S, because the NaC14S:Me2HtolBr/W system was transparent over the examined range of cScD-1, with ηmax at cScD-1 ) 0.8 close to unity. However, ηmax for this system was not the highest among the NaC14S:R3tolBr/W mixtures examined. It was concluded that the size of the cationic group had a critical effect on micelle formation in these assemblies. Because the Krafft temperature of NaC16S is 31 °C,17,18 NaC16S is basically insoluble in water at 25 °C. Therefore, all aqueous NaC16S solutions with cationic hydrotropes, NaC16S:R3tolBr/W
and NaC16S:C5R3NBr/W, were turbid at cScD-1 < 0.6 at room temperature, 25 °C. However, both these systems were transparent and showed remarkable viscoelasticity even at cScD-1 > 0.6 when they were formulated with hydrotropes having relatively large cationic groups such as Me2EtN+. Since the NaC16S: Me3tolI/W and NaC16S:Me2EttolBr/W systems showed significant viscoelasticity and because their maximum viscosities occurred at cScD-1 ) 1, these micelles likely maintained a 1:1 molar ratio between NaC16S and hydrotropes. NaC16S:Me3tolI/W was turbid at 25 °C over the cScD-1 range examined, but became clear above 30 °C irrespective of cScD-1. These results indicated that the electrostatic interactions between C16S- and Me3tol+ were rather stronger than in the transparent solution at 25 °C. Consequently, the NaC16S:Me2EttolBr/W system, shaded in Table 1, was the most promising combination of NaC16S and R3tolBr for the formation of threadlike micelles with 1:1 composition. Another series of cationic hydrotropes, C5R3NBr, strongly affected the stability and viscoelasticity of the NaC16S:C5Me2EtNBr/W systems, whose maximum viscosities were about 300 times higher than those of the NaC16S:R3tolBr/W systems with the same cationic groups in the hydrotropes. The NaC16S:C5Me2EtNBr/W systems that were transparent and isotropic when stationary at room temperature became slightly turbid at the onset of shear flow during rheological measurements. Similar shear-induced phase transitions have been observed in certain threadlike micellar systems close to the phase boundaries.19,20 The NaC16S:Me2EttolBr/W system, however, remained transparent even under shear flow. Among NaC16S:C5R3NBr/W systems, NaC16S:C5Et2MeNBr/W (also shaded in Table 1) was presumably the most appropriate combination of NaC16S and C5Et2MeNBr (17) Weil, J. K.; Smith, F. S.; Stirton, A. J.; Bistline, R. G. J. Am. Oil Chem. Soc. 1963, 40, 538. (18) Lange, H.; Schwnger, M. J. Kolloid Z. Z. Polym. 1968, 223, 145. (19) Wheeler, E. K.; Fischer, P.; Full, G. G. J. Non-Newtonian Fluid Mech. 1998, 75, 193. (20) Schubert, B. A.; Wagner, N. J.; Kaler, E. W.; Raghavan, S. R. Langmuir 2004, 20, 3564.
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Figure 1. Relationship between the specific electric conductance κ0 and cationic hydrotrope salt concentration cS for the NaC16S: Me2EttolBr/W and NaC16S:C5Et2MeNBr/W systems at surfactant concentration cD ) 100 mM. Solid and dotted lines represent straight lines fitted to the data at cS < cD and cS > cD, respectively.
for the construction of anionic threadlike micelles with 1:1 molar composition, because this system remained transparent even under shear flow and because of its remarkably high viscosity. Although there were some differences in solution behavior between NaC16S:R3tolBr/W and NaC16S:C5R3NBr/W, their chief characteristics of phase and viscosity dependence on cScD-1 were similar. These results suggested that careful control of electrostatic interactions between surfactant molecules and also between surfactant and hydrotrope molecules by an appropriate choice of the size of cationic groups is one of the key factors in constructing anionic threadlike micelles in aqueous solutions of the relatively long sodium alkyl sulfates NaC14S and NaC16S. On the basis of the above observations, it is likely that aqueous mixtures of cationic and anionic surfactants form stable threadlike micelles at certain combinations of surfactant species and composition.21,22 More than three decades ago, Barker et al. found a substantial viscosity jump in an aqueous mixture of octyltrimethylammonium bromide and NaC12S,21 possibly caused by the formation of threadlike micelles in their mixture of cationic and anionic surfactants. Viscoelastic Behavior. A detailed investigation was conducted on the viscoelastic behavior of NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W, which were transparent irrespective of the cScD-1 ratio and showed profound viscoelasticity. We first confirmed the fact that NaC16S formed threadlike micelles with Me2EttolBr and C5Et2MeNBr in a 1:1 molar ratio using conductivity measurements.23 Figure 1 shows the relationship between specific electric conductance (κ0) and cS for the NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W systems with cD ) 100 mM at 25 °C. For cS less than cD, κ0 increased with cS with the same slope in both systems. A clear break point was found at cS ) cD in the κ0 vs cS plot. The slope obviously decreased for both systems as shown by the dotted and broken lines in the figure. These data strongly suggested that NaC16S and the cationic hydrotropes formed 1:1 threadlike micelles. Na+ ions from NaC16S were totally replaced by cationic hydrotropes with increasing cS up to cS ) cD, after which the dissociated Na+ and Br- ions were released into the bulk aqueous phase. Threadlike micelles with 1:1 composition formed at cD ) cS, after which excess cationic hydrotropes yielded cations with a mobility lower than that of Na+ in the bulk aqueous phase, accounting for the conductivity behavior in the range cS > cD. Since the network formed by threadlike micelles obstructed the diffusion of all (21) Barker, C. A.; Saul, D.; Tiddy, G. J. T.; Wheeler, B. A.; Willis, E. J. Chem. Soc., Faraday Trans. 1 1974, 70, 154. (22) Oda, R.; Narayanan, J.; Hassan, P. A.; Manohar, C.; Salkar, R. A.; Kern, F.; Candau, S. J. Langmuir 1998, 14, 4364. (23) Shikata, T.; Imai, S. J. Phys. Chem. B 1999, 103, 8694.
Figure 2. Dependence of the storage and loss moduli G′ and G′′ on ω for (a) NaC16S:Me2EttolBr/W and (b) NaC16S:C5Et2MeNBr/W at various cD ()cS) values. Solid lines represent the best-fit curves calculated from eq 1 assuming behavior of a single Maxwell element.
dissociated ions, the specific conductance of each ion was reduced by the same factor. Strongly supporting the above explanation was the fact that the ratios of the specific conductance of NaBr to κ0 (cS < 100 mM in Figure 1) and that of Me2EttolBr (or C5Et2MeNBr) to κ0 (cS > 100 mM) were very similar, 1.44 and 1.41, respectively. Parts a and b of Figure 2 show the dependence of G′ and G′′ on ω for NaC16S:Me2EttolBr/W and NaC16S:MeC5Et2NBr/W, respectively, at various values of the concentration cD ()cS). Solid lines represent the best fit curves calculated assuming a single Maxwell element with a single relaxation time (τ) and strength (GN) for G′ and G′′:
G′ )
GNω2τ2 1+ω τ
2 2
G′′ )
GNωτ 1 + ω2τ2
(1)
The distribution of the relaxation time for both systems at cD ()cS) ) 50 mM seemed rather broader than that of the Maxwell element. The viscoelastic spectra at this concentration looked similar to those of high molecular weight polymer solutions with full entanglements.23 For both systems, when cD > 75 mM, the viscoelasticity was reasonably approximated by a single Maxwell element, as closely as for entangled cationic threadlike micellar systems such as CTAB:NaSal/W6,7 and CTAB:NapTS/W.7,8 The dependence of GN and τ on cD for the systems whose behavior is depicted in Figure 2 is shown in Figure 3. When the
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Figure 3. Dependence of the plateau modulus GN and the relaxation time τ on cD ()cS) for the NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W systems.
measured viscoelastic spectra were not well fitted by a single Maxwell element, such as at cD ) 50 mM, the average relaxation time (τw) and the reciprocal of the steady-state compliance (Je-1), τw ) Jeη0, where η0 represents the zero-shear viscosity, were estimated instead of τ and GN. From the rheology of fully entangled polymer systems and the classical theory of rubber elasticity,24 the magnitude of GN is known to be proportional to the number density of entanglement points in the system. The relation GN ∝ cD2 found in Figure 3 strongly suggested that enormously long anionic threadlike micelles were as fully entangled as in analogous polymer systems24 and also as in cationic threadlike micellar systems. Since the GN values for NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W were almost the same at each cD, the mesh size, length, and rigidity of the entangled threadlike micelles must have been similar. On the other hand, the τ values for NaC16S:C5Et2MeNBr/W were about 10 times longer than those for NaC16S:Et2MetolBr/W, as seen in Figure 3, though τ for both systems decreased with increasing cD. Parts a and b of Figure 4 show the dependence of G′ and G′′ on ω for NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W obtained at a constant cD of 100 mM and varying cS. The viscoelastic spectra of both systems showed constant GN values and maintained a single Maxwell element-type behavior irrespective of cScD-1, indicating that the number density of entanglement points in these systems depended only on the surfactant concentration cD and that τ was strongly influenced by cS. These points are important in understanding entanglement release mechanisms operating in these systems and will be discussed in detail in relation to the concentration of free dissociated cationic hydrotrope ions.
Discussion Relaxation Mechanisms. It is well-known that the relaxation mechanism for entangled polymers is essentially governed by the reptation process,25 whose relaxation time is described by the relationship τ ∝ M3cP1.5, where M and cP represent the molar mass and concentration of the polymer, respectively. Since τ for (24) Ferry, J. D. Viscoelastic Properities of Polymers, 3rd ed.; John Wiley: New York, 1980. (25) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics, 3rd ed.; John Wiley: New York, 1980.
Figure 4. Dependence of G′ and G′′ on ω for (a) NaC16S:Me2EttolBr/W and (b) NaC16S:C5Et2MeNBr/W at cD ) 100 mM and various cS values. Solid lines represent the best fit curves calculated from eq 1.
the NaC16S systems in this study decreased with increasing cD, as seen in Figure 3, the reptation process could not have been the main relaxation mechanism in these anionic threadlike micellar systems. The concentration dependence of τ observed in cationic systems such as CTAB:NaSal/W6,7 and CTAB:NapTS/W7,8 was similar to that of NaC16S:Me2EttolBr/W and NaC16S:C5Et2MeNBr/W, indicating that the relaxation processes in both families of micelles were essentially identical. In the cationic threadlike micellar systems, the phantom-crossing model26 provided a plausible entanglement release mechanism, in which entanglements between two distinct threadlike micelles are assumed to cross through each other, as schematically depicted in Scheme 2, after an entanglement lifetime equal to the mechanical relaxation time τ. According to this model, the relaxation time τ depends only on the concentration of free dissociated additive (hydrotrope) ions such as salicylate (Sal-) or p-toluenesulfonate (pTS-) ions in the bulk aqueous phase.7,8,26 Hence, the freely dissociated anions Sal- or pTS- act as catalysts for the crossing-through reaction at entanglement points in cationic threadlike micellar systems. If a similar relaxation mechanism were operative in NaC16S:Me2EttolBr/W and NaC16S: C5Et2MeNBr/W, τ for these systems would be governed only by the concentration (cS*) of free dissociated cationic hydrotrope ions Me2Ettol+ and C5Et2MeN+ in the bulk aqueous phase. To confirm the validity of the phantom-crossing mechanism in the anionic threadlike micellar systems, we determined the value of cS* and examined the relationship between τ and cS* for both systems. (26) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354.
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Figure 5. 1H NMR spectra for NaC16S/D2O at cD ) 50 mM, NaC16S:Me2EttolBr/D2O at cD ) 50 mM and various cS values, and Me2EttolBr/D2O at cS ) 50 mM. “AN” represents the signal of acetonitrile added as the reference standard. Scheme 2
When threadlike micellar systems did involve aromatic hydrotropes (such as NaSal in the CTAB:NaSal/W system), cS* could be determined using 1H NMR from the chemical shift change of ring protons whose signals did not overlap with those of protons belonging to hydrocarbon chains in the surfactants.8,26 Figure 5 shows 1H NMR spectra for NaC16S:Me2EttolBr/D2O at cD ) 50 mM and various cS values, for NaC16S in D2O at cD ) 50 mM, and for Me2EttolBr in D2O at cS ) 50 mM. Comparing the spectrum of NaC16S:Me2EttolBr/D2O to that of NaC16S, it could be seen that signals attributed to protons of hydrocarbon chains in NaC16S became much broader after micelle formation, whereas signals attributed to protons of Me2EttolBr were sharp irrespective of micelle formation. These results suggested that molecular motions of the surfactant anion C16S- were highly restricted but that those of the Me2Ettol+ cation were still rather fast even in the interior of the micelles. Rapid exchange of Me2Ettol+ between the bulk aqueous phase and the micellar interior probably sustained the dynamic structure of the enormously long micelles and prevented their precipitation. To evaluate cS* for the NaC16S:Me2EttolBr/D2O system, the chemical shifts of the meta proton of Me2Ettol+ were carefully recorded and were found to shift to higher magnetic field upon the formation of micelles. The values of cS* were evaluated from the chemical shifts (δ) of the meta proton in Me2EttolBr as a function of cS using eq 2,6,7
cS* ) cS
δ - δmic δaq - δmic
(2)
Figure 6. Relationship between τ and the free hydrotrope ion concentration in the bulk aqueous phase, cS*, for NaC16S:Me2EttolBr/W at various surfactant concentrations cD ()cS).
where δmic and δaq represent the chemical shifts of the meta protons in the micellar interior and in the bulk aqueous phase, respectively. Figure 6 shows the relationship between the mechanical relaxation time τ and cS* determined from NMR measurements for NaC16S:Me2EttolBr/D2O, found using the procedure described above at various surfactant concentrations cD from 50 to 200 mM. The value of τ likely depended only on cS* and was independent of cD. Consequently, one could conclude that the phantom-crossing mechanism governed entanglement release relaxation in NaC16S:Me2EttolBr/W and that free dissociated Me2Ettol+ ions acted as catalysts for entanglement release in these anionic threadlike micellar systems. NMR techniques could not be employed to determine cS* in NaC16S:C5Et2MeNBr/W because this system did not contain aromatic protons. The proton signals assigned to C5Et2MeNBr reflecting the chemical environment of the micellar interior densely overlapped those of protons in the hydrocarbon chain of C16S-. Nevertheless, the conductance measurements shown in Figure 1 implied that NaC16S and C5Et2MeNBr formed threadlike micelles with 1:1 composition, so the value of cS*
Threadlike Micelle Formation of Anionic Surfactants
Figure 7. Relationship between τ and cS* for the NaC16S:C5Et2MeNBr/W system at various surfactant concentrations cD ()cS).
could be calculated assuming the relationship cS* ) cS - cD for cS > cD. Figure 7 shows the relationship between the relaxation time τ and cS* () cS - cD) in the NaC16S:C5Et2MeNBr/W system for various surfactant concentrations cD. The value of τ depended only on cS - cD and was independent of cD, suggesting that the phantom-crossing mechanism accounted for entanglement release in this system as well. The procedure used to evaluate cS* was undoubtedly also valid for NaC16S:Me2EttolBr/W when cS > cD. It is worth noting that the living polymer model,27-29 proposed as a relaxation mechanism in threadlike micellar systems and applied to many systems such as CTAB:KBr/W30,31 and CTAB: NaSal/W32 (only at cScD-1 < 0.5), was not applicable to the viscoelastic behavior of NaC16S:Me2EttolBr/W or NaC16S:C5Et2MeNBr/W. In this model, the viscoelastic parameters GN and τ are expressed as functions of the volume fraction or concentration cD of threadlike micelles and have been calculated for three types of entanglement release mechanisms: reversible scission, bond interchange, and end interchange. According to this model, the relaxation time τ is proportional to cDR, withthe exponent R varying between 0.3 and 1.7 depending on the mechanism.29 Since the value of τ for both NaC16S:Me2EttolBr/W and NaC16S: (27) Cates, M. E. Macromolecules 1987, 20, 2289. (28) Cates, M. E.; Candau, S. J. J. Phys.: Condens. Matter 1990, 2, 6869. (29) Turner, M. S.; Marques, C.; Cates, M. E. Langmuir 1993, 9, 695. (30) Kern, F.; Lamarechal, P.; Candau, S. J.; Cates, M. E. Langmuir 1992, 8, 437. (31) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (32) Berret, J.-F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851.
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C5Et2MeNBr/W decreased with increasing cD as seen in Figure 3, the relaxation mechanism for these systems was not explained by the living polymer model but rather conformed to the phantomcrossing model. Finally, we could conclude that hydrotropes appropriate for the formation of threadlike micelles with sulfate-type anionic surfactants such as NaC16S must possess cationic groups of adequate size to generate stoichiometric 1:1 electrostatic interactions with sulfate groups and enough mobility to exchange their partner sulfate groups and also their site in the micellar interior for that in the bulk water phase when cS > cD. Me2EttolBr, C5Me2EtNBr, and C5Et2MeNBr reasonably satisfied these conditions for NaC16S at 25 °C.
Conclusions In this study, we have established that aqueous anionic micellar systems containing enormously long, stable threadlike micelles formed fully entangled networks and showed remarkable viscoelastic behavior. We employed combinations of common anionic linear chain surfactants, sodium tetradecyl sulfate or sodium hexadecyl sulfate, and cationic hydrotropes such as N,N-dimethylN-ethyl-p-toluidine bromide and n-pentyl-N,N-diethyl-N-methylammonium bromide, which possessed an appropriately sized cationic group. In these systems, anionic threadlike micelles were constructed in 1:1 molar composition of anionic surfactant and cationic hydrotrope ions. The viscoelastic behavior of these systems was similar to that of fully entangled cationic threadlike micellar systems such as the aqueous solution of cetyltrimethylammonium bromide and sodium salicylate. Viscoelastic spectra of these systems were adequately described with a simple Maxwell element possessing a single relaxation strength and time. The relaxation strength was proportional to the square of the surfactant concentration, and the relaxation time depended only on the concentration of free dissociated hydrotrope ions in the bulk aqueous phase. The relaxation mechanism in the anionic threadlike micellar systems was presumably governed by a phantom-crossing mechanism at each point of entanglement between threadlike micelles and activated by the presence of free dissociated hydrotrope ions. Acknowledgment. T.S. thanks Dainippon Ink and Chemicals Inc. Ltd. for their financial support of this study. K.N. expresses special thanks to the Center of Excellence (21 COE) program “Creation of Integrated EcoChemistry of Osaka University”. LA061031W
