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Effect of polycarbonic acids on the molecular mobility of cationic surfactants in micelles
Colloids and Surfaces A: Physicochemical and Engineering Aspects 147 (1999) 169 – 178
Effect of polycarbonic acids on the molecular mobility of cationic surfactants in micelles V.A. Kasaikin a,*, A.M. Wasserman b, J.A. Zakharova a, M.V. Motyakin b, A.D. Kolbanovskly c b
a Department of Chemistry, Moscow State Uni6ersity, Moscow 119899, Russia Semeno6 Institute of Chemical Physics, Russian Academy of Sciences, ul.Kosygina 4, Moscow, Russia c Department of Chemistry, New York Uni6ersity, New York 10003, USA
Abstract The rotational mobility of a spin probe in the micelles of alkyltrimethylammonium bromides and in their complexes with poly(acrylic acid) and poly(methacrylic acid) has been studied at pH 6. Formation of the polyelectrolyte– surfactant complexes (PSC) causes the decrease in the molecular mobility of surfactant ions in the PSC micelles as compared with the micelles of the free surfactant. It was found that the surfactant ion mobility in PSC micelles is almost independent of the complex composition and even its phase state. The chemical structure of the polyacid has little effect on the molecular mobility of the surfactant ions in the micelles of the complexes. It was shown that the rotational mobility of the surfactant ions in micelles depends on the hydrocarbon chain length of surfactant molecules. Increase in surfactant ion alkyl chain length causes the decrease of their molecular mobility in micelles. Dissociation of the ionic bonds between polyelectrolyte and the surfactant as a result of the increase in the concentration of NaCl in the solution leads to the enhancement of the mobility of the surfactant ions in PSC micelles. It was shown that solubilization of isopropyl alcohol in complex micelles leads to an increase of surfactant ion mobility even at a very low (B10%) alcohol concentration, i.e. at the conditions when the complexes are still stable. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Micelles; Molecular mobility; Surfactants
1. Introduction Cooperative binding of ionic surfactants to oppositely charged linear polyelectrolytes is known to result in formation of polyelectrolyte –surfactant complexes (PSC) [1 – 3]. The general feature of PSCs is aggregation of surfactant ions and * Corresponding author.
generation of micellar phase inside the PSC particle. The formation of surfactant micelles in the presence of polyelectrolyte occurs on the polymer chain which may cause considerable changes in the process of micelle formation as well as in the morphology of the micellar phase [2–11]. Despite the fact that the binding of ionic surfactants to oppositely charged polyelectrolytes was extensively studied [8,9,12–20], systematic data eluci-
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dating polyelectrolyte influence on the structure of PSC micellar phase are deficient. Our previous studies with the spin probe technique [21,22] have shown that the molecular mobility of dodecyltrimethylammonium bromide in micelles is markedly decreased upon the complex formation with sodium polyacrylate. In the present paper the effects of hydrophobic and electrostatic interactions in the complexes formed of poly(acrylic acid) or poly(methacrylic acid) and alkyltrimethylammonium bromides homologous at pH 6 on the morphology of PSC micelles have been studied using a spin probe technique. A spin probe rotation correlation time (t) is a function of the molecular mobility and packing density of the surrounding surfactant ions, while the hyperfine coupling constant (aN) is determined by the polarity of the environment of a spin probe [23]. Thus, monitoring the changes of EPR spectra of a spin probe solubilized in micelles of the free surfactant or in PSC micelles allows to study the effect of the polyacid on the molecular mobility of surfactant ions in micelles. Previously this technique was extensively used to study the processes of micelle formation for direct and reversed micelles [24–27] as well as for studying the complexes of nonionic polymers with surfactants [28,29].
2. Experimental section Synthesis and fractionation of poly(acrylic acid) (PA) and poly(methacrylic acid) (PMA) were described elsewhere [30,31]. The polymer fractions used in this study had the following molecular weights: Mw= 1.65 ×105 g mol − 1 with Mw/ Mn= 1.15 (PA) and Mw = 3.0 ×105 g mol − 1 (PMA). Surfactants-dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB) and dodecylpyridinium chloride (DPC) (Aldrich-Chemie, Germany) were used without further purification. Synthesis of a spin probe-2,2,6,6,-tetramethyl4(2-hydroxyethyl)-piperidin-1-oxyl (I) was described previously [21].
Other reagents used in the study were of chemically pure grade, water was twice distilled.
The reaction mixture composition, Z, is determined as the ratio of surfactant molar concentration to the molar concentration of polyanion residues. The composition of PSC, 8, was determined as the ratio of the amount of surfactant ions to the number of polyanion residues in the complex particle. The samples of complexes were prepared as follows: weighed sample of the spin probe was dissolved in an aqueous solution of the surfactant with its concentration above critical micelles concentration (CMC) and with the mole ratio of probe to surfactant of 1/50. This stock solution was added to aqueous solutions of PA or PMA of certain concentrations with or without NaCl under extensive stirring. All experiments were performed at pH 6. EPR spectra were recorded 24 h after sample preparation. When insoluble complexes were studied, the precipitant of PSC was prepared at the mixture composition Z= 1 and separated by centrifugation on microcentrifuge ELMI (Latvia) at 10 000 rpm for 10 min. The samples of insoluble PSC were used for EPR experiments without any additional treatment. The spin probe solutions in methanol, hexane and aqueous solutions of polyacids or surfactants were prepared simply by dissolving the probe in the corresponding solvent. The study of the effect of added isopropyl alcohol on the spin probe molecular dynamics was carried out using two different procedures: 1. The samples of stoichiometric PSC precipitant were prepared by addition of water and the stock surfactant solution containing the spin probe to water-alcohol solutions of PA or PMA. All the samples had the constant final volume. In 24 h the PSC precipitant was separated with centrifugation and EPR spectra were recorded.
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2. The samples of PSC precipitant were prepared in aqueous solutions and were separated by centrifugation. Isopropyl alcohol – water mixtures were added to PSC precipitants to reach the same final volume as in the first case for various concentrations of the alcohol. After 24 h the precipitant was separated by centrifugation and EPR spectra were recorded. EPR spectra of the spin probe were recorded using a serial X-range EPR spectrometer Radiopan SE/X 2544 (Poland). The values of the spin probe rotation correlation time (t) were calculated using the following equation [32,33]: t=6.65 · DH + 1 · ( I + 1/I − 1 −1) · 10 − 10 s
(1)
where DH + 1 is a width of the low field component (G), I + 1, and I − 1 are the intensities of a low and a high field component of an EPR spectrum, respectively. The experimental error in the t measurement is not more than 10% [33]. Hyperfine coupling constant values (aN) were calculated by the distance between spectrum components using Gauss units (G). The experimental error in the hyperfine coupling constant measurement is 0.1 G [33,34]. The high-speed sedimentation data were obtained using ultracentrifuge Spinco-E (Beckman, USA) equipped with a UV scanning system. The speed of rotation was 6000 s − 1. Optical density of the solution was monitored at the wavelength of 265 nm.
3. Results and discussion Polyelectrolyte – surfactant complexes formed with polycarbonic acids and alkyltrimethylammonium bromides homologues have been extensively studied [6,35 – 41]. Previously, it was shown [35,37,41] that for the mixtures of polycarbonic acids with cationic micelle forming surfactants at pH 6 the formation of water-soluble complexes is limited to the range of the reaction mixture composition Z = [surfactant]/[polyelectrolyte] less than Zlim. At the mixture composition above Zlim the system undergoes a phase separation. The composition of water-soluble PSC, 8, was shown
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to be very close to the reaction mixture composition, Z, when the polyacid concentration exceeds 0.015 M [35,41]. On the basis of this findings, we can postulate that at a concentration of polyacids higher than 0.015 M the complex composition is equal to the reaction mixture composition with not more than 10% error. The studies of such complexes by Elastic (ELLS) and quasi-elastic laser light scattering (QELLS) have revealed that a complex particle contains only one polyelectrolyte molecule and that the amount of the surfactant ions in PSC micelle is a linear function of the complex composition [37]. When the reaction mixture composition exceeds Zlim, the system undergoes the phase separation accompanying by a nonuniform distribution of surfactant ions between polyelectrolyte molecules (disproportionation): the precipitant consists of PSC of the stoichiometric composition (i.e. molar ratio of surfactant ions to polyacid units in PSC particle is 1:1) and supernatant contains watersoluble PSC of the composition similar to Zlim. Increase in surfactant concentration in the range of the reaction mixture compositions from Zlim to 1 changes only the mass ratio of the water-soluble to insoluble PSCs of the constant compositions [35,37,41]. In the present paper we have studied soluble and insoluble PSCs, as well as the solutions of alkyltrimethylammonium bromides (AlkTAB) with various alkyl chain lengths in a wide concentration range using a spin probe technique. The spin probe does not dissolve in water but dissolves in either polar or nonpolar organic solvents as well as in aqueous solutions of PA, PMA and AlkTAB, even at a surfactant concentration considerably lower than CMC. Typical EPR spectra of the spin probe are presented on Fig. 1 as an example. The spin probe rotation correlation time (t) and hyperfine coupling constant (aN) for aqueous solutions of PA and PMA are identical: t= 0.8×10 − 10 s and aN = 17.0 G. These values are similar to those of the nitroxide radical in aqueous solutions [34] and indicates almost free probe rotation in aqueous environment. Al1 the observed experimental spectra correspond to a fast motion of the probe (i.e. in all cases t values
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are lower than 10 − 9 s). At the same time, it is necessary to justify the possibility of using Eq. (1) for evaluation of t values for all studied systems. The main problem is the anisotropy of the micellar media either for free micelles or intramolecular micelles of PSC, in which partial orientation of probe molecules is probable. The methods of the analysis of EPR spectra for anisotropic media environment are now well developed [42]. It was shown [33,43,44] that if EPR spectrum is poorly asymmetric and there are no additional extreme (which indicate that the degree of a probe orientation in anisotropic environment is insignificant), the values of spin probe rotation correlation time can be evaluated from the equations offered for isotropic environments, i.e. by Eq. (1) in particular. In this case, the value of hyperfine coupling constant also may be obtained by a common method for isotropic motion, i.e. as a distance between EPR spectrum components in Gauss units.
Fig. 1. The typical EPR spectra of the spin probe in the aqueous solutions of PMA at the concentration of 0.3 M at pH 6 (1), DTAB at 7.5 10 − 4 M (2), DTAB at 0.045 M (3), complex PMA–DTAB of the composition 8= 0.2, surfactant concentration is 0.045 M (4).
From Fig. 1 one can see that for all the systems studied, the asymmetry of the spectra is poor and there are no additional extreme. This leads to conclusion that Eq. (1) can be reasonably used for evaluation of t values either for free surfactant micelles or micelles of PSC.
3.1. Molecular dynamics of the surfactant ions in micelles Solubilization of the probe in the surfactant micelles results in the decrease in the polarity of the probe microenvironment. At the same time the correlation time of the spin probe rotation significantly increases, which indicates the decrease in the molecular mobility of the surfactant ions in the micelles in comparison with those in the solution. Fig. 2 shows the spin probe rotation correlation time (t) and hyperfine coupling constant (aN) as a function of the surfactant concentration. It is clear that at a surfactant concentration (Csur) lower than CMC, the values of t and aN do not depend on the surfactant concentration and their values are equal to those of nitroxide radical in water [33]. When a surfactant concentration exceeds CMC, the spin probe solubilizes in micelles which leads to the increase of the spin probe rotation correlation time and decrease of the hyperfine coupling constant. At surfactant concentrations considerably higher than CMC, both of the parameters (as t and aN) reach a plateau, i.e. do not depend on the surfactant concentration. It suggests that the contribution from the probe which is not solubilized in micelles becomes negligible, the probe is located predominantly in the micelles and its mobility reflects the molecular mobility of the surfactant ions in the micelles. All experiments described below have been performed at the surfactant concentration near 3 CMC, i.e. in the concentration range corresponding to the plateau. Table 1 shows the values of the spin probe rotation correlation time and hyperfine coupling constant in micelles of alkyltrimethylammonium bromides with different alkyl chain lengths. It is clear that at Csur CMC, the values of aN do
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3.2. Effect of complex formation on the molecular mobility of the surfactant ions
Fig. 2. Dependence of the spin probe rotation correlation time (a) and the hyperfine coupling constant (b) on DTAB (1) and TTAB (2) concentration. CMC of DTAB and TTAB are indicated with an arrow signs.
not depend on the alkyl chain length of the surfactant molecule and coincide with those for a spin probe in methanol (aN =16.2 G). Hence, the nitroxide moiety of the probe is localized in the polar part of the micelles, which have a dielectric permeability similar to that of methanol. The localization of the probe appears to be independent of the alkyl chain length. On the contrary, the spin probe rotation correlation time values increase with the increase in the surfactant ions alkyl chain length (Table 1). This suggests that surfactant ion molecular mobility in micelles is determined by the alkyl chain length.
Polycarbonic acids essentially alter micelle formation and structure [1–3]. Fig. 3 shows the spin probe rotation correlation time as a function of alkyltrimethylammonium bromide (AlkTAB) concentration in the system for water-soluble PMA–AlkTAB complexes (the similar data for PA–AlkTAB complexes are not shown). A surfactant concentration was varied by changing either the PSC concentration at a fixed value of the reaction mixture composition or by changing the mixture composition at a fixed polyion concentration. It appears that t values do not depend on the PSC composition and are determined by the total concentration of the surfactant ions in the system. It should be emphasized that in presence of the polyelectrolyte the increase in t is observed at a considerably lower surfactant concentration (compare data on Figs. 2 and 3). This can be explained by CMC decrease in the presence of a polyanion. At the surfactant concentration above C* the values of t and aN within the experimental errors do not depend on the surfactant concentration. (i.e. the values of t and aN reach a plateau at the surfactant concentration, C*, which is indicated with an arrow sign on the Fig. 3). This suggests that the probe is located predominantly in the PSC micelles, and the t value reflects the local mobility of the surfactant ions in PSC micelles. It is necessary to emphasize that all further experiments have been performed at a surfactant concentrations higher than C*. Table 2 shows the influence of the complex composition on the rotational mobility and hyperfine coupling constant for the probe solubilized in water-soluble PSCs and in water-insoluble stoichiometric PSCs. Fig. 3 and Table 2 show the common behavior for all used surfactants: the correlation time of the spin probe rotation in the PSC micelles is considerably higher than that in ‘free’ micelles. This suggests that the molecular mobility of the surfactant ions in PSC micelles is substantially lower than in the surfactant micelles formed without polyanion.
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Table 1 Spin probe rotation correlation time, t, and hyperfine coupling constant, aN, in aqueous solutions of alkyltrimethylammonium bromides Surfactant
DTAB TTAB CTAB
CMC·103 (M47)
15.0 3.5 0.92
t 1010 (s)
aN (G)
t1 (CsurBCMC)
t2 (CsurCMC)
CsurBCMC
CsurCMC
0.8 1.0 1.0
3.3 4.5 6.0
16.9 17.1 17.0
16.3 16.3 16.2
The similarity of the hyperfine coupling constant values for the probe solubilized in the free surfactant micelles and in the micelles of watersoluble and insoluble PSCs could indicate that location of a probe in the micellar phase is not greatly influenced by the polyanion. At the same time, the values of aN for the spin probe in water-soluble PSCs are slightly higher than those for insoluble PSCs and ‘free’ surfactant micelles (compare the data in Tables 1 and 2). This indicates that the polarity of the probe microenvironment in micelles of water-soluble complexes is slightly higher than that in free micelles or in water-insoluble PSCs. A possible reason for this behavior is higher hydration degree in the micelles of soluble PSCs.
A comparison of a spin probe rotation correlation time for PSCs of different compositions shows that the t values for the soluble PSCs are almost independent of the complex composition. Moreover, the molecular mobility of the surfactant ions remains nearly constant upon transition from water soluble PSC to water insoluble stoichiometric complexes. It should be noted that the probe mobility in this case depends on the mobility of the surfactant ions surrounding the probe molecule in micelle but not on the local mobility of the macromolecule. Thus, the experimental data described above suggest that the process of phase separation in these systems does not cause any essential change in the morphology of micellar phase of PSCs. The spin probe rotation correlation time increases with the increase in the surfactant ion alkyl chain length (Table 2). This suggests that the surfactant ion alkyl chain length has a strong effect on the molecular mobility of the surfactants in PSC micelles, as well as in polymer-free micellar solutions.
3.3. The influence of NaCl on the surfactant ions mobility in micelles formed with or without a polymer
Fig. 3. The dependence of the spin probe rotation correlation time on the surfactant concentration in solution. (a) Water-soluble PMA – DTAB complexes of the different complex compositions 8=0.1 (1), 8 =0.2 (2), 8 =0.25 (3); (b) Water-soluble PMA – TTAB complexes 8=0.05(4), 8 =0.1 (5).
The Coulombic binding between ionized units of a polyanion and oppositely charged surfactant ions is known to be the additional factor for micelle stabilization in PSC particles besides hydrophobic interactions between aliphatic groups of the surfactant ions. It is probably the main reason for the remarkable decrease in the molecular mobility of the surfactant ions in the complex particles. It was shown [28] that formation of the
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Table 2 Spin probe rotation correlation time, t, and hyperfine coupling constant, aN, for water-soluble and insoluble (i.e. for 8 =1) polyelectrolyte-surfactant complexes at pH 6 Polyelectrolyte
Surfactant
The complex composition, 8
t 1010 (s)
aN (G)
PMA
DTAB
0.1 0.15 0.2 0.25 1 0.05 0.1 1 1
5.5 5.6 5.9 6.2 4.5 7.4 7.9 7.5 10.0
16.5 16.5 16.6 16.55 16.2 16.5 16.4 16.2 16.2
TTAB
CTAB PA
DTAB
0.1 0.15 0.2 0.25 1
complexes of non ionic polymers — polyethylenoxide and polypropylenoxide — with cetyltrimethylammonium bromide causes the exactly opposite effect—the mobility of surfactant ions in the complex micelles increases as compared with those in ‘free’ surfactant micelles. Moreover, low molecular weight electrolytes shift the equilibrium of polyelectrolyte-surfactant binding towards complex dissociation [1,15 – 17,36]. An influence of ionic strength of the solution on the molecular mobility of surfactant ions was studied for PMA – dodecyltrimethylammonium bromide (DTAB) complexes. Fig. 4 represents the dependence of t and aN versus a concentration of NaCl. It is clear that the presence of the salt at the concentration below 0.15 M does not affect the molecular mobility of the spin probe solubilized in PSC micelles. Increase in the salt concentration above 0.2 M leads to the substantial increase in the surfactant ion molecular mobility. At the NaCl concentration near 0.8 M the values of t for the probe in water-soluble PSCs are almost equal to those for the probe in DTAB micelles in polymer-free solution. This indicates that at high concentrations of the salt the polyeletrolyte does not affect the molecular mobility of the surfactant ions in micelles.
6.2 6.14 5.9 5.9 6.5
16.55 16.6 16.6 16.5 16.2
In polymer-free solutions, the decrease in the surfactant CMC and growth of the surfactant ions aggregation numbers in micelles upon the increase of the salt concentration [45–47] does not affect the spin probe rotation correlation time and consequently the molecular mobility of the surfactant ions in micelles (Fig. 4).
Fig. 4. Effect of NaCl concentration on the spin probe rotation correlation time (t) and hyperfine coupling constant (aN) in solutions of DTAB at the concentration of 0.045 M (a) and in the PMA – DTAB complex solution of the composition 8 = 0.25 (b).
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It should be noted that the values of aN for the spin probe in DTAB micelles formed with and without PMA do not depend on the salt concentration. This indicates that the addition of the salt does not induce the probe relocation in a micelle. These results show that suppression of Coulombic binding of the surfactant ions to oppositely charged polyelectrolyte at the concentration of NaCl above 0.2 M leads to gradual dissociation of the complex, and at the concentration of NaCl near 0.8 M the surfactant micelles are not bound to polyelectrolyte molecules. In this case the polyelectrolyte does not have any effect on the molecular mobility of the surfactant ions in micelles.
3.4. The effect of isopropyl alcohol on the morphology of PSC micelles It is well-known that the low-molecular weight aliphatic alcohols are amphiphilic compounds and affect both the electrostatic and hydrophobic interactions. It was shown [48 – 50] that the addition of the small (i.e. up to 15 – 20%) amounts of aliphatic alcohols results in the decrease of the surfactant CMC, while at the higher concentrations of alcohols, the micelle formation does not occur [51–53]. The effect of isopropyl alcohol on the properties of the soluble PA – dodecylpyridinium chloride (DPC) complexes has been studied using high-speed sedimentation. Fig. 5 shows the ratio of the concentration of unbound DPC to its total concentration in solution versus isopropyl alcohol concentration. It is clear that the increase in the alcohol concentration up to 20% (vol.) does not lead to pronounced increase in the equilibrium concentration of DPC. However, at the concentration of the alcohol above 40% (vol.) all the surfactant ions are not bound to the polymer which indicates complex dissociation. Fig. 6 represents the dependence of the spin probe rotation correlation time versus isopropyl alcohol concentration for the insoluble PA-DTAB complex. It is apparent that the addition of even small amounts of isopropyl alcohol causes a substantial decrease in the T value for the probe included in PSC particles. It should be noted that
Fig. 5. The relative concentration on the unbound surfactant in solution as a function of isopropyl alcohol concentration in the reaction mixture (PA – DPC complex of the composition 8 =0.25).
this effect does not depend on the method of sample preparation (Section 2). The decrease in the t values unambiguously indicates that isopropanol is solubilized in micellar phase of PSC. Intercolation of the alcohol molecules between surfactant ions results in increase in surfactant ion mobility because of weakening of hydrophobic interactions between alkyl chains and perturbation of the micelle. The similar ‘disordering’ of surfactant micelle interface structure upon addition of very small (down to 5%) amounts of
Fig. 6. The spin probe rotation correlation time as a function of isopropyl alcohol concentration in the solution for water-insoluble PA – DTAB complex.
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aliphatic alcohols was shown previously [53]. It should be noted that at the alcohol concentration of 15%, when the high-speed sedimentation data indicate that the complexes are still quite stable, the mobility of surfactant ions in PSC micelles increases : 2-fold. The similar effect of isopropyl alcohol was found for the complexes of PMA with DTAB, TTAB and CTAB (the data are not shown). Thus, the study of the influence of added isopropanol on the stability of PSC and molecular mobility of the surfactant ions in PSC micelles shows that the complexes are stable when the alcohol concentration is less than 20%. At the same time, the solubilization of the alcohol in the micelles causes a pronounced increase in molecular mobility of surfactant ions even when the alcohol concentration in the surrounding solution is extremely low. The disturbance of the hydrophobic interactions of aliphatic chains of the surfactant ions at the isopropyl alcohol concentration above 40% causes the dissociation of complexes.
4. Conclusions The study of the molecular mobility of the surfactant ions in micelles formed with or without polycarbonic acids shows that the polymer has a strong impact on the molecular mobility of surfactant ions in micelles. Coulombic binding of the surfactant ions with oppositely charged polyelectrolyte units results in the decrease of molecular mobility of surfactant ions in PSC micelles. Addition of NaCl to the system causes the dissociation of Coulombic bonds between the surfactant and the polyacid which leads to enhancement in the surfactant ions mobility. It was revealed that the molecular mobility of the surfactant ions in micelles of either free surfactants or of the polyelectrolyte–surfactant complexes depends on the surfactant alkyl chain length, increase of which results in the decrease of the surfactant ions molecular mobility. Solubilization of aliphatic alcohols causes the increase of the mobility of surfactant ions in the micelles of PSC even at the low (i.e. below 10% vol.) alcohol concentration when
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the complexes are still quite stable. The composition and even the phase state of the polyelectrolyte–surfactant complexes do not have strong effect on the morphology of micellar phase in the polyelectrolyte –surfactant complexes.
Acknowledgements This research is supported by Russian Foundation for Fundamental Research (Project codes 96-03-32612a and 96-03-32900a).
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[36] K. Hayakawa, J.P. Santerre, J.C.T. Kwak, Macromolecules 16 (1983) 1642. [37] Z.H. Ibragimova, V.A. Kasaikin, A.B. Zezin, V.A. Kabanov, Polym. Sci. USSR 28 (1986) 1826. [38] K. Thalberg, E. Lindman, K. Bergfeidt, Langmuir 7 (1991) 2893. [39] J.J. Kiefer, P. Somasundaran, K.P. Ananthapadmanabhan, Langmuir 9 (1993) 1187. [40] P. Hansson, M. Almpren, Langmuir 10 (1994) 2115. [41] V.A. Kasaikin, Polymer – colloid complexes. Generation, structure, properties. Dissertation, Moscow State University, Moscow, 1988 (in Russian). [42] D.J. Schneider, J.H. Freed, in: L. Bereiner, J. Reubeu (Eds.), Biological Magnetic Resonance, 1979, p. 1. [43] A.N. Kuznetsov, V.A. Livshits, Chem. Phis. Lett. 20 (1973) 534. [44] A.N. Kuznetsov, V.A. Livshits, J. Phys. Chem. Russ. 48 (1974) 2995 (in Russian). [45] M.L. Corrin, W.P. Harkins, J. Am. Chem. Soc. 69 (1947) 683. [46] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes (ch. 7), Wiley, New York, 1980, p. 7. [47] E.J. Fendler, J.H. Fendler, Adv. Phis. Org. Chem. 8 (1970) 271. [48] K. Shirahama, T. Kashiwabara, J. Colloid Interface Sci. 36 (1971) 65. [49] M. Abu-Hanidiyyach, L.A. Rachman, J. Phys. Chem. 89 (1985) 2377. [50] R. Zana, S. Yiv, C. Strazielle, P. Lianos, J. Colloid Interface Sci. 80 (1981) 208. [51] A.W. Ralston, C.W. Hoerz, J. Am. Chem. Soc. 68 (1946) 2460. [52] A.W. Ralston, D.N. Eggenberger, J. Phys. Chem. 52 (1948) 1494. [53] P. Baglioni, L. Kevan, J. Phys. Chem. 91 (1987) 1516.
.
Effect of polycarbonic acids on the molecular mobility of cationic surfactants in micelles V.A. Kasaikin a,*, A.M. Wasserman b, J.A. Zakharova a, M.V. Motyakin b, A.D. Kolbanovskly c b
a Department of Chemistry, Moscow State Uni6ersity, Moscow 119899, Russia Semeno6 Institute of Chemical Physics, Russian Academy of Sciences, ul.Kosygina 4, Moscow, Russia c Department of Chemistry, New York Uni6ersity, New York 10003, USA
Abstract The rotational mobility of a spin probe in the micelles of alkyltrimethylammonium bromides and in their complexes with poly(acrylic acid) and poly(methacrylic acid) has been studied at pH 6. Formation of the polyelectrolyte– surfactant complexes (PSC) causes the decrease in the molecular mobility of surfactant ions in the PSC micelles as compared with the micelles of the free surfactant. It was found that the surfactant ion mobility in PSC micelles is almost independent of the complex composition and even its phase state. The chemical structure of the polyacid has little effect on the molecular mobility of the surfactant ions in the micelles of the complexes. It was shown that the rotational mobility of the surfactant ions in micelles depends on the hydrocarbon chain length of surfactant molecules. Increase in surfactant ion alkyl chain length causes the decrease of their molecular mobility in micelles. Dissociation of the ionic bonds between polyelectrolyte and the surfactant as a result of the increase in the concentration of NaCl in the solution leads to the enhancement of the mobility of the surfactant ions in PSC micelles. It was shown that solubilization of isopropyl alcohol in complex micelles leads to an increase of surfactant ion mobility even at a very low (B10%) alcohol concentration, i.e. at the conditions when the complexes are still stable. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Micelles; Molecular mobility; Surfactants
1. Introduction Cooperative binding of ionic surfactants to oppositely charged linear polyelectrolytes is known to result in formation of polyelectrolyte –surfactant complexes (PSC) [1 – 3]. The general feature of PSCs is aggregation of surfactant ions and * Corresponding author.
generation of micellar phase inside the PSC particle. The formation of surfactant micelles in the presence of polyelectrolyte occurs on the polymer chain which may cause considerable changes in the process of micelle formation as well as in the morphology of the micellar phase [2–11]. Despite the fact that the binding of ionic surfactants to oppositely charged polyelectrolytes was extensively studied [8,9,12–20], systematic data eluci-
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dating polyelectrolyte influence on the structure of PSC micellar phase are deficient. Our previous studies with the spin probe technique [21,22] have shown that the molecular mobility of dodecyltrimethylammonium bromide in micelles is markedly decreased upon the complex formation with sodium polyacrylate. In the present paper the effects of hydrophobic and electrostatic interactions in the complexes formed of poly(acrylic acid) or poly(methacrylic acid) and alkyltrimethylammonium bromides homologous at pH 6 on the morphology of PSC micelles have been studied using a spin probe technique. A spin probe rotation correlation time (t) is a function of the molecular mobility and packing density of the surrounding surfactant ions, while the hyperfine coupling constant (aN) is determined by the polarity of the environment of a spin probe [23]. Thus, monitoring the changes of EPR spectra of a spin probe solubilized in micelles of the free surfactant or in PSC micelles allows to study the effect of the polyacid on the molecular mobility of surfactant ions in micelles. Previously this technique was extensively used to study the processes of micelle formation for direct and reversed micelles [24–27] as well as for studying the complexes of nonionic polymers with surfactants [28,29].
2. Experimental section Synthesis and fractionation of poly(acrylic acid) (PA) and poly(methacrylic acid) (PMA) were described elsewhere [30,31]. The polymer fractions used in this study had the following molecular weights: Mw= 1.65 ×105 g mol − 1 with Mw/ Mn= 1.15 (PA) and Mw = 3.0 ×105 g mol − 1 (PMA). Surfactants-dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB) and dodecylpyridinium chloride (DPC) (Aldrich-Chemie, Germany) were used without further purification. Synthesis of a spin probe-2,2,6,6,-tetramethyl4(2-hydroxyethyl)-piperidin-1-oxyl (I) was described previously [21].
Other reagents used in the study were of chemically pure grade, water was twice distilled.
The reaction mixture composition, Z, is determined as the ratio of surfactant molar concentration to the molar concentration of polyanion residues. The composition of PSC, 8, was determined as the ratio of the amount of surfactant ions to the number of polyanion residues in the complex particle. The samples of complexes were prepared as follows: weighed sample of the spin probe was dissolved in an aqueous solution of the surfactant with its concentration above critical micelles concentration (CMC) and with the mole ratio of probe to surfactant of 1/50. This stock solution was added to aqueous solutions of PA or PMA of certain concentrations with or without NaCl under extensive stirring. All experiments were performed at pH 6. EPR spectra were recorded 24 h after sample preparation. When insoluble complexes were studied, the precipitant of PSC was prepared at the mixture composition Z= 1 and separated by centrifugation on microcentrifuge ELMI (Latvia) at 10 000 rpm for 10 min. The samples of insoluble PSC were used for EPR experiments without any additional treatment. The spin probe solutions in methanol, hexane and aqueous solutions of polyacids or surfactants were prepared simply by dissolving the probe in the corresponding solvent. The study of the effect of added isopropyl alcohol on the spin probe molecular dynamics was carried out using two different procedures: 1. The samples of stoichiometric PSC precipitant were prepared by addition of water and the stock surfactant solution containing the spin probe to water-alcohol solutions of PA or PMA. All the samples had the constant final volume. In 24 h the PSC precipitant was separated with centrifugation and EPR spectra were recorded.
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2. The samples of PSC precipitant were prepared in aqueous solutions and were separated by centrifugation. Isopropyl alcohol – water mixtures were added to PSC precipitants to reach the same final volume as in the first case for various concentrations of the alcohol. After 24 h the precipitant was separated by centrifugation and EPR spectra were recorded. EPR spectra of the spin probe were recorded using a serial X-range EPR spectrometer Radiopan SE/X 2544 (Poland). The values of the spin probe rotation correlation time (t) were calculated using the following equation [32,33]: t=6.65 · DH + 1 · ( I + 1/I − 1 −1) · 10 − 10 s
(1)
where DH + 1 is a width of the low field component (G), I + 1, and I − 1 are the intensities of a low and a high field component of an EPR spectrum, respectively. The experimental error in the t measurement is not more than 10% [33]. Hyperfine coupling constant values (aN) were calculated by the distance between spectrum components using Gauss units (G). The experimental error in the hyperfine coupling constant measurement is 0.1 G [33,34]. The high-speed sedimentation data were obtained using ultracentrifuge Spinco-E (Beckman, USA) equipped with a UV scanning system. The speed of rotation was 6000 s − 1. Optical density of the solution was monitored at the wavelength of 265 nm.
3. Results and discussion Polyelectrolyte – surfactant complexes formed with polycarbonic acids and alkyltrimethylammonium bromides homologues have been extensively studied [6,35 – 41]. Previously, it was shown [35,37,41] that for the mixtures of polycarbonic acids with cationic micelle forming surfactants at pH 6 the formation of water-soluble complexes is limited to the range of the reaction mixture composition Z = [surfactant]/[polyelectrolyte] less than Zlim. At the mixture composition above Zlim the system undergoes a phase separation. The composition of water-soluble PSC, 8, was shown
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to be very close to the reaction mixture composition, Z, when the polyacid concentration exceeds 0.015 M [35,41]. On the basis of this findings, we can postulate that at a concentration of polyacids higher than 0.015 M the complex composition is equal to the reaction mixture composition with not more than 10% error. The studies of such complexes by Elastic (ELLS) and quasi-elastic laser light scattering (QELLS) have revealed that a complex particle contains only one polyelectrolyte molecule and that the amount of the surfactant ions in PSC micelle is a linear function of the complex composition [37]. When the reaction mixture composition exceeds Zlim, the system undergoes the phase separation accompanying by a nonuniform distribution of surfactant ions between polyelectrolyte molecules (disproportionation): the precipitant consists of PSC of the stoichiometric composition (i.e. molar ratio of surfactant ions to polyacid units in PSC particle is 1:1) and supernatant contains watersoluble PSC of the composition similar to Zlim. Increase in surfactant concentration in the range of the reaction mixture compositions from Zlim to 1 changes only the mass ratio of the water-soluble to insoluble PSCs of the constant compositions [35,37,41]. In the present paper we have studied soluble and insoluble PSCs, as well as the solutions of alkyltrimethylammonium bromides (AlkTAB) with various alkyl chain lengths in a wide concentration range using a spin probe technique. The spin probe does not dissolve in water but dissolves in either polar or nonpolar organic solvents as well as in aqueous solutions of PA, PMA and AlkTAB, even at a surfactant concentration considerably lower than CMC. Typical EPR spectra of the spin probe are presented on Fig. 1 as an example. The spin probe rotation correlation time (t) and hyperfine coupling constant (aN) for aqueous solutions of PA and PMA are identical: t= 0.8×10 − 10 s and aN = 17.0 G. These values are similar to those of the nitroxide radical in aqueous solutions [34] and indicates almost free probe rotation in aqueous environment. Al1 the observed experimental spectra correspond to a fast motion of the probe (i.e. in all cases t values
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are lower than 10 − 9 s). At the same time, it is necessary to justify the possibility of using Eq. (1) for evaluation of t values for all studied systems. The main problem is the anisotropy of the micellar media either for free micelles or intramolecular micelles of PSC, in which partial orientation of probe molecules is probable. The methods of the analysis of EPR spectra for anisotropic media environment are now well developed [42]. It was shown [33,43,44] that if EPR spectrum is poorly asymmetric and there are no additional extreme (which indicate that the degree of a probe orientation in anisotropic environment is insignificant), the values of spin probe rotation correlation time can be evaluated from the equations offered for isotropic environments, i.e. by Eq. (1) in particular. In this case, the value of hyperfine coupling constant also may be obtained by a common method for isotropic motion, i.e. as a distance between EPR spectrum components in Gauss units.
Fig. 1. The typical EPR spectra of the spin probe in the aqueous solutions of PMA at the concentration of 0.3 M at pH 6 (1), DTAB at 7.5 10 − 4 M (2), DTAB at 0.045 M (3), complex PMA–DTAB of the composition 8= 0.2, surfactant concentration is 0.045 M (4).
From Fig. 1 one can see that for all the systems studied, the asymmetry of the spectra is poor and there are no additional extreme. This leads to conclusion that Eq. (1) can be reasonably used for evaluation of t values either for free surfactant micelles or micelles of PSC.
3.1. Molecular dynamics of the surfactant ions in micelles Solubilization of the probe in the surfactant micelles results in the decrease in the polarity of the probe microenvironment. At the same time the correlation time of the spin probe rotation significantly increases, which indicates the decrease in the molecular mobility of the surfactant ions in the micelles in comparison with those in the solution. Fig. 2 shows the spin probe rotation correlation time (t) and hyperfine coupling constant (aN) as a function of the surfactant concentration. It is clear that at a surfactant concentration (Csur) lower than CMC, the values of t and aN do not depend on the surfactant concentration and their values are equal to those of nitroxide radical in water [33]. When a surfactant concentration exceeds CMC, the spin probe solubilizes in micelles which leads to the increase of the spin probe rotation correlation time and decrease of the hyperfine coupling constant. At surfactant concentrations considerably higher than CMC, both of the parameters (as t and aN) reach a plateau, i.e. do not depend on the surfactant concentration. It suggests that the contribution from the probe which is not solubilized in micelles becomes negligible, the probe is located predominantly in the micelles and its mobility reflects the molecular mobility of the surfactant ions in the micelles. All experiments described below have been performed at the surfactant concentration near 3 CMC, i.e. in the concentration range corresponding to the plateau. Table 1 shows the values of the spin probe rotation correlation time and hyperfine coupling constant in micelles of alkyltrimethylammonium bromides with different alkyl chain lengths. It is clear that at Csur CMC, the values of aN do
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3.2. Effect of complex formation on the molecular mobility of the surfactant ions
Fig. 2. Dependence of the spin probe rotation correlation time (a) and the hyperfine coupling constant (b) on DTAB (1) and TTAB (2) concentration. CMC of DTAB and TTAB are indicated with an arrow signs.
not depend on the alkyl chain length of the surfactant molecule and coincide with those for a spin probe in methanol (aN =16.2 G). Hence, the nitroxide moiety of the probe is localized in the polar part of the micelles, which have a dielectric permeability similar to that of methanol. The localization of the probe appears to be independent of the alkyl chain length. On the contrary, the spin probe rotation correlation time values increase with the increase in the surfactant ions alkyl chain length (Table 1). This suggests that surfactant ion molecular mobility in micelles is determined by the alkyl chain length.
Polycarbonic acids essentially alter micelle formation and structure [1–3]. Fig. 3 shows the spin probe rotation correlation time as a function of alkyltrimethylammonium bromide (AlkTAB) concentration in the system for water-soluble PMA–AlkTAB complexes (the similar data for PA–AlkTAB complexes are not shown). A surfactant concentration was varied by changing either the PSC concentration at a fixed value of the reaction mixture composition or by changing the mixture composition at a fixed polyion concentration. It appears that t values do not depend on the PSC composition and are determined by the total concentration of the surfactant ions in the system. It should be emphasized that in presence of the polyelectrolyte the increase in t is observed at a considerably lower surfactant concentration (compare data on Figs. 2 and 3). This can be explained by CMC decrease in the presence of a polyanion. At the surfactant concentration above C* the values of t and aN within the experimental errors do not depend on the surfactant concentration. (i.e. the values of t and aN reach a plateau at the surfactant concentration, C*, which is indicated with an arrow sign on the Fig. 3). This suggests that the probe is located predominantly in the PSC micelles, and the t value reflects the local mobility of the surfactant ions in PSC micelles. It is necessary to emphasize that all further experiments have been performed at a surfactant concentrations higher than C*. Table 2 shows the influence of the complex composition on the rotational mobility and hyperfine coupling constant for the probe solubilized in water-soluble PSCs and in water-insoluble stoichiometric PSCs. Fig. 3 and Table 2 show the common behavior for all used surfactants: the correlation time of the spin probe rotation in the PSC micelles is considerably higher than that in ‘free’ micelles. This suggests that the molecular mobility of the surfactant ions in PSC micelles is substantially lower than in the surfactant micelles formed without polyanion.
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Table 1 Spin probe rotation correlation time, t, and hyperfine coupling constant, aN, in aqueous solutions of alkyltrimethylammonium bromides Surfactant
DTAB TTAB CTAB
CMC·103 (M47)
15.0 3.5 0.92
t 1010 (s)
aN (G)
t1 (CsurBCMC)
t2 (CsurCMC)
CsurBCMC
CsurCMC
0.8 1.0 1.0
3.3 4.5 6.0
16.9 17.1 17.0
16.3 16.3 16.2
The similarity of the hyperfine coupling constant values for the probe solubilized in the free surfactant micelles and in the micelles of watersoluble and insoluble PSCs could indicate that location of a probe in the micellar phase is not greatly influenced by the polyanion. At the same time, the values of aN for the spin probe in water-soluble PSCs are slightly higher than those for insoluble PSCs and ‘free’ surfactant micelles (compare the data in Tables 1 and 2). This indicates that the polarity of the probe microenvironment in micelles of water-soluble complexes is slightly higher than that in free micelles or in water-insoluble PSCs. A possible reason for this behavior is higher hydration degree in the micelles of soluble PSCs.
A comparison of a spin probe rotation correlation time for PSCs of different compositions shows that the t values for the soluble PSCs are almost independent of the complex composition. Moreover, the molecular mobility of the surfactant ions remains nearly constant upon transition from water soluble PSC to water insoluble stoichiometric complexes. It should be noted that the probe mobility in this case depends on the mobility of the surfactant ions surrounding the probe molecule in micelle but not on the local mobility of the macromolecule. Thus, the experimental data described above suggest that the process of phase separation in these systems does not cause any essential change in the morphology of micellar phase of PSCs. The spin probe rotation correlation time increases with the increase in the surfactant ion alkyl chain length (Table 2). This suggests that the surfactant ion alkyl chain length has a strong effect on the molecular mobility of the surfactants in PSC micelles, as well as in polymer-free micellar solutions.
3.3. The influence of NaCl on the surfactant ions mobility in micelles formed with or without a polymer
Fig. 3. The dependence of the spin probe rotation correlation time on the surfactant concentration in solution. (a) Water-soluble PMA – DTAB complexes of the different complex compositions 8=0.1 (1), 8 =0.2 (2), 8 =0.25 (3); (b) Water-soluble PMA – TTAB complexes 8=0.05(4), 8 =0.1 (5).
The Coulombic binding between ionized units of a polyanion and oppositely charged surfactant ions is known to be the additional factor for micelle stabilization in PSC particles besides hydrophobic interactions between aliphatic groups of the surfactant ions. It is probably the main reason for the remarkable decrease in the molecular mobility of the surfactant ions in the complex particles. It was shown [28] that formation of the
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Table 2 Spin probe rotation correlation time, t, and hyperfine coupling constant, aN, for water-soluble and insoluble (i.e. for 8 =1) polyelectrolyte-surfactant complexes at pH 6 Polyelectrolyte
Surfactant
The complex composition, 8
t 1010 (s)
aN (G)
PMA
DTAB
0.1 0.15 0.2 0.25 1 0.05 0.1 1 1
5.5 5.6 5.9 6.2 4.5 7.4 7.9 7.5 10.0
16.5 16.5 16.6 16.55 16.2 16.5 16.4 16.2 16.2
TTAB
CTAB PA
DTAB
0.1 0.15 0.2 0.25 1
complexes of non ionic polymers — polyethylenoxide and polypropylenoxide — with cetyltrimethylammonium bromide causes the exactly opposite effect—the mobility of surfactant ions in the complex micelles increases as compared with those in ‘free’ surfactant micelles. Moreover, low molecular weight electrolytes shift the equilibrium of polyelectrolyte-surfactant binding towards complex dissociation [1,15 – 17,36]. An influence of ionic strength of the solution on the molecular mobility of surfactant ions was studied for PMA – dodecyltrimethylammonium bromide (DTAB) complexes. Fig. 4 represents the dependence of t and aN versus a concentration of NaCl. It is clear that the presence of the salt at the concentration below 0.15 M does not affect the molecular mobility of the spin probe solubilized in PSC micelles. Increase in the salt concentration above 0.2 M leads to the substantial increase in the surfactant ion molecular mobility. At the NaCl concentration near 0.8 M the values of t for the probe in water-soluble PSCs are almost equal to those for the probe in DTAB micelles in polymer-free solution. This indicates that at high concentrations of the salt the polyeletrolyte does not affect the molecular mobility of the surfactant ions in micelles.
6.2 6.14 5.9 5.9 6.5
16.55 16.6 16.6 16.5 16.2
In polymer-free solutions, the decrease in the surfactant CMC and growth of the surfactant ions aggregation numbers in micelles upon the increase of the salt concentration [45–47] does not affect the spin probe rotation correlation time and consequently the molecular mobility of the surfactant ions in micelles (Fig. 4).
Fig. 4. Effect of NaCl concentration on the spin probe rotation correlation time (t) and hyperfine coupling constant (aN) in solutions of DTAB at the concentration of 0.045 M (a) and in the PMA – DTAB complex solution of the composition 8 = 0.25 (b).
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It should be noted that the values of aN for the spin probe in DTAB micelles formed with and without PMA do not depend on the salt concentration. This indicates that the addition of the salt does not induce the probe relocation in a micelle. These results show that suppression of Coulombic binding of the surfactant ions to oppositely charged polyelectrolyte at the concentration of NaCl above 0.2 M leads to gradual dissociation of the complex, and at the concentration of NaCl near 0.8 M the surfactant micelles are not bound to polyelectrolyte molecules. In this case the polyelectrolyte does not have any effect on the molecular mobility of the surfactant ions in micelles.
3.4. The effect of isopropyl alcohol on the morphology of PSC micelles It is well-known that the low-molecular weight aliphatic alcohols are amphiphilic compounds and affect both the electrostatic and hydrophobic interactions. It was shown [48 – 50] that the addition of the small (i.e. up to 15 – 20%) amounts of aliphatic alcohols results in the decrease of the surfactant CMC, while at the higher concentrations of alcohols, the micelle formation does not occur [51–53]. The effect of isopropyl alcohol on the properties of the soluble PA – dodecylpyridinium chloride (DPC) complexes has been studied using high-speed sedimentation. Fig. 5 shows the ratio of the concentration of unbound DPC to its total concentration in solution versus isopropyl alcohol concentration. It is clear that the increase in the alcohol concentration up to 20% (vol.) does not lead to pronounced increase in the equilibrium concentration of DPC. However, at the concentration of the alcohol above 40% (vol.) all the surfactant ions are not bound to the polymer which indicates complex dissociation. Fig. 6 represents the dependence of the spin probe rotation correlation time versus isopropyl alcohol concentration for the insoluble PA-DTAB complex. It is apparent that the addition of even small amounts of isopropyl alcohol causes a substantial decrease in the T value for the probe included in PSC particles. It should be noted that
Fig. 5. The relative concentration on the unbound surfactant in solution as a function of isopropyl alcohol concentration in the reaction mixture (PA – DPC complex of the composition 8 =0.25).
this effect does not depend on the method of sample preparation (Section 2). The decrease in the t values unambiguously indicates that isopropanol is solubilized in micellar phase of PSC. Intercolation of the alcohol molecules between surfactant ions results in increase in surfactant ion mobility because of weakening of hydrophobic interactions between alkyl chains and perturbation of the micelle. The similar ‘disordering’ of surfactant micelle interface structure upon addition of very small (down to 5%) amounts of
Fig. 6. The spin probe rotation correlation time as a function of isopropyl alcohol concentration in the solution for water-insoluble PA – DTAB complex.
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aliphatic alcohols was shown previously [53]. It should be noted that at the alcohol concentration of 15%, when the high-speed sedimentation data indicate that the complexes are still quite stable, the mobility of surfactant ions in PSC micelles increases : 2-fold. The similar effect of isopropyl alcohol was found for the complexes of PMA with DTAB, TTAB and CTAB (the data are not shown). Thus, the study of the influence of added isopropanol on the stability of PSC and molecular mobility of the surfactant ions in PSC micelles shows that the complexes are stable when the alcohol concentration is less than 20%. At the same time, the solubilization of the alcohol in the micelles causes a pronounced increase in molecular mobility of surfactant ions even when the alcohol concentration in the surrounding solution is extremely low. The disturbance of the hydrophobic interactions of aliphatic chains of the surfactant ions at the isopropyl alcohol concentration above 40% causes the dissociation of complexes.
4. Conclusions The study of the molecular mobility of the surfactant ions in micelles formed with or without polycarbonic acids shows that the polymer has a strong impact on the molecular mobility of surfactant ions in micelles. Coulombic binding of the surfactant ions with oppositely charged polyelectrolyte units results in the decrease of molecular mobility of surfactant ions in PSC micelles. Addition of NaCl to the system causes the dissociation of Coulombic bonds between the surfactant and the polyacid which leads to enhancement in the surfactant ions mobility. It was revealed that the molecular mobility of the surfactant ions in micelles of either free surfactants or of the polyelectrolyte–surfactant complexes depends on the surfactant alkyl chain length, increase of which results in the decrease of the surfactant ions molecular mobility. Solubilization of aliphatic alcohols causes the increase of the mobility of surfactant ions in the micelles of PSC even at the low (i.e. below 10% vol.) alcohol concentration when
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the complexes are still quite stable. The composition and even the phase state of the polyelectrolyte–surfactant complexes do not have strong effect on the morphology of micellar phase in the polyelectrolyte –surfactant complexes.
Acknowledgements This research is supported by Russian Foundation for Fundamental Research (Project codes 96-03-32612a and 96-03-32900a).
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