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Self-organization in complexes of polyacids with oppositely charged surfactants
Colloids and Surfaces A: Physicochemical and Engineering Aspects 147 (1999) 107 – 114
Self-organization in complexes of polyacids with oppositely charged surfactants V.A. Kasaikin *, J.A. Zakharova Department of Polymer Science, Faculty of Chemistry, Moscow State Uni6ersity, Leninskie Gory, Moscow 119899, Russia
Abstract Formation and structure of water-soluble complexes of alkyltrimethylammonium bromide homologues (AlkTAB) with poly(acrylic acid) (PA) of different polymerization degrees at pH 5.7 have been examined by elastic and quasi-elastic laser light-scattering and high-speed sedimentation technique. It was experimentally shown that generation of intramolecular micellar phase is the necessary condition for formation of PA-AlkTAB complexes. Minimum aggregation number of the surfactant ions in the complex micelle was found to be close to that of the surfactant micelles in polymer-free solution. The structure of the polyelectrolyte-surfactant complexes (i.e. a phase state of the complex, conformation of the polyion coil and the surfactant ion aggregation number) was shown to be largely determined by PA polymerization degree. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Surfactants; Polyacids; Self-organisation
1. Introduction Cooperative interaction between micelle-forming surfactant ions and oppositely charged linear macromolecules is known to result in formation of polyelectrolyte-surfactant complexes (PSC) which exhibit a set of extremely interesting and practically important properties. Polyelectrolyte-surfactant complexes form as a result of the ion-exchange reaction between surfactant ions and ionized units of a macromolecule [1–3]. These compounds are stabilized by hydrophobic interactions of the surfactant ion alkyl chains [1,2] and may be considered as a special class of surface-active polyelectrolytes. There are * Corresponding author.
two types of complexes: water-soluble nonstoichiometric and insoluble in water stoichiometric complexes which may form depending on the binding conditions, such as: pH, ionic strength, concentration and chemical structure of the components, the reaction mixture composition [1,2,4]. It is well known that in PSC surfactant ions segregate and form intramolecular micellar phase [1,2]. Previously, it was shown that the morphology of the PSC micelles differ dramatically from that of the corresponding surfactant ‘free’ micelles [5]. Theoretical examination of polyelectrolytesurfactant interactions [10] shows, that the necessary condition for the formation of PSC is generation of the intramolecular micellar phase. If the amount of a surfactant ion in the system is not enough for formation of the micellar phase
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inside each macroion, the non-uniform distribution of surfactant ions between macromolecules have to take place: polycomplex particles of certain composition have to coexist with the molecules of free polyelectrolyte [10]. By now, data concerning surfactant aggregation number values in PSC micelles are extremely inconsistent [6–9], and the question on the minimum number of surfactant ions capable to form intra-chain micellar phase is practically not studied. The present paper is devoted to the study of the minimum aggregation number of the surfactant ions in PSC micelles, with the special interest on the influence of polyelectrolyte polymerization degree (Pw) on the structure of PSC particles.
2. Experimental section Synthesis and fractionation of poly(acrylic acid) (PA) were described elsewhere [11]. Fractions of PA with weight average degree of polymerization of Pw =140, 600 and 3000 were used. Surfactants-alkyltrimethylammonium bromide homologues (AlkTAB) of various alkyl chain lengths: dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), (Aldrich-Chemie, Germany) were used without further purification. The high-speed sedimentation studies were carried out at 20°C in presence of 0.1 M NaCl with a Spinco-E (Beckman, USA) ultracentrifuge supplied with Filpot – Swenson optical system for the refractive index gradient determination. Rotation frequency was 58000 rpm. The solvent was layered over the sample solution. The values of sedimentation coefficients (S) were calculated using the following equation [12]: S =d(ln x)/v 2d(t), where x is the distance between the sedimentation peak maximum and the rotation axis, v is the angular speed of the rotor rotation and t is the time of the rotation. The experimental error of the sedimentation coefficient measurements was not more than 10%.
The concentration of PA in solution was measured by potentiometry with AUTOCAL pH M83 (Radiometer, Denmark). Molecular characteristics of water-soluble nonstoichiometric complexes were obtained by static light scattering with small-angle laser photometer KMX-6/DC (Milton Roy, USA) with a 4 mW He–Ne laser as a light source (l= 630 nm, the scattering angle 6.5°). The RSD value was 5% for all measurements of the molecular weight and the second virial coefficient. The refractive index increment ( dn/dc) was measured using KMX-16 differential refractometer (Milton Roy, USA) with 0.5 mW He–Ne laser as a light source (l=630 nm.). The solvent equilibrated with the complex solution by dialysis was used as the reference sample. Autocorrelation function of the scattering intensity fluctuations was measured by quasi-elastic light scattering (QELS) using a 1096 correlator (Langly Ford, USA). The obtained data were analyzed by the cumulants method. For all studied systems the dependence of (Kc/ Ru) versus c obey the Debye equation: Kc/Ru = 1/Mw + 2A2c where c is weight concentration, K is optical constant, Ru is the Rayleigh ratio, and u is an angle of light scattering (6.5° in our case). The concentration dependencies of (Kc/Ru) as well as of Dz for solutions of poly(acrylic acid) and water soluble complexes are linear within the studied range of concentrations (0.05–0.5 wt.%) [3]. This means that at the experimental conditions the dimension and the molecular weight of the PSC particles remains constant as PSC concentration decreases. In other words, in the studied concentration range the dissociation of PSC as well as polyelectorlyte swelling are not observed. Average hydrodynamic radii of appropriate equivalent spheres (Re) were calculated using the Stokes equation. The reaction mixture composition Z is determined as the ratio of surfactant molar concentration to the molar concentration of polyanion units in the solution Z= [surfactant]/[polyelectrolyte]. The composition of PSC, 8, is determined as the ratio of the number of surfactant ions to the
V.A. Kasaikin, J.A. Zakharo6a / Colloids and Surfaces A: Physicochem. Eng. Aspects 147 (1999) 107–114
number of polyanion monomer units in the complex particle 8 = [surfactant]psc/[polyacid units]psc. All investigations were performed at pH 5.7, T= 20°C.
3. Results and discussion Interaction of linear polyelectrolytes with oppositely charged surfactant ions proceeds as the cooperative binding at the surfactant concentration 1–3 order of magnitude lower than CMC [13–15]. This process is known to be accompanied by association of the surfactant ions and formation of micelles. Depending on the conditions (pH, ionic strength, chemical nature of the components etc), the yielding PSC are either soluble or insoluble in water. The possibility of the formation of water-soluble nonstoichiometric complexes is defined by the degree of the polyelectrolyte polymerization and the length of the surfactant ion alkyl chain, with other parameters being equal. Fig. 1 shows the relative concentrations of poly(acrylic acid) (i.e. the ratio of the PA concentration in the supernatant after separation of the precipitate from the bulk solution to the initial
Fig. 1. Relative concentration of PA in supernatant versus the reaction mixture composition. (0.1 M NaCl). PA (Pw =140), DTAB ( + ), TTAB ( × ), CTAB (*); PA (Pw = 600), DTAB ( ), TTAB (), CTAB (); PA (Pw = 3000), DTAB ( ), TTAB (), CTAB () mixtures
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PA concentration) as a function of the mixture composition (Z) for three samples of sodium polyacrylate with different molecular weight. The interaction of PA of Pw = 140 with alkyltrimethylammonium bromide solutions leads to formation of insoluble complexes followed by decreasing in PA concentration in supernatant at any mixture composition for all surfactants studied. The dependence of the relative concentration of PA in the supernatant is a linear function of the mixture composition, Z, extrapolated to Z=1 (Fig. 1(a)). It suggests that for all Z values in the range from 0 to 1 water-insoluble stoichiometric (i.e. of the complex composition of 8= 1) complex forms. Completely other pattern is observed for PA fractions of Pw ] 600 (Fig. 1(b–d)). In this case the dependencies of the relative concentrations of PA in the supernatant versus Z are characterized by rather wide intervals of Z in the range from 0 to Zlim. (arrow signs on the Fig. 1), in which the precipitant does not form, and the system remains externally homogeneous. At the reaction mixture composition above Zlim for PA of Pw = 600 and 3000 a linear decrease of polyelectrolyte concentration in the supernatant is observed. The corresponding plots of (C/C0) versus Z at Z above Zlim. (Fig. 1(b–d)) can also be extrapolated to Z= 1, what indicates the formation of insoluble stoichiometric complex at Z above Zlim. Hence, Zlim is the maximal value of Z which limits the homogeneous state of the system; when Z exceeds Zlim, phase separation is observed. It is necessary to emphasize that in this case (Pw ] 600) the value of Zlim does not depend on the PA polymerization degree but is determined by the length of the surfactant molecule alkyl chain length. Increase of the surfactant ion alkyl chain length lead to the decrease of Zlim from 0.35 to 0.22 for dodecyltrimethylammonium and cetyltrimethylammonium ions, respectively. The homogeneous mixtures of PA of Pw =600 with the surfactants were studied by high-speed sedimentation technique at the reaction mixture composition below Zlim. Fig. 2 represents the sedimentation profiles of the studied mixtures. In the range of Z values from 0 to 0.1 and from 0 to 0.15 for TTAB and CTAB respectively, two peaks
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Fig. 2. Sedimentation profiles for PA of Pw = 600 and PATTAB (a) and PA-CTAB (b) mixtures (0.1 M NaCl).
can be observed. The existence of two peaks on the sedimentogram unambiguously indicates that there are two different types of particles in the reaction mixture [12]. Comparison of these sedimentation coefficients with that of PA (Table 1) shows that the ‘slow’ peak corresponds to a free polyion, and the ‘fast’-to the soluble PSC particles. The increase of the surfactant concentration in the reaction mixture is accompanied by decrease of the free PA peak area and increase of the area of the complex peak with the same values of sedimentation coefficients for both peaks (see Fig. 2 and Table 1). At the composition of the reaction mixture Z= Zmin on the sedimentograms one can see only one peak attributed to the sedimentation of PSC particles of 8 = 8min. The results of sedimentation studies unambiguously indicate that the binding of TTAB and CTAB ions with PA for the reaction mixture composition below Zmin is characterized by the non-uniform distribution of the surfactant ions between macromolecules (i.e. disproportionation). In this range of Z, PSC particles coexist with the molecules of free polyacid, and increase of Z results only in the change of a mass ratio between free PA and water-soluble complexes of constant composition of 8 =8min. We can conclude that in
such PSC particles surfactant ions form intramolecular micelles of the minimum size. Moreover, formation of the complex of 8B 8min (i.e. with the micelle of lower size) at given conditions is impossible. In other words, 8min represents the minimum number of the surfactant ions necessary for PSC formation (nmin). The existence of only one peak attributed to PSC particles sedimentation upon increasing of the reaction mixture composition above Zmin indicates that the concentration of the surfactant ions in solution finally becomes high enough for the formation of a micelle of the minimum size inside each macromolecule coil. Therefore, in order to obtain a homogeneous system containing only a complex of the composition of 8min, the mixture composition should be equal Zmin. For evaluation of nmin, the molecular weights of PSC (Mw) were measured in the homogeneous systems at Z similar to Zmin by elastic laser light scattering. The technique of such measurements is described [3]. From the obtained values of the weight average molecular masses, the average number of the polyelectrolyte chains, N, and the average number of surfactant ions, n, incorporated into the complex particle have been calculated [3]. The results are summarized in Table 2. Evidently, a particle of water-soluble complex of the minimum composition includes one macromolecule of PA and approximately 130 surfactant ions. This suggests that PA-surfactant interaction is not accompanied by the association of macromolecules, and hence, PSC solutions are molecularly dispersed in terms of polyelectrolyte and can be considered as individual molecules. In concordance with aforesaid, the obtained values of n correspond nmin and are all around 102. This suggests that the minimum aggregation numbers of the surfactant ions in PSC micelles are close to the aggregation numbers of spherical micelles of corresponding surfactants in aqueous solutions [16,17]. The similar values of aggregation degree in intra-chain micelles were shown by time-resolved fluorescence measurements (see for example [18]). These results make it clear why poly(acrylic acid) of Pw = 140 unlike with PA of Pw = 600 and 3000, does not form soluble complexes with the
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Table 1 Sedimentation coefficients (S) for PA (Pw = 600) and NPSC, 0.1 M NaCl Surfactant
TTAB
CTAB
Mixture composition Z
S1 1013/s
S2 1013/s
S1 1013/s
S2 1013/s
0 0.05 0.10 0.15 0.20
1.7 1.8 1.7 – –
– 2.8 2.7 2.8 3.1
1.7 1.9 2.0 1.9 –
– 3.4 3.4 3.4 3.2
studied surfactants. In fact, the necessary condition of cooperative binding of surfactant ions to polyelectrolyte molecule is formation of micellar phase with an aggregation number not less than nmin : 102 with a certain number of oppositely charged polyelectrolyte units absorbed on a micelle surface. For maintenance of solubility of PSC, it is necessary for the polyion to conserve a sufficient number of free units capable to make PSC particle hydrophilic. The hydrophilic/hydrophobic balance of PSC particles, necessary for their solubility, is determined by the structure of the polyelectrolyte and the surfactant and is characterized by 8lim value which, in the first approximation, does not depend on the length of the polyion. This suggests that the soluble PSC will be formed if 8min B8lim. Since 8min = [surfactant]psc,min/[PA]=nmin/P, the necessary condition of water-soluble complex formation can be written as following: P\Pmin =nmin/8lim,
(1)
i.e. the polyion polymerization degree, P, should exceed a minimum value, Pmin, determined by the ratio (Eq. (1)). For PA-TTAB and PA-CTAB mixtures Pmin are approximately 500 and 600 accordingly. Thus, PA of Pw =600 and 3000 satisfies this criterion, while PA of Pw =140, does not satisfy it. This conclusion is illustrated by Fig. 3 which schematically represents a diagram of poly(acrylic acid) (pH 5.7)-oppositely charged surfactant interactions. In the first region at the reaction mixture composition below Zmin free polyacid molecules coexist with PSC particles of the minimum
composition 8min, which contain the minimum number of the surfactant ions capable to form an intramolecular micelle. Increase in the concentration of the surfactant ions in this region of Z results only in the change of proportion between free PA and PSC particles of the constant composition of 8min. At the reaction mixture composition equal to Zmin all PA molecules are bound with the surfactant ions into the complex of the minimum composition 8min. In the second region in the range of the reaction mixture composition from Zmin to Zlim the only water-soluble PSC are found in the system. In the third region at the reaction mixture composition above Zlim the phase separation occurs. In the concentrated phase, the insoluble stoichiometric complex was found, in which virtually all the monomer units of a polymer chain form salt bonds with the surfactant ions [3], while the diluted phase contains PSC of the composition of 8lim. Increase in the surfactant concentration within the range from Zlim to 1 only shifts the mass ratio between water-soluble and insoluble PSC while the compositions of the complexes in both phases remain constant. When the reaction mixture composition is equal to the composition of the stoichiometric PSC, i.e. Z= 1, both components are bound into a polycomplex, and the only surfactant ions at the concentration of 1–3 order of magnitude lower than CMC are found in solution. The region where the water-soluble complexes form (i.e. the value of Zlim) is defined by the surfactant ion alkyl chain length and does not depend on the PA polymerization degree. On the contrary, the value of Zmin decreases with the
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Table 2 Molecular characteristics of PA (Pw = 600) and NPSCa
PA PA-TTAB PA-CTAB a
Z
Mw 10−4/g mol−1
N
n
(Dz)0 107/sm2 s−1
Re/nm
– 0.25 0.2
5.5 8.3 10.6
1 0.9 1.2
– 130 140
2.1 4.6 2.9
10 5 7
0.1 M NaBr, N-the number of PA molecules, n-the number of S-ions the NPSC particle.
increase of PA polymerization degree since 8min =nmin/P. Thus, for PSC, formed by PA of Pw much more than Pmin, 8min is so small that the properties of PSC solution are indistinguishable from that of PA solution by the used experimental techniques. At the same time, an employment of the high-molecular weight PA enables us to study the formation and the structure of PSC in the second region. One should note that such investigations can not be performed for PA of Pw = 600 because of the closeness of 8min and 8lim values in this case. Fig. 4 represents the sedimentation data for PA of Pw = 3000 and PA-surfactant mixtures. The data show that in the second region the uniform distribution of the surfactant ions between PA molecules is observed. Since the equilibrium concentration of the free surfactant ions in solution is negligible (10 – 100 folds lower than their total concentration in the system) we can approximate that the PSC composition is similar to the composition of the reaction mixture. Table 3 summarizes the values of the weight-average molecular mass (Mw) of the PSC particles and the second virial coefficients (A2) for PSC solutions obtained by static light scattering. Obviously, the molecular mass of soluble PSC increases with the increase of the surfactant concentration, and the dependence of Mw versus 8 is linear within the whole range of the polycomplex concentrations. Evidently, a PSC particle includes one macromolecule, while the number of surfactant ions increases linearly with the increase of the reaction mixture composition. The same conclusion is correct for DTAB as well as for TTAB and CTAB. Increase of the number of the surfactant ions incorporated into a PSC particle results in decrease of the second virial coefficient
of the PSC solutions. Hence, the increase in the number of the polyanion chain units occupied by the surfactant ions deteriorates the thermodynamic quality of water as the solvent for PSC particles. The drop in A2 results from the reduction of the chain charge due to the neutralization of the carboxylic groups by the positively charged amphiphilic surfactant ions. Values of A2 remain, however, positive in the whole range of the complex compositions where PSC solutions are homogeneous. The conformation changes of the PA molecules occurring due to PSC formation, can be estimated from the data obtained by the quasi-elastic laser light scattering technique. Fig. 5 shows the dependence of the diffusion coefficient (Dz )0 of the polycomplexes PA-AlkTAB vs. the reaction mixture composition for PA of different polymerization degrees. One can see that for the complexes of poly(acrylic acid) of Pw = 3000 the increase in the concentration of surfactant ions does not affect (Dz )0. Hence, in the range of Z from 0 to Zlim, the average hydrodynamic size of a macromolecule coil is independent of the complex composition and the alkyl chain length of the surfactant ions and, within the experimental errors, is equal to those of free PA molecule. At the same time, for PA of Pw =600, formation of the intramolecular micelle of the minimum size (at the reaction mixture composition similar to Zmin) results in the essential compactization of the polyion coil (Table 2). In the case of water-soluble polyelectrolyte-surfactant complexes, the hydrophobic counterions are able to migrate along the polymer chain participating in the exchange reaction with other low-molecular ions presented in the bulk solution. The ionic bonds can be easily stretched and are
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Fig. 3. Schematic drawing of the PA-oppositely charged surfactant interaction.
not strictly oriented what allows the additional adjustment of the hydrophobic groups. In the case when the total amount of surfactant ions bounded with the polyion chain is close to nmin, formation of a symmetrical micelle, like a spherical one, is the only possibility to afford the maximum number of hydrophobic contacts between the surfactant ion alkyl chains. This leads to a collapse of macromolecule coil which results in the decrease of polymer conformational mobility and hence to the decrease of system entropy [19]. In case of high molecular weight PA where the total amount of surfactant ions is much more higher than nmin, the collapse of a polyelectrolyte molecule is not necessary for the achievement of the maximum number of hydrophobic contacts
Fig. 4. Sedimentation profiles for PA of Pw = 3000 (1) and PA-DTAB (a), PA-TTAB (b) and PA-CTAB (c) mixtures (2), Z= 0.05; (3), Z = 0.1; (4), Z= 0.2 (0.1 M NaCl).
between alkyl chains of the surfactant ions. We assume that in such PSC particle an asymmetric micelle which is stabilized by the oppositely charged polyelectrolyte molecule forms. In this case the conformational restrictions for a polyelectrolyte molecule are minimum. Increase of the surfactant ion concentration in the reaction mixture only causes the increase in such asymmetric micelle size.
4. Conclusion The study of the formation and structure of polyelectrolyte-oppositely charged surfactant complexes shows that such complexes can be considered as the polymeric surfactants of a distinct type, a kind of self-organizing systems. The necessary condition for the formation of PSC is aggregation of the surfactant ions and generation of intramolecular micellar phase. Minimum aggregation numbers of the surfactant ions in PSC micelles were shown to be close to that in the polymer-free solutions. If the amount of a surfactant ions in the system is not enough for the formation of the micellar phase inside each macromolecule coil, a non-uniform distribution of surfactant ions between macromolecules take place: polycomplex particles coexist with the molecules of the free polyelectrolyte. The structure of PSC is determined by the compromise between the maximization of hydrophobic contacts between alkyl chains of the surfactant ions and the minimization of conformational restrictions of the polyelectrolyte molecule. Realization of this compromise for PA
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Table 3 Molecular characteristics of water-soluble complexes formed by AlkTAB and PA of Pw =3000 in 0.1 M NaBr Surfactant
Complex composition, 8
Mw 10−5/g mol−1
A2 103
N
n
DTAB
0.10 0.15 0.20 0.25
3.05 2.85 3.84 4.15
0.85 0.84 0.34 0.25
0.90 1.0 0.85 0.94
270 460 500 710
TTAB
0.1 0.15 0.2 0.25
3.92 3.86 4.71 4.76
0.32 0.18 0.13 0.05
1.1 1.0 1.1 1.0
330 450 670 760
CTAB
0.1 0.15 0.2
3.76 4.5 5.85
0.3 0.2 0.13
0.98 1.1 1.2
300 500 790
References
Fig. 5. z-Average translational diffusion coefficient for watersoluble PA-AlkTAB complexes. (1), Pw = 600; (2), Pw =3000; (a), DTAB; (b), TTAB; (c), CTAB (0.1 M NaBr).
fractions of different molecular weight results in formation of PSC of different composition and structure. The conformation of the polyion coil and the surfactant ions aggregation number in the soluble PSC are largely determined by the degree of the polyelectrolyte polymerization.
Acknowledgements This research is supported by Russian Foundation for Fundamental Research (Project Code 9603-32900a).
[1] E.D. Goddard, Colloid. Surf. 19 (1986) 301 – 329. [2] B. Lindman, K. Thalberg, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interaction of surfactants with polymer and proteins, CRS, USA, 1993, p. 203. [3] Z.Kh. Ibragimova, E.M. Ivleva, N.V. Pavlova, et al., Polym. Sci. 34 (1992) 808. [4] Z.Kh. Ibragimova, V.A. Kasaikin, A.B. Zezin, V.A. Kabanov, Polym. Sci. USSR 28 (1986) 1826. [5] Yu.A. Zakharova, A.D. Kolbanovskiy, L.A. Krinitskaya, V.A. Kasaikin, A.M. Wasserman, Polym. Sci. Ser. B 37 (1995) 439. [6] P. Hansson, M. Almgrem, Langmuir 10 (1994) 2115. [7] K. Thalberg, J. van Stam, C. Lindblad, M. Almgren, B. Lindman, J. Phys. Chem. 95 (1991) 8975. [8] E.B. Abuin, J.C. Scaiano, J. Am. Chem. Soc. 106 (1984) 6274. [9] D.E. Chu, J.K. Thomas, J. Am. Chem. Soc. 108 (1986) 6270. [10] V.A. Efremov, A.R. Khohlov, Yu.V. Shikina, Polym. Sci. 34 (1992) 484. [11] O.A. Harenko, A.V. Harenko, R.I. Kalyuzhnaya, et al., Polym. Sci. USSR 21 (1979) 3002. [12] Ch. Tanford, Physical chemistry of macromolecules, Moscow, Chemistry, 1965 (in Russian). [13] K. Hayakawa, J.P. Santerre, J.C.T. Kwak, Macromolecules 16 (1983) 1642. [14] K. Thalberg, B. Lindman, K. Bergfeldt, Langmuir 7 (1991) 2893. [15] K. Kalyanasundaram, J.K. Thomas, J. Am. Chem. Soc. 99 (1977) 2039. [16] G.N. Krukova, V.A. Kasaikin, A.V. Sinyova, Z.N. Markina, Colloid. J. 40 (1978) 457. [17] G.N. Krukova, V.A. Kasaikin, Z.N. Markina, A.V. Sinyova, Colloid. J. 43 (1981) 660. [18] P. Hansson, M. Almgren, Langmuir 10 (1994) 2115. [19] L.M. Lifshits, A.Yu. Grosberg, Zh. Exsp. Teor. Fiz. 65 (1973) 2399.
Self-organization in complexes of polyacids with oppositely charged surfactants V.A. Kasaikin *, J.A. Zakharova Department of Polymer Science, Faculty of Chemistry, Moscow State Uni6ersity, Leninskie Gory, Moscow 119899, Russia
Abstract Formation and structure of water-soluble complexes of alkyltrimethylammonium bromide homologues (AlkTAB) with poly(acrylic acid) (PA) of different polymerization degrees at pH 5.7 have been examined by elastic and quasi-elastic laser light-scattering and high-speed sedimentation technique. It was experimentally shown that generation of intramolecular micellar phase is the necessary condition for formation of PA-AlkTAB complexes. Minimum aggregation number of the surfactant ions in the complex micelle was found to be close to that of the surfactant micelles in polymer-free solution. The structure of the polyelectrolyte-surfactant complexes (i.e. a phase state of the complex, conformation of the polyion coil and the surfactant ion aggregation number) was shown to be largely determined by PA polymerization degree. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Surfactants; Polyacids; Self-organisation
1. Introduction Cooperative interaction between micelle-forming surfactant ions and oppositely charged linear macromolecules is known to result in formation of polyelectrolyte-surfactant complexes (PSC) which exhibit a set of extremely interesting and practically important properties. Polyelectrolyte-surfactant complexes form as a result of the ion-exchange reaction between surfactant ions and ionized units of a macromolecule [1–3]. These compounds are stabilized by hydrophobic interactions of the surfactant ion alkyl chains [1,2] and may be considered as a special class of surface-active polyelectrolytes. There are * Corresponding author.
two types of complexes: water-soluble nonstoichiometric and insoluble in water stoichiometric complexes which may form depending on the binding conditions, such as: pH, ionic strength, concentration and chemical structure of the components, the reaction mixture composition [1,2,4]. It is well known that in PSC surfactant ions segregate and form intramolecular micellar phase [1,2]. Previously, it was shown that the morphology of the PSC micelles differ dramatically from that of the corresponding surfactant ‘free’ micelles [5]. Theoretical examination of polyelectrolytesurfactant interactions [10] shows, that the necessary condition for the formation of PSC is generation of the intramolecular micellar phase. If the amount of a surfactant ion in the system is not enough for formation of the micellar phase
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inside each macroion, the non-uniform distribution of surfactant ions between macromolecules have to take place: polycomplex particles of certain composition have to coexist with the molecules of free polyelectrolyte [10]. By now, data concerning surfactant aggregation number values in PSC micelles are extremely inconsistent [6–9], and the question on the minimum number of surfactant ions capable to form intra-chain micellar phase is practically not studied. The present paper is devoted to the study of the minimum aggregation number of the surfactant ions in PSC micelles, with the special interest on the influence of polyelectrolyte polymerization degree (Pw) on the structure of PSC particles.
2. Experimental section Synthesis and fractionation of poly(acrylic acid) (PA) were described elsewhere [11]. Fractions of PA with weight average degree of polymerization of Pw =140, 600 and 3000 were used. Surfactants-alkyltrimethylammonium bromide homologues (AlkTAB) of various alkyl chain lengths: dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), (Aldrich-Chemie, Germany) were used without further purification. The high-speed sedimentation studies were carried out at 20°C in presence of 0.1 M NaCl with a Spinco-E (Beckman, USA) ultracentrifuge supplied with Filpot – Swenson optical system for the refractive index gradient determination. Rotation frequency was 58000 rpm. The solvent was layered over the sample solution. The values of sedimentation coefficients (S) were calculated using the following equation [12]: S =d(ln x)/v 2d(t), where x is the distance between the sedimentation peak maximum and the rotation axis, v is the angular speed of the rotor rotation and t is the time of the rotation. The experimental error of the sedimentation coefficient measurements was not more than 10%.
The concentration of PA in solution was measured by potentiometry with AUTOCAL pH M83 (Radiometer, Denmark). Molecular characteristics of water-soluble nonstoichiometric complexes were obtained by static light scattering with small-angle laser photometer KMX-6/DC (Milton Roy, USA) with a 4 mW He–Ne laser as a light source (l= 630 nm, the scattering angle 6.5°). The RSD value was 5% for all measurements of the molecular weight and the second virial coefficient. The refractive index increment ( dn/dc) was measured using KMX-16 differential refractometer (Milton Roy, USA) with 0.5 mW He–Ne laser as a light source (l=630 nm.). The solvent equilibrated with the complex solution by dialysis was used as the reference sample. Autocorrelation function of the scattering intensity fluctuations was measured by quasi-elastic light scattering (QELS) using a 1096 correlator (Langly Ford, USA). The obtained data were analyzed by the cumulants method. For all studied systems the dependence of (Kc/ Ru) versus c obey the Debye equation: Kc/Ru = 1/Mw + 2A2c where c is weight concentration, K is optical constant, Ru is the Rayleigh ratio, and u is an angle of light scattering (6.5° in our case). The concentration dependencies of (Kc/Ru) as well as of Dz for solutions of poly(acrylic acid) and water soluble complexes are linear within the studied range of concentrations (0.05–0.5 wt.%) [3]. This means that at the experimental conditions the dimension and the molecular weight of the PSC particles remains constant as PSC concentration decreases. In other words, in the studied concentration range the dissociation of PSC as well as polyelectorlyte swelling are not observed. Average hydrodynamic radii of appropriate equivalent spheres (Re) were calculated using the Stokes equation. The reaction mixture composition Z is determined as the ratio of surfactant molar concentration to the molar concentration of polyanion units in the solution Z= [surfactant]/[polyelectrolyte]. The composition of PSC, 8, is determined as the ratio of the number of surfactant ions to the
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number of polyanion monomer units in the complex particle 8 = [surfactant]psc/[polyacid units]psc. All investigations were performed at pH 5.7, T= 20°C.
3. Results and discussion Interaction of linear polyelectrolytes with oppositely charged surfactant ions proceeds as the cooperative binding at the surfactant concentration 1–3 order of magnitude lower than CMC [13–15]. This process is known to be accompanied by association of the surfactant ions and formation of micelles. Depending on the conditions (pH, ionic strength, chemical nature of the components etc), the yielding PSC are either soluble or insoluble in water. The possibility of the formation of water-soluble nonstoichiometric complexes is defined by the degree of the polyelectrolyte polymerization and the length of the surfactant ion alkyl chain, with other parameters being equal. Fig. 1 shows the relative concentrations of poly(acrylic acid) (i.e. the ratio of the PA concentration in the supernatant after separation of the precipitate from the bulk solution to the initial
Fig. 1. Relative concentration of PA in supernatant versus the reaction mixture composition. (0.1 M NaCl). PA (Pw =140), DTAB ( + ), TTAB ( × ), CTAB (*); PA (Pw = 600), DTAB ( ), TTAB (), CTAB (); PA (Pw = 3000), DTAB ( ), TTAB (), CTAB () mixtures
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PA concentration) as a function of the mixture composition (Z) for three samples of sodium polyacrylate with different molecular weight. The interaction of PA of Pw = 140 with alkyltrimethylammonium bromide solutions leads to formation of insoluble complexes followed by decreasing in PA concentration in supernatant at any mixture composition for all surfactants studied. The dependence of the relative concentration of PA in the supernatant is a linear function of the mixture composition, Z, extrapolated to Z=1 (Fig. 1(a)). It suggests that for all Z values in the range from 0 to 1 water-insoluble stoichiometric (i.e. of the complex composition of 8= 1) complex forms. Completely other pattern is observed for PA fractions of Pw ] 600 (Fig. 1(b–d)). In this case the dependencies of the relative concentrations of PA in the supernatant versus Z are characterized by rather wide intervals of Z in the range from 0 to Zlim. (arrow signs on the Fig. 1), in which the precipitant does not form, and the system remains externally homogeneous. At the reaction mixture composition above Zlim for PA of Pw = 600 and 3000 a linear decrease of polyelectrolyte concentration in the supernatant is observed. The corresponding plots of (C/C0) versus Z at Z above Zlim. (Fig. 1(b–d)) can also be extrapolated to Z= 1, what indicates the formation of insoluble stoichiometric complex at Z above Zlim. Hence, Zlim is the maximal value of Z which limits the homogeneous state of the system; when Z exceeds Zlim, phase separation is observed. It is necessary to emphasize that in this case (Pw ] 600) the value of Zlim does not depend on the PA polymerization degree but is determined by the length of the surfactant molecule alkyl chain length. Increase of the surfactant ion alkyl chain length lead to the decrease of Zlim from 0.35 to 0.22 for dodecyltrimethylammonium and cetyltrimethylammonium ions, respectively. The homogeneous mixtures of PA of Pw =600 with the surfactants were studied by high-speed sedimentation technique at the reaction mixture composition below Zlim. Fig. 2 represents the sedimentation profiles of the studied mixtures. In the range of Z values from 0 to 0.1 and from 0 to 0.15 for TTAB and CTAB respectively, two peaks
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Fig. 2. Sedimentation profiles for PA of Pw = 600 and PATTAB (a) and PA-CTAB (b) mixtures (0.1 M NaCl).
can be observed. The existence of two peaks on the sedimentogram unambiguously indicates that there are two different types of particles in the reaction mixture [12]. Comparison of these sedimentation coefficients with that of PA (Table 1) shows that the ‘slow’ peak corresponds to a free polyion, and the ‘fast’-to the soluble PSC particles. The increase of the surfactant concentration in the reaction mixture is accompanied by decrease of the free PA peak area and increase of the area of the complex peak with the same values of sedimentation coefficients for both peaks (see Fig. 2 and Table 1). At the composition of the reaction mixture Z= Zmin on the sedimentograms one can see only one peak attributed to the sedimentation of PSC particles of 8 = 8min. The results of sedimentation studies unambiguously indicate that the binding of TTAB and CTAB ions with PA for the reaction mixture composition below Zmin is characterized by the non-uniform distribution of the surfactant ions between macromolecules (i.e. disproportionation). In this range of Z, PSC particles coexist with the molecules of free polyacid, and increase of Z results only in the change of a mass ratio between free PA and water-soluble complexes of constant composition of 8 =8min. We can conclude that in
such PSC particles surfactant ions form intramolecular micelles of the minimum size. Moreover, formation of the complex of 8B 8min (i.e. with the micelle of lower size) at given conditions is impossible. In other words, 8min represents the minimum number of the surfactant ions necessary for PSC formation (nmin). The existence of only one peak attributed to PSC particles sedimentation upon increasing of the reaction mixture composition above Zmin indicates that the concentration of the surfactant ions in solution finally becomes high enough for the formation of a micelle of the minimum size inside each macromolecule coil. Therefore, in order to obtain a homogeneous system containing only a complex of the composition of 8min, the mixture composition should be equal Zmin. For evaluation of nmin, the molecular weights of PSC (Mw) were measured in the homogeneous systems at Z similar to Zmin by elastic laser light scattering. The technique of such measurements is described [3]. From the obtained values of the weight average molecular masses, the average number of the polyelectrolyte chains, N, and the average number of surfactant ions, n, incorporated into the complex particle have been calculated [3]. The results are summarized in Table 2. Evidently, a particle of water-soluble complex of the minimum composition includes one macromolecule of PA and approximately 130 surfactant ions. This suggests that PA-surfactant interaction is not accompanied by the association of macromolecules, and hence, PSC solutions are molecularly dispersed in terms of polyelectrolyte and can be considered as individual molecules. In concordance with aforesaid, the obtained values of n correspond nmin and are all around 102. This suggests that the minimum aggregation numbers of the surfactant ions in PSC micelles are close to the aggregation numbers of spherical micelles of corresponding surfactants in aqueous solutions [16,17]. The similar values of aggregation degree in intra-chain micelles were shown by time-resolved fluorescence measurements (see for example [18]). These results make it clear why poly(acrylic acid) of Pw = 140 unlike with PA of Pw = 600 and 3000, does not form soluble complexes with the
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Table 1 Sedimentation coefficients (S) for PA (Pw = 600) and NPSC, 0.1 M NaCl Surfactant
TTAB
CTAB
Mixture composition Z
S1 1013/s
S2 1013/s
S1 1013/s
S2 1013/s
0 0.05 0.10 0.15 0.20
1.7 1.8 1.7 – –
– 2.8 2.7 2.8 3.1
1.7 1.9 2.0 1.9 –
– 3.4 3.4 3.4 3.2
studied surfactants. In fact, the necessary condition of cooperative binding of surfactant ions to polyelectrolyte molecule is formation of micellar phase with an aggregation number not less than nmin : 102 with a certain number of oppositely charged polyelectrolyte units absorbed on a micelle surface. For maintenance of solubility of PSC, it is necessary for the polyion to conserve a sufficient number of free units capable to make PSC particle hydrophilic. The hydrophilic/hydrophobic balance of PSC particles, necessary for their solubility, is determined by the structure of the polyelectrolyte and the surfactant and is characterized by 8lim value which, in the first approximation, does not depend on the length of the polyion. This suggests that the soluble PSC will be formed if 8min B8lim. Since 8min = [surfactant]psc,min/[PA]=nmin/P, the necessary condition of water-soluble complex formation can be written as following: P\Pmin =nmin/8lim,
(1)
i.e. the polyion polymerization degree, P, should exceed a minimum value, Pmin, determined by the ratio (Eq. (1)). For PA-TTAB and PA-CTAB mixtures Pmin are approximately 500 and 600 accordingly. Thus, PA of Pw =600 and 3000 satisfies this criterion, while PA of Pw =140, does not satisfy it. This conclusion is illustrated by Fig. 3 which schematically represents a diagram of poly(acrylic acid) (pH 5.7)-oppositely charged surfactant interactions. In the first region at the reaction mixture composition below Zmin free polyacid molecules coexist with PSC particles of the minimum
composition 8min, which contain the minimum number of the surfactant ions capable to form an intramolecular micelle. Increase in the concentration of the surfactant ions in this region of Z results only in the change of proportion between free PA and PSC particles of the constant composition of 8min. At the reaction mixture composition equal to Zmin all PA molecules are bound with the surfactant ions into the complex of the minimum composition 8min. In the second region in the range of the reaction mixture composition from Zmin to Zlim the only water-soluble PSC are found in the system. In the third region at the reaction mixture composition above Zlim the phase separation occurs. In the concentrated phase, the insoluble stoichiometric complex was found, in which virtually all the monomer units of a polymer chain form salt bonds with the surfactant ions [3], while the diluted phase contains PSC of the composition of 8lim. Increase in the surfactant concentration within the range from Zlim to 1 only shifts the mass ratio between water-soluble and insoluble PSC while the compositions of the complexes in both phases remain constant. When the reaction mixture composition is equal to the composition of the stoichiometric PSC, i.e. Z= 1, both components are bound into a polycomplex, and the only surfactant ions at the concentration of 1–3 order of magnitude lower than CMC are found in solution. The region where the water-soluble complexes form (i.e. the value of Zlim) is defined by the surfactant ion alkyl chain length and does not depend on the PA polymerization degree. On the contrary, the value of Zmin decreases with the
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Table 2 Molecular characteristics of PA (Pw = 600) and NPSCa
PA PA-TTAB PA-CTAB a
Z
Mw 10−4/g mol−1
N
n
(Dz)0 107/sm2 s−1
Re/nm
– 0.25 0.2
5.5 8.3 10.6
1 0.9 1.2
– 130 140
2.1 4.6 2.9
10 5 7
0.1 M NaBr, N-the number of PA molecules, n-the number of S-ions the NPSC particle.
increase of PA polymerization degree since 8min =nmin/P. Thus, for PSC, formed by PA of Pw much more than Pmin, 8min is so small that the properties of PSC solution are indistinguishable from that of PA solution by the used experimental techniques. At the same time, an employment of the high-molecular weight PA enables us to study the formation and the structure of PSC in the second region. One should note that such investigations can not be performed for PA of Pw = 600 because of the closeness of 8min and 8lim values in this case. Fig. 4 represents the sedimentation data for PA of Pw = 3000 and PA-surfactant mixtures. The data show that in the second region the uniform distribution of the surfactant ions between PA molecules is observed. Since the equilibrium concentration of the free surfactant ions in solution is negligible (10 – 100 folds lower than their total concentration in the system) we can approximate that the PSC composition is similar to the composition of the reaction mixture. Table 3 summarizes the values of the weight-average molecular mass (Mw) of the PSC particles and the second virial coefficients (A2) for PSC solutions obtained by static light scattering. Obviously, the molecular mass of soluble PSC increases with the increase of the surfactant concentration, and the dependence of Mw versus 8 is linear within the whole range of the polycomplex concentrations. Evidently, a PSC particle includes one macromolecule, while the number of surfactant ions increases linearly with the increase of the reaction mixture composition. The same conclusion is correct for DTAB as well as for TTAB and CTAB. Increase of the number of the surfactant ions incorporated into a PSC particle results in decrease of the second virial coefficient
of the PSC solutions. Hence, the increase in the number of the polyanion chain units occupied by the surfactant ions deteriorates the thermodynamic quality of water as the solvent for PSC particles. The drop in A2 results from the reduction of the chain charge due to the neutralization of the carboxylic groups by the positively charged amphiphilic surfactant ions. Values of A2 remain, however, positive in the whole range of the complex compositions where PSC solutions are homogeneous. The conformation changes of the PA molecules occurring due to PSC formation, can be estimated from the data obtained by the quasi-elastic laser light scattering technique. Fig. 5 shows the dependence of the diffusion coefficient (Dz )0 of the polycomplexes PA-AlkTAB vs. the reaction mixture composition for PA of different polymerization degrees. One can see that for the complexes of poly(acrylic acid) of Pw = 3000 the increase in the concentration of surfactant ions does not affect (Dz )0. Hence, in the range of Z from 0 to Zlim, the average hydrodynamic size of a macromolecule coil is independent of the complex composition and the alkyl chain length of the surfactant ions and, within the experimental errors, is equal to those of free PA molecule. At the same time, for PA of Pw =600, formation of the intramolecular micelle of the minimum size (at the reaction mixture composition similar to Zmin) results in the essential compactization of the polyion coil (Table 2). In the case of water-soluble polyelectrolyte-surfactant complexes, the hydrophobic counterions are able to migrate along the polymer chain participating in the exchange reaction with other low-molecular ions presented in the bulk solution. The ionic bonds can be easily stretched and are
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Fig. 3. Schematic drawing of the PA-oppositely charged surfactant interaction.
not strictly oriented what allows the additional adjustment of the hydrophobic groups. In the case when the total amount of surfactant ions bounded with the polyion chain is close to nmin, formation of a symmetrical micelle, like a spherical one, is the only possibility to afford the maximum number of hydrophobic contacts between the surfactant ion alkyl chains. This leads to a collapse of macromolecule coil which results in the decrease of polymer conformational mobility and hence to the decrease of system entropy [19]. In case of high molecular weight PA where the total amount of surfactant ions is much more higher than nmin, the collapse of a polyelectrolyte molecule is not necessary for the achievement of the maximum number of hydrophobic contacts
Fig. 4. Sedimentation profiles for PA of Pw = 3000 (1) and PA-DTAB (a), PA-TTAB (b) and PA-CTAB (c) mixtures (2), Z= 0.05; (3), Z = 0.1; (4), Z= 0.2 (0.1 M NaCl).
between alkyl chains of the surfactant ions. We assume that in such PSC particle an asymmetric micelle which is stabilized by the oppositely charged polyelectrolyte molecule forms. In this case the conformational restrictions for a polyelectrolyte molecule are minimum. Increase of the surfactant ion concentration in the reaction mixture only causes the increase in such asymmetric micelle size.
4. Conclusion The study of the formation and structure of polyelectrolyte-oppositely charged surfactant complexes shows that such complexes can be considered as the polymeric surfactants of a distinct type, a kind of self-organizing systems. The necessary condition for the formation of PSC is aggregation of the surfactant ions and generation of intramolecular micellar phase. Minimum aggregation numbers of the surfactant ions in PSC micelles were shown to be close to that in the polymer-free solutions. If the amount of a surfactant ions in the system is not enough for the formation of the micellar phase inside each macromolecule coil, a non-uniform distribution of surfactant ions between macromolecules take place: polycomplex particles coexist with the molecules of the free polyelectrolyte. The structure of PSC is determined by the compromise between the maximization of hydrophobic contacts between alkyl chains of the surfactant ions and the minimization of conformational restrictions of the polyelectrolyte molecule. Realization of this compromise for PA
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Table 3 Molecular characteristics of water-soluble complexes formed by AlkTAB and PA of Pw =3000 in 0.1 M NaBr Surfactant
Complex composition, 8
Mw 10−5/g mol−1
A2 103
N
n
DTAB
0.10 0.15 0.20 0.25
3.05 2.85 3.84 4.15
0.85 0.84 0.34 0.25
0.90 1.0 0.85 0.94
270 460 500 710
TTAB
0.1 0.15 0.2 0.25
3.92 3.86 4.71 4.76
0.32 0.18 0.13 0.05
1.1 1.0 1.1 1.0
330 450 670 760
CTAB
0.1 0.15 0.2
3.76 4.5 5.85
0.3 0.2 0.13
0.98 1.1 1.2
300 500 790
References
Fig. 5. z-Average translational diffusion coefficient for watersoluble PA-AlkTAB complexes. (1), Pw = 600; (2), Pw =3000; (a), DTAB; (b), TTAB; (c), CTAB (0.1 M NaBr).
fractions of different molecular weight results in formation of PSC of different composition and structure. The conformation of the polyion coil and the surfactant ions aggregation number in the soluble PSC are largely determined by the degree of the polyelectrolyte polymerization.
Acknowledgements This research is supported by Russian Foundation for Fundamental Research (Project Code 9603-32900a).
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