J. Phys. Chem. 1992, 96, 2345-2348
2345
Interaction between Polyelectrolyte and Surfactant of Opposite Charge. Hydrodynamic Effects in the Sodium Hyaluronate/Tetradecyltrimethylammonium Bromide/ Sodium Chloride/Water System Asa Herslof,* Lars-Olof Sundelof, Department of Physical and inorganic Pharmaceutical Chemistry, Uppsala University, Biomedical Center, P.O. Box 574, S-751 23 Uppsala, Sweden
and Katarina Edsman Department of Polymer Physics and Chemistry, Kabi Pharmacia AB, S-751 82 Uppsala, Sweden (Received: March 22, 1991)
The interaction between the negatively charged polyelectrolyte sodium hyaluronate and the positively charged ion of the surfactant tetradecyltrimethylammonium bromide (TTAB) has been investigated in water solution. The properties of the system have been monitored by measuring the solution viscosity, surface tension, and phase equilibrium properties for increasing sodium chloride concentrations. On addition of TTAB and at low ionic strength the solution separates into two phases. If the salt (sodium chloride) concentration is sufficiently high (in the present case approximately 200 mM), a homogeneous system is obtained. Just above this critical salt concentration the solution viscosity passes through a marked minimum as the TTAB concentration is increased. At higher salt concentration the viscosity behavior becomes almost identical to that of a solution without TTAB. The effect is tentatively explained as an initial decharging of the polyelectrolyte accompanied by contraction of the coil and partial aggregation. At higher TTAB contents the adsorption leads to expanded chains and possibly deaggregation.
Introduction For polyelectrolytes the specific feature as opposed to that of uncharged polymers is to a large degree governed by the distribution of charges along the polymer chain.’S2 Thus, the conformational properties and the dynamic behavior of the polyelectrolyte chain are determined by the degree of ionization of the polyelectrolyte and of the counterion concentration and distrib ~ t i o n . ~ ”In aqueous solution the solvation properties and the hydrophobic interactions are also important factors that strongly influence the conformational properties of the polymer backbone. Polysaccharides are especially intriguing in this respect due to the steric variability of their monomeric units and the mixture of hydrophilic and hydrophobic structures. Solutions of ionized polysaccharides thus present complex problems from the theoretical point of view. At the same time they often furnish properties of high biological specificity, and many features are of considerable interest for applications. A polyelectrolyte derives its solution behavior partly from the “backbone properties” and partly from interaction with solvent and other solutes, charged or uncharged. The counterions may be solvated in different ways and hence influence the polymer solvation in a manner depending upon which ions are present. Low-molecular-weight amphiphiles may either redistribute to the polymer segment region or adsorb to the backbone by electrostatic and/or hydrophobic bonds. This will in effect change the chemical properties of the polymer, and conformational changes can be expected. In the case of a neutral polymer and an anionic surfactant, binding of the latter to the polymer is cooperative and normally starts at a well-defined surfactant concentration, somewhat below the normal cmc. The aggregates are supposed to consist of clusters For a system of surfactant adsorbed on the polymer with a polyelectrolyte and an oppositely charged surfactant, the binding is also cooperati~e’~-’~ but sets in already at a surfactant concentration much lower than the cmc. For systems with polyelectrolyte and surfactant of the same charge or neutral polymer and cationic surfactant, only weak or no interaction at all has been reported.20*z1A review article on systems with polyelectrolytes and oppositely charged surfactants has been published by God-
* To whom correspondence should be addressed.
dard,22and more recently, Hayakawa, Kwak, and co-workersz3 presented a review of the systems of polymers and cationic surfactants. Conformational changes in a polymer are normally reflected in its hydrodynamic volume. Hence, viscosity measurementsz4
(1) Morawetz, H. Macromolecules in Solution; Wiley: New York, 1975. (2) Cleland, R. L.; Wang, J. L.; Detweiler, D. M. Macromolecules 1982, 15, 386. (3) Wang, L.; Yu, H. Macromolecules 1988, 21, 3498. (4) Wang, L.; Bloomfield, V. A. Macromolecules 1990, 23, 194. (5) Wang, L.; Bloomfield, V. A. Macromolecules 1990, 23, 804. (6) Tam, K. C.; Tiu, C. Polym. Commun. 1989, 30, 114. (7) Cabane, B. J . Phys. Chem. 1977, 81, 1639. (8) Cabane, B.; Duplessix, R. In ‘Solution Behav. Surfactanis: Theor. Appl. Aspects”, (Proc. Int. Symp.) Meeting Date 1980, 1, 661-664, Ed. by Mittal, K. L. and Fendler, E. J., Plenum: New York, N.Y. (9) Cabane, B.; Duplessix, R. J . Phys. (Paris) 1982, 43, 1529. (10) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (1 1) Cabane, B.; Duplessix, R. J . Phys. (Paris) 1987, 48, 65 1 . (12) Nagarajan, R. Colloids Surf. 1985, 13, 1 . (13) Nagarajan, R. J . Chem. Phys. 1989, 90, 1980. (14) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987, 3, 382. (15) Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1982,86, 3866. (16) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (17) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1984, 88, 1930. (18) Santerre, J. P.; Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1985, 13, 35. (19) Shirahama, K.; Tashiro, M. Bull. Chem. SOC.Jpn. 1984, 57, 377. (20) Shirahama, K.; Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987, 265, 96. (21) Shirahama, K.; Oh-Ishi, M.; Takisawa, N. Colloids Surf. 1989, 40, 261. (22) Goddard, E. D. Colloids Surf. 1986, 19, 301. (23) Hayakawa, K.; Kwak, J. C. In Cationic Surfactants. Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Chapter 5, pp 189-247. (24) Cleland, R. Biopolymers 1984, 23, 647.
0022-3654/92/2096-2345%03.00/00 1992 American Chemical Society
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2346 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
5-e p 8 g
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TTAB concentration (mM) Figure 1. Phase diagram showing the critical electrolyte concentrations (ccc)of NaCl in samples of 0.1%(w/w) NaHy (M, 559000) and TTAB as a function of TTAB concentration. Open symbols refer to clear one-phase solutions and filled symbols to two-phase solutions. The temperature was 20 OC.
in dilute solution constitute a simple and sensitive method by which various aspects of the polyelectrolytic behavior indicated above can be monitored. The present work deals with the effect of a positively charged surfactant, tetradecyltrimethylammoniumbromide (TTAB), on the negatively charged polysaccharide, sodium hyaluronate (NaHy),r2*28 for various ionic strengths (as regulated by sodium
1
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chloride) and at neutral pH where the carboxylic groups on the polymer chain are completely ionized. This means that there is one charge on every disaccharide unit, i.e. on the glucuronate unit (the repeating disaccharide unit being glucuronate and Nacetylglucosamine). In the absence of salt (apart from the Na+ and Br- counterions from NaHy and TTAB), the system NaHy/TTAB/water forms a onephase solution only for very low or very high TTAB content and phase separation occurs in the intermediate range. By addition of a suitable electrolyte (NaCl, NaBr, etc.), this solubility gap can be s u p p r d and the phase separation eventually eliminated. Figure 1 shows the phase diagram for the actual system. Phase diagrams for similar systems have earlier been thoroughly studied by Thalberg et al.29-36 The full drawn line denotes the critical (25) Laurcnt, T. In Chemistry and Molecular Biology of the Intercellular Matrix; Balas E. A., Ed.; Academic Press: London, 1970; Chapter 5, pp 703-732. (26) Laurent, T. Acta Of@Luryngol.,Suppl. 1987, No. 442, 7. (27) The Biology of Hyaluronan; Ciba Foundation Symposium 143; John Wiley & Sons: London, 1989. (28) Moms, E.R.;Rees, D. A.; Welsh, E. J. J. Mol. Biol. 1980,138, 383. (29) Thalberg, K.; Lindman, B. J . Phys. Chcm. 1989, 93, 1478. (30) Thalberg, K.; Lindman, B.; Karlstrom, G. J . Phys. Chem. 1991,95, 3370. (31) Thalberg, K.; Lindman, B.; Karlstrom, G. J . Phys. Chem. 1990, 94, 4289. (32) Thalberg, K.; Lindman, B.; Karlstrom, G. Electrolyte dependent phase separation in aqueous mixtures of a polyelectrolyte and an ionic surfactant. Submitted for publication in Prog. Colloid Polym. Sci. (33) Thalberg, K.; Lindman, B.; Karlstrbm, G. J . Phys. Chem. 1991, 93, 6004. (34) Thalberg, K.; Lindman, B.; Bergfeldt, K. Phase behavior of systems of polyacrylate and cationic surfactant. Imngmuir, in press. (35) Thalberg, K.; Lindman, B. Gel formation in aqueous systems of a polyanion and an oppositely charged surfactant. Lungmuir 1991, 7, 277.
v,
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Figure 2. Surface tension as a function of TTAB concentration for three sets of data: a pure "TAB solution (A); a solution containing TTAB and 200 mM NaCl (0); a solution containing TTAB, 200 mM NaCl and 0.1%(w/w) NaHy (0). The temperature was 25 OC.
electrolyte concentration (cec)needed to prevent phase separation. Earlier studies30 show the influence on the cec values of the surfactant chain length and the molecular weight of the polymer. The longer this alkane chain and the higher the molecular weight of the polymer, the higher are the cec values. The present study is concemed with this homogeneous solution region, with appropriately adjusted salt concentrations. The main aim has been to map the overall conformational changes as reflected in the hydrodynamic properties of the polymer at different concentrations as a function of TTAB and NaCl concentration.
Experimental Section MateMs. Sodium hyaluronate (NaHy), Batch No. L.M.W-1, was kindly supplied by Kabi Pharmacia AB, Uppsala, Sweden. The weight-average molecular weight was 559 OOO as determined by low-angle laser light scattering (LALLS).The protein content was less than 0.2 mg/g. Tetradecyltrimethylammonium bromide ("TAB) was obtained from Sigma Chemical Co., St. Louis, MO. Sodium chloride (NaCl) was of analytical grade and obtained from Merck, Darmstadt, Germany. All chemicals were used without further purification. Prepontion of Solutrona All solutions were prepared by weight from stock solutions of polyelectrolyte, surfactant, and salt in deionized and redistilled water. The order of adding the components when preparing the solutions was of great importance. To reduce problems with phase separation, the salt had to be added before TTAB. For the lowest salt concentrations (Le. close to the phase equilibrium line), metastable phase separation could not always be avoided. However, by careful mixing of stock solution the amount of initially formed concentrated phase could be decreased and the time of achieving a clear solution reduced. Once prepared, the solutions were allowed to stand with magnetic stirring overnight or longer to be sure of homogeneous solutions. The hyaluronate concentration in the stock solutions was determined by optical rotation measurements. Methods. The viscosity measurements were carried out in an automatic, low-shear capillary viscometer (designed in the laboratory of Kabi Pharmacia AB). The measurements were made in series starting with the solution with the highest (or lowest) concentration and then diluting (or concentrating) the solutions by adding solvent (or "solution") from a Methrom dosimat (Model 645 Multi-Dosimat). Both the viscometer and the dosimat were controlled by a computer. All measurements were performed at 25 OC. For the optical rotation measurements a Perkin-Elmer 241 polarimeter was used. The determinations were performed with a mercury lamp with the wavelength 436 nm, where hyaluronate has a specific rotation of -160.7. Density measurements were made in a digital densitometer (Model DMA 02C from Anton Paar K.G., Graz Austria). For the surface tension measurements a du Nouy tensiometer (Kruss Tensiometer nach Le(36) Thalberg, K.; van Stam, J.; Lindblad, C.; Almgren, M.; Lindman, B. Timeresolved fluorescence and self-diffusionstudies in systems of a cationic surfactant and an anionic polyelectrolyte. Submitted for publication in J . Phys. Chem.
Interaction between Polyelectrolyte and Surfactant
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2341
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NaCl concentration m M Figure 3. Viscosity measurements showing the NaCl dependence for a pure NaHy solution (0.07% (w/w)) (0)compared to a solution of the same NaHy concentration but also containing TTAB (15 mM) (H). Relative viscosity as a function of NaCl concentration. The temperature was 25 OC. Solvents is here the polymer-free solutions.
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TTAB concentration (mM) Figure 4. Relative viscosity as a function of TTAB concentration for solutions of the same NaHy concentration (0.1% (w/w)) but with different NaCl concentrations: 200 (*), 210 (0),220 (X), 230 (+), 250 (A), 300 (0),400 (A),and 770 mM (R). Relative viscosity is here taken as the viscosity of the solution relatively to the viscosity of a similar solution but without TTAB. The temperature was 25 OC.
comte du Noiiy) from A. Kruss, Hamburg, was used. The phase diagram was determined by visual observations of opacity of solutions with varying TTAB and salt concentration. The latter was varied in sufficiently small steps to allow fairly accurate estimation of the phase equilibrium composition.
Results and Discussion Surface tension measurements, reported in Figure 2, indicate that in the presence of 200 mM NaCl (the amount of supporting electrolyte needed to assure single-phase conditions in the NaHy/TTAB/NaCl system for the TTAB region of interest) the critical micelle concentration (cmc) occurs at [TTAB],,, around 0.5 mM. Here [TTAB],, denotes the total surfactant concentration. The cmc for pure TTAB was determined to be 3 mM. Addition of polymer to the surfactantsalt solution does not seem to affect the cmc further. From Figure 1 it is seen that most of the viscosity measurements presented in this paper are performed at TTAB concentrations above the cmc. Figure 3 compares the change in relative viscosity with increasing salt concentration of a solution containing only NaHy with that of a NaHy solution also containing TTAB. The NaHy concentrations are identical for the two curves. For the pure NaHy solution the viscosity initially decreases rapidly and then approaches an asymptotic value. The viscosity decrease reflects the screening of charges on the polymer backbone, resulting in a more flexible chain, but obviously a rather high salt concentration is required to screen out most of the polyelectrolyte behavior. For the solution which contains also TTAB-starting with a salt concentration just sufficient to bring the system to one-phase conditions (200 mM NaC1)-the viscosity is seen to be considerably lower than in the pure NaHy solution. When the salt concentration is increased, the viscosity initially increases rapidly and at slightly more than 300 mM salt the viscosity of the pure NaHy solution is reached. The basic explanation of the effects observed is likely to be the following. The viscosity of a polymer solution is governed by the spatial segment distribution of the individual molecules, by intermolecular interaction, and by entanglements and aggregation. The reduction in viscosity, when TTAB is added, can then be ascribed either to a contraction of the spatial distribution of chain segments in individual molecules or to strong interaction in the form of aggregation, or both. The results presented in Figure 1 have indicated that there is sufficient interaction in the system NaHy/TTAB/NaCl/water to give rise to phase separation in certain composition ranges. The viscosity data in Figure 3 indicate that even in a one-phase region there is considerable interaction between polymer and surfactant. To allow a more precise interpretation, it is necessary to understand how the interaction relates to the TTAB and NaCl content on a molar scale, as well as to the polymer concentration. Results from such experiments are presented in Figures 4 and 5 and are to be discussed in what follows. Figure 4 gives a more complete picture of the viscosity variation as a function of [TTAB] for increasing salt content. For the low
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TTAB concentration (mM) Figure 5. Viscosity (flow time) as a function of TTAB concentration for solutions of varying NaHy and NaCl concentrations. Full-drawn curves refer to 0.01% (w/w) NaHy, dotted curves to 0.05% (w/w) NaHy, and dashed curves to 0.1% (w/w) NaHy. Open symbols refer to 200 mM NaCl and filled symbols to 770 mM NaCI. The temperature was 25 OC.
TTAB and salt concentration there is a rapid decrease in viscosity when the TTAB concentration is increased until a flat minimum is reached. Upon further addition of TTAB the viscosity starts to increase. Increasing the salt concentration gradually suppresses the minimum until a continually increasing relationship is obtained. In Figure 5 the same kind of viscosity measurements are shown for different concentrations of the polymer, but now only for the lowest and the highest salt concentration (200 and 770 mM, respectively). The curves follow the same pattern as in Figure 4, but the viscosity changes become more pronounced for increasing polymer concentration. Since the actual system is a four-component system, it is not easy to rigorously isolate the contribution from each of the components to the observed hydrodynamic effects. However, from the results obtained so far, the following explanation may be suggested. Adding TTAB effectively means adding TTAB micelles to the system (because of the lowered cmc when salt and polymer are present). The viscosity decrease for the lowest salt concentrations is believed to reflect an overall reduction of the hydrodynamic volume of the polymer. However, the hydrodynamic data alone do not allow any unambiguous conclusions to be drawn concerning the more detailed molecular mechanism. As already mentioned, the volume reduction could be either a consequence of a polymer chain contraction when TTAB micelles "clusters/adsorbs" on single chains or be due to aggregation when two or more polymer chains share the same micelles. The conformation of the polymersurfactant "complex" would be a compromise between the preferred structure of the polymer and the preferred packing and size of the surfactant micelles, where each of the components will influence the other. When the TTAB concentration is low, there
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J. Phys. Chem. 1992,96, 2348-2355
is probably more than one polymer chain sharing the same micelle, at least if the polymer concentration is not too low. This explanation seems to be in accord with results from light scattering studies (both classical and quasielastic) which will be published soon. The aggregation theory is also supported by the results presented in Figure 5. The relative decrease in flow time from its initial value for [TTAB] = 0 to its minimum value is approximately 0.17 for the two higher polymer concentrations. For the lowest polymer concentration the relative decrease is only of the order of 0.05. This seems to indicate that the decrease in viscosity primarily could be related to some type of polymerpolymer interaction rather than to some intramolecular conformational change. Adding more TTAB will lead to further clustering and therefore reduction of the hydrodynamic volume. At some point increasing the number of micelles does not give any further reduction of the viscosity. Rather, the viscosity passes through a minimum and begins to increase. Intuitively, it seems likely that increasing the ratio of number of micelles to number of polymer molecules per volume unit would lead to dissolution of polymer aggregates and an increase in the overall hydrodynamic volume, as observed (see Figure 4). For higher salt concentrations the tendency for the polymer chains to “anchor” to the micelles would be less pronounced (but not absent) due to the higher amount of the supporting electrolyte. This electrostatic nature of the interaction is demonstrated by the decreased binding as the amount of NaCl
is increased. To deepen the molecular understanding of the phenomena discussed, investigations are in progress by classical as well as dynamic light scattering.
Conclusion The viscosity changes upon addition of l T A B are due to binding of micelles to the polymer. For low TTAB concentrations this binding gives rise to either association products of one polymer chain contracted around one or more micelles or aggregates of more than one polymer sharing the same micelles. The effect of this contraction and/or aggregation is seen as a decrease in viscosity. Most probably there will be more than one polymer chain sharing the same micelles (due to the low TTAB concentration). As the TTAB concentration increases, there will be enough free micelles for the polymer chains to expand or “deaggregate”. This results in a viscosity increase. The electrostatic nature of the interaction is demonstrated by the decreased binding as the amount of NaCl is increased. Acknowledgment. Financial support from Kabi Pharmacia AB, the Swedish National Board For Technical Development, and the Swedish Natural Science Research Council is gratefully acknowledged. We also thank Dr. Per MBnsson, Kabi Pharmacia AB, for helpful discussions. Registry No. TTAB, 1119-97-7; sodium hyaluronate, 9067-32-7; sodium chloride, 7647-14-5.
Ultrasonic Relaxation Studies of Mixed Micelles Formed from Propanol-Decyltrimethylammonium Bromide-Water Emilio Aicart; David J. Jobe, Bohdan Skalski,t and Ronald E. VerraU* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO (Received: June 5, 1991)
Ultrasonic absorption (0.6 < f < 210 MHz) and conductance studies of decyltrimethylammonium bromide (DTAB), water, and propanol (Pr) mixtures have been carried out as a function of surfactant and alcohol concentrations at 25 OC. Two relaxation frequencies (rates) are found for all systems and are assigned to exchange processes involving monomer surfactant Cf,,) and alcohol (f2)species between mixed micellar aggregates and the bulk phase. The lower frequency relaxation is believed to be due to the exchange of monomer surfactant, and the higher frequency relaxation, due to the exchange of the alcohol. The ultrasonic results were analyzed according to the theory of Aniansson to obtain information about the polydispersity of the mixed micelles (al*and u j ) , the rate constants ( k ; , k l , k;, and kj), and the change of volumes of the exchange processes (AV, and AV,). Time resolved fluorescence measurements were made to obtain estimates of the mean aggregation number of the surfactant in the micelles, directly, and of the alcohol, indirectly, from the binding constant of the alcohol to the micelles and the free alcohol in the bulk phase. The exit rates of the DTAB monomer (k;) and alcohol molecule (k;) from a mixed micelle decrease with increasing stoichiometric alcohol concentration. Both the variance in the size distribution (uI2)and the mean aggregation number of the surfactant (ii) decrease as the Pr concentration is increased. These results can be explained by the change in the packing of the micelle with increased partitioning of Pr into the mixed micelle which results in a decrease in the charge density at the micelle surface. Analysis of the relaxation amplitude data gives rather large volumes for the surfactant exchange which appear to be consistent with a two-step process: its transfer from the mixed micelle to the bulk phase followed by its transfer to a propanol-water microphase.
Introduction Ultrasonic relaxation spectroscopy has been shown to be an important kinetic tool in the investigation of many chemical equilibria. Chemical relaxation processes associated with the formation of alcohol mixed and, more recently, complexes of cyclodextrins and surfactant^^^ have all been successfully studied using this technique. Although Present address: Departamento de Quimica-Fisica, Facultad de Ciencias Quimicas, Universidad Complutense, Madrid 28040, Spain. 1Present address:
Poznan, Poland.
Faculty of Chemistry, A. Mickiewicz University, 60-780,
0022-365419212096-2348$03.00/0
some studies of mixed micelles of surfactant-alcohols have been reported using ultrasonic relaxation methods, only a few studies (1) Kato, S.; Nomura, H.; Honda, H.; Zielinski, R.; Ikeda, S.J . Phys. Chem. 1988, 92, 2305. (2) Rassing, J.; Sam, P. J.; Wyn-Jones, E. J . Chem. Soc., Faraday Trans. 2 1974, 70, 1247. ( 3 ) Lang, J. J . Phys. Chem. 1982,86,992. (4) Madigosky, W. M.; Warfield, R. W. J . Chem. Phys. 1987,86, 1491. ( 5 ) Nishikawa, S.;Mashima, M.; Yasunaga, T. Bull. Chem. SOC.Jpn. 1975, 48, 661. ( 6 ) Zana, R.; Michels, B. J . Phys. Chem. 1989, 93, 2643. (7) Nishikawa, S.; Matsuo, F. J . Phys. Chem. 1991, 95, 437.
0 1992 American Chemical Society
