Oct., 1957
COUNTERION BINDING BY POLYELECTROLYTES
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COUNTERION BINDING BY POLYELECTROLYTES. I. EXPLORATORY ELECTROPHORESIS, SOLUBILITY AND VISCOSITY STUDIES OF THE INTERACTION BETWEEN POLYPHOSPHATES AND SEVERAL UNIVALENT CATIONS1 BY ULRICH P. STRAUSS, DANIEL WOODSIDE AND PHILIP WINEMAN School of Chemistry, Rutgere University, New Brunswiclc, N . J . Received February 66, 1967
The nature of the binding of lithium, sodium, potassium and tetramethylammonium ions by polyphosphates, ranging in degree of polymerization from 160 to 27,000 was explored by electrophoresis, salting-out and viscosity studies. With tetramethylammonium bromide as the supporting electrolyte, the electro horetic mobilit is large and changes only little with increasing ionic strength, indicating the absence of site-binding. $owever, both t i e magnitude and relative order of the mobility and intrinsic viscosity values obtained in the presence of lithium, sodium and potassium salts suggest sitebinding of these cations. The polyphos hates can be precipitated by lithium, sodium and potassium bromide, but not by tetramethylammonium salts. Several oiservations concerning the precipitation behavior with potassium ion lead to the suspicion that the precipitated potassium polyphosphate is semi-crystalline; the sodium and lithium polyphosphates separate as gels. The large difference in the lithium and sodium concentrations necessary to cause hase separation is explained by the hypothesis that the degrees of solvent incompatibility of the site-bound LiPOs and %aPOa groups are different. This hypothesis is also supported by the intrinsic viscosity results.
The low activity of counterions which characterizes the osmotic and transport properties of polyelectrolytes has been explained in two ways. The earlier point of view considers the counterions attracted to and associated with the polyion due to the high net charge of the latter.2-6 A more recent theoretical treatment is based on the idea that the counterions are bound a t specific ionic sites of the p~lyion.~.*It is the purpose of this series of papers to investigate the problem of ionpair formation by polyelectrolytes from an experimental viewpoint by comparing the properties of a given polyion species in environments of different counterions. Because the electrophoresis technique has previously proved successful in giving evidence for site-binding between bromide ions and quaternization products of p~lyvinylpyridine,~ we have used it here to explore the binding of Li+, Na+, K + and TMA+ to long-chain polyphosphates. These measurements have been supplemented by exploratory salting-out and viscosity studies in order to examine possible consequences of the binding phenomenon on the solvent affinity and molecular dimensions of the polyelectrolyte. Experimental
pre ared by ion exchange from Graham salts. All samples w i d P , >500 were prepared by ion exchange from potassium Kurrol salts.ll Freeze-drying of the samples prepared by ion exchange did not remove the last few per cent. of solvent. The true phosphate content was determined by acid degradation of the samples and analysis for orthophosphate by potentiometric acid-base titration. Degrees of polymerization were determined by viscometry, using NaBr solutions as solvents. Intrinsic viscosity-molecular weight relationships in these solvents are based on light scattering.'* Electrophoretic mobilities were measured at 0" in a Perkin-Elmer Model 38 Tiselius apparatus by the method previously de~cribed.~Solutions containing from 0.1 to 1.0% of sodium or tetramethylammonium polyphosphate were dialyzed for several days against large volumes of the electrolyte solution against which the boundaries were to be formed. At electrolyte normalities higher than 0.1, ascending and descending boundaries moved a t approximately the same rate. Increasing differences between the boundary velocities, amounting to as much as 30% in a few instances, were observed as the concentration of supporting electrolyte was reduced below 0.1 N . In such cases average values were used. The pH was kept between 6 and 8, and over this range had no effect on the electrophoretic mobility. It made no difference whether NaCl or NaBr was used as the supporting electrolyte.1s Viscosities were measured at 25.0" in a Bingham viscometer" operating a t driving premures ranging from 30 to 200 g./cm.*. Viscometer calibrations were made with sucrose solutions according to the method of Fuoss and Cathers.16
The polymer samples used were sodium and tetramethylammonium polyphosphates ranging in weight-average degree of polymerization, P,, from 160 to 27,000. Sodium polyphosphate (NaPP) samples with P, N a t > K+. (26) J. R. Van Wazer and D. A. Campanella, J. A m . Chem. Sou., 7 2 , 655 (1950).
(27) R. M. Smith and R. A. Alberty, THIB JOURNAL, 60, 180 (1956). (28) This idea was apparently first employed by Teunissen and Bungenberg de Jong [P. H. Teunissen and H. G. Bungenberg de Jong, Kolloid Beihefte, 48, 33 (1938)] to explain the order of binding for the alkali metal ions by colloidal phospnates and carboxylates. According t o this hypothesis, some anions have greater affinities for small cations than has water, and these anions will penetrate through and replace water molecules from the hydration sphere. There is recent evidence that this type of binding is accompanied b y a substantial entropy increase [E. L. King, J. Chem. Ed., 90, 71 (1953)l. An alternate concept involving "localized hydrolysis" which has been used to account for the unusual order of the activity coefficients of the alkali metal salts of weak acids [R. A. Robinson and H. 8. Harned, Chem. Revs., 28, 419 (1941)l would not be expected t o apply to the salts of strong acids, with which we are dealing here. (29) If one admits the possibility that a cation may be bound by two non-adjacent phosphate groups, the greater compactness of the polyphosphate chain in a n environment of sodium ions may be such a factor. (30) Because of the evidence for crystallinity in the potassium precipitate, the precipitation molarity of KBr is not included in this comparison. (31) The fact that the zeta-potentials were determined a t 0' does not affect our argument, because incidental observations have shown that the differences between the precipitation molarities of LiBr and NaBr are in the same order and of the same magnitude a t 0' as a t 25".
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U. P. STRAUSS,D. WOODSIDE AND P. WINEMAN
than it does with lithium ion.32 The polyelectrolyte may be viewed as essentially a copolymer of ionized groups and ion-pairs, whose composition may vary with changes in the environment and whose properties depend on the relative amount and the characteristics of both these functional groups. If, as is the case here, the aqueous electrolyte solution is a non-solvent for the ion-pairs, the polyelectrolyte is kept in solution only by its ionized groups. As the ratio of ionized groups to ion-pairs becomes smaller, a critical ratio will be reached where the polyelectrolyte becomes insoluble. The intrinsic viscosity results in Fig. 2 furnish further evidence for our hypothesis. According to current theories, a 3 the molecular dimensions of a polymer should be strongly influenced by the relative solvent affinities of its functional groups. Accordingly, the large difference between the lithium and sodium intrinsic viscosity curves, which on the basis of the electrophoresis results should be close together, can be explained with the same assumption which was used to explain the large difference between these ions in the salting-out effect. Similarly, the observation that the polyphosphate has a higher intrinsic viscosity in a Li+ than in a K+ environment despite its greater ionization in the latter can be understood if we assume that the (32) Speculation concerning the causes of the different solvent a5nities of the ion-pairs is of interest. One point to be considered is the difference in the solvents. Thus, sodium saltlte usually salt out non-electrolytes more efficiently than do lithium salts [F. A. Long and W. F. McDevit, Chem. Reus., 61, 119 (1952)l. However, while this effect may be a contributing factor, it would not be expected to be large enough to account for the observed difference between our precipitation molarities. An explanation which seemn more likely is that while the interactions between two ion-pairs or between an ion-pair and an ionized phosphate group are quite similar in the sodium and lithium cases, the bound lithium ion has a considerably greater attraction for water molecules than has the bound sodium ion. (38) P. J. Flory. “Principles of Polymer Chemistry,” Cornell University Press. Ithaca, N. Y., 1953, Chap. XIV.
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LiPOa ion-pairs are less incompatible with the solvent than the KPOa ion-pairs. Conclusions.-We have shown that there is strong evidence for the existence of specific interactions between the solvent and ion-pairs of polyphosphates. Several conclusions may be drawn from these findings. First, disregard of such effects may be the cause for many of the reported failures of theoretical tIeatments to account for the observed molecular dimensions and interactions of polyelectrolytes. Second, the frequently employed procedure of using the concentration a t which a counterion salts out a polyelectrolyte as a relative measure of the binding capacity of the polyelectrolyte for the counterion is not valid. Finally, specific solvent interactions of site-bound groups may also be expected to play an important role in classical colloid chemistry and to affect such properties as the stability, gel-forming tendency and coagulation of colloidal particles other than polyelectrolytes. DISCUSSION MAX BENDER.-The question was asked whether, experimentally, in comparing electrophoresis measurements on the polyelectrolytes with diffusion measurements, differences in the apparent degree of ion binding would be obvious and to what degree. U. P. STRAuSs.-The answer to this question depends on whether the diffusion measurements are to be carried out with respect to the counterions or with respect to the polymer species. In the former case, diffusion experiments with radioactive sodium ion in solutions containing polyacrylate ion indicate the same degree of counterion binding as do electrical transference experiments [ J. R. Huisenga, P. R. Grieger and F. T. Wall, J . Am. Chem. SOC.,72,4228 (1950)l I n the latter case, the diffusion constant, unlike the electrophoretic mobility, depends on the molecular dimensions of the polymer chain and would not be an independent measure of the degree of ion binding. I n general, various methods have been used to estimate counterion binding by polyelectrolytes, and with the assumptions usually made -namely of ascribing all deviations from ideality to counterion binding-the degree of binding appears larger than that obtained with electrophoresis.
