Thermodynamic Properties of Strong Electrolyte — Strong Polyelectrolyte Mixtures at 25°C [1]

Abstract

Isopiestic vapor pressure comparison measurements were performed with aqueous mixtures of sodium chloride (B) with sodium polystyrenesulfonate, NaPSS, (C) and with polyvinylbenzyltri-methyl ammonium chloride, PVR4NCI, (C) to determine molal osmotic coefficients, q" for typical strong electrolyte-strong polye\ectrolyte systems. The McKay-Perring method was applied to compute the stoichiometric mean molal activity coefficients,)'B, of NaCI and)"c of NaPSS or PVR4NCI, respectively, in the mixtures as a function of the osmolal concentration, m, and of the osmolal fraction of polyelectrolyte, yc. The activity coefficient of the NaCI was strongly decreased by the addition of polyelectrolyte at constant m, while the activity coefficients of the polyelectrolytes were increased or decreased only slightly by the addition of salt. The mean molal activity coefficients, r' B, of the polyelectrolytes in their binary solutions also were computed relative to an arbitrary concentration of 0.001 equiv.(kg H20 where the activity coefficient was defined as unity. The values of 10gr'B decreased linearly with increasing logm. The measured efo values were employed to test the predictions of the Additivity Rule for mixtures at constant water activity. Small but significant departures from additivity dependent on Yc were found at aII concentration. The variations of-10g)'B and-10g)"B with yc in mixtures of constant osmolality showed that Harned's Rule was not obeyed. The e\ectrolyte-polyelectrolyte interaction coefficients, aB, ac, PB, and PB computed from the variations of 10g)'B and log)" B with yc were much larger than for simple electrolyte-electrolyte mixtures, especiaIly in dilute solutions. The thermodynamic properties of aqueous electrolyte-polyelectrolyte mixtures are quite unusual, and therefore, a subject of interest because apparently they can be described empirically as a superposition of the individual properties of the pure components [2-6]. For example, the osmotic pressure, exerted by a mixture is equal to the sum of the osmotic pressure of the salt-free polyelectrolyte solution, 7r p ' and of the polyelectrolyte-free salt solution, 7r., each at the same concentration as when it was present in the mixture: (A) The activity of the counter-ions, a+, in a mixture likewise appears to be given as the sum of the independent contributions of the counter-ions from the polyelectrolyte, a~, and from the salt, a"t, respective1y: (B) Additivity of thermodynamic properties is not generally shown by aqueous mixtures of simple strong electrolytes, and, while it might be expected for dilute solutions of non-interacting solutes, this behavior is surprising for mixtures of salts with highly charged polyions. Several attempts have been made to supply a theoretical explanation for the Addi-tivity Rule. Katchalsky and Alexandrowicz [7] have demonstrated that (A) and (B) above can be derived thermodynamically from the Gibbs-Duhem equation for a Eric Selegny (ed.) , Polyelectrolytes, 135-155. AII Rights Reserved.