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 polyvinylbenzyltrimethyl ammonium chloride, PVR₄NCI, (C) to determine molal osmotic coefficients, φ, for typical strong electrolyte-strong polyelectrolyte systems. The McKay-Perring method was applied to compute the stoichiometric mean molal activity coefficients, γ _B, of NaCl and γ′_C of NaPSS or PVR₄NCI, respectively, in the mixtures as a function of the osmolal concentration, m, and of the osmolal fraction of polyelectrolyte, γ _C.

The activity coefficient of the NaCl 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, Γ′_B, of the polyelectrolytes in their binary solutions also were computed relative to an arbitrary concentration of 0.001 equiv./kg H₂O where the activity coefficient was defined as unity. The values of logΓ′B decreased linearly with increasing logm. The measured φ 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 γ _C were found at all concentration. The variations of — logγ _B and —logγ′_B with γ _C in mixtures of constant osmolality showed that Harned’s Rule was not obeyed. The electrolyte-polyelectrolyte interaction coefficients, α _B, α _C, β _B, and β _B computed from the variations of LOGγ _B and logγ′_B with γ _C were much larger than for simple electrolyte-electrolyte mixtures, especially in dilute solutions.