The effect of ion‐site interactions on the apparent molal volume of polystyrenesulfonic acid and its salts in aqueous solutions

Abstract Apparent molal volumes of two samples of polystyrenesulfonic acid and p‐toluenesulfonic acid and their alkaline salts in water have been determined at 25°C using the magnetic float technique. The results show that the monomeric acid and salts behave like ordinary strong electrolytes. In the case of the polystyrenesulfonates, the variation of the apparent molal volume with molar concentration does not follow the simple Masson's equation, i.e., φv decreases rapidly at concentration below 0.005 mole/liter. Furthermore, the additivity rule (applied to the ionic partial molal volumes at infinite dilution) does not hold too well for the strong polyelectrolytes investigated in this work. This anomalous behavior is discussed in terms of persisting ion‐binding at infinite dilution. Theoretical calculations on the variation of degree of ion‐binding with concentration indicate that the number of bound cations at infinite dilution is a function of their size.