Osmotic coefficients were measured by the vapor pressure osmometry technique for dilute aqueous solutions of Me4NBr, Et4NBr, Pr4NBr, Bu4NBr, Me4NCl, and Me4NI at 308.15 K. From the experimental osmotic coefficient data, the water activity, water activity coefficient and vapor pressure values were evaluated. The experimental osmotic coefficient data were fitted to the Pitzer equation from which the values of the mean molal activity coefficients were calculated. The values of the obtained osmotic and mean molal activity coefficient data for the bromides decreased with the size of the cation: Bu4N+ < Pr4N+ < Et4N+ < Me4N+. The results for the tetramethylammoniums show that the osmotic and mean molal activity coefficient varies in the following way: Me4NCl > Me4NBr > Me4NI. However the values of the water activity, vapor pressure depression, and the water activity coefficient data follow the order Me4NBr < Et4NBr < Pr4NBr < Bu4NBr and Me4NCl < Me4NBr < Me4NI, which implies that the smaller ions hydrate more water molecules than the ions with larger size.
