Shape Effects in the Partial Molar Volume of Tetraethylphosphonium and Ammonium Iodides in Six Nonaqueous Solvents

Densities of dilute solutions of the electrolytes tetraethylphosphonium iodide and tetraethylammonium iodide in the nonaqueous solvents methanol, ethanol, 1-propanol, 1-butanol, acetonitrile, and 2-propanone were measured at 25°C. Using published values for the Debye-Hückel limiting slopes A V, apparent molar volume data were fitted to the Pitzer equation yielding infinite-dilution partial molar volumes $$\overline V _{\text{i}}^\infty$$ and deviation parameters B V. It is found that the variation in the van der Waals volume of the cationic central atoms is about one half the experimental volume change in 2-propanone solutions, but twice that value in the other five solvents. This finding is interpreted in terms of openness of solute structure and solvent penetration. Parameter B V for each salt is shown to be solvent dependent. An interesting approximate linear variation between A V and B V parameters is suggested by the data. This empirical relationship would entail correlation of short- and long-range interionic interactions in solution.