Effect of charge on the standard partial molar volumes and heat capacities of organic electrolytes in methanol and water

Abstract

Previously developed additivity schemes for nonelectrolytes have been used to estimate$\bar V_2^{\text{o}}$ and$\bar C_{{\text{p}},2}^{\text{o}}$ for tetraalkyl and tetraphenyl methanes in methanol and water. Corrections have been applied to the thermodynamic values of these model compounds to account for a variation in size of the central atom, and these were used to ascertain the effect of charge on$\bar V_2^{\text{o}}$ and$\bar C_{{\text{p}},2}^{\text{o}}$ of alkyl and phenyl quaternary ions having N, P and B as central atoms. Investigations of R₄NBr, (R=methyl to heptyl) salts show that the charge effect on$\bar V_2^{\text{o}}$ and$\bar C_{{\text{p}},2}^{\text{o}}$ of R₄N⁺ ions is large and relatively independent of ion size suggesting that the solvent molecules penetrate the ions. The ability to estimate$\bar V_2^{\text{o}}$ and$\bar C_{{\text{p}},2}^{\text{o}}$ of the quaternary ions in the bromide salt solutions has made it possible to make ionic assignments with some confidence;$\bar V_2^{\text{o}}$ (Br⁻) has been evaluated as 19.7±2 and 30.2±7 cm³-mol⁻¹ and$\bar C_{{\text{p}},2}^{\text{o}}$ (Br⁻) as −83±7 and −68±30 J-K⁻¹-mol⁻¹ in methanol and water, respectively. The use of organic ions for making ionic assignments of$\bar V_2^{\text{o}}$ and$\bar C_{{\text{p}},2}^{\text{o}}$ is critically examined and comparisons with other assignments are made. The scaled particle theory is employed to divide the heat capacities of electrolytes into cavity and interaction contributions.