Ion-Solvent Interactions Investigated by Isentropic Compressibility Measurements of Lithium and Sodium Salts in Binary Mixtures of Acetonitrile and Nitromethane at 298.15 K

Ultrasonic velocities (u) and densities (ρ) of lithium perchlorate (LiClO4), sodium perchlorate (NaClO4), sodium tetraphenylborate (NaBPh4), tetrabutylammonium tetraphenylborate (Bu4NBPh4) and tetrabutylammonium perchlorate (Bu4NClO4) were measured in the concentration range 0.001-0.25 mol kg-1 in acetonitrile (AN) and nitromethane (NM) binary mixtures consisting of 0, 20, 40, 60, 80 and 100 mol%NM in AN at 298.15 K. The isentropic compressibility (Ks) and apparent molal isentropic compressibility (Ks,f) values of the above salts in all solvent mixtures have been evaluated from experimental data. Limiting apparent molal isentropic compressibilities (Ko s,φ) for various salts were evaluated and split into the ionic contributions (Ko s,φ)±. The variation of (Ko s,φ)± with solvent compositions shows that Li+ and Na+ ions have very large negative (Ko s,φ)± values indicating strong solvation of both Li+ and Na+ ions in AN + NM mixtures over whole binary solvent composition range. Stronger solvation was further observed in the intermediate compositions of AN + NM mixtures. Li+ ions, however, showed much higher solvation as compared to Na+ ions at all compositions. The ClO4 – ions showed feeble solvation in AN + NM mixtures through some interaction with AN binary mixtures with higher mol% of AN. The positive values of (Ko s,φ)± with large magnitude for Bu4N+ and Ph4B– ions indicate solvophobic type of interaction with the solvent molecules, which is stronger in the intermediate compositions of AN + NM mixtures.