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

Abstract

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 (K_s) and apparent molal isentropic compressibility (K_s,f) values of the above salts in all solvent mixtures have been evaluated from experimental data. Limiting apparent molal isentropic compressibilities (K^o_s,_f) for various salts were evaluated and split into the ionic contributions (K^o_s,_f)_±. The variation of (K^o_s,_f)_± with solvent compositions shows that Li⁺ and Na⁺ ions have very large negative (K^o_s,_f)_± 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 (K^o_s,_f)_± 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.