Compressibility Studies of Solvation Behaviour of Lithium and Sodium Ions in Nitromethane + Dimethylsulfoxide Binary Mixtures at 298.15 K

Abstract

The isentropic compressibilities (K_s) and apparent molal isentropic compressibilities (K_s,f) of lithium perchlorate (LiClO₄), sodium perchlorate (NaClO₄), sodium tetraphenylborate (NaBPh₄), tetrabutylammonium tetraphenylborate (Bu₄NBPh₄) and tetrabutylammonium perchlorate (Bu₄NClO₄), have been calculated from measured ultrasonic velocity (u) and density (ρ) of these salts in the concentration range 0.001 to 0.60 mol Kg^-1 in the mixed binary solutions of nitromethane (NM) and dimethyl sulfoxide (DMSO) containing 0, 20, 40, 60, 80 and 100 mol% of DMSO in NM at 298.15 K. Limiting values of apparent molal isentropic compressibilities (K°_s,f) for various salts were evaluated and split into the contributions (K°_s,f)_± of the individual ions. Exceptionally large and negative (K°_s,f)_± values were obtained whose variation with solvent composition shows strong solvation of both Li⁺ and Na⁺ in NM + DMSO mixtures at all compositions of the mixed binary solutions. Solvation of Li⁺ and Na⁺ increases as mol% of DMSO in NM increases. Solvation exhibited by Li⁺, however, is much stronger than Na⁺ in all compositions of the mixtures. Solvation exhibited by ClO₄^– in NM + DMSO mixtures is weak which perchlorate ion shows by having some interactions with DMSO in the DMSO-rich compositions of the mixtures. The large and positive values for Bu₄N⁺ and Ph₄B^– are indicative of some special type of interactions, mainly solvophobic with the solvent molecules.