Ultrasonic Velocity and Compressibility Studies of Tetraalkylammonium Salts in Acetonitrile + Methanol Binary Mixtures at 298.15 K

Abstract

Abstract The ultrasonic velocities (u) and densities (ρ) of tetraalkylammonium perchlorates (R4NClO4, where R = Methyl, Ethyl, Propyl or Butyl) and tetrabutylammonium tetraphenylborate (Bu4NPh4B) solutions have been measured at different salt concentrations in the range (30–500) × 10−4 mol kg –1 in binary mixtures of acetonitrile (AN) with methanol (MeOH) containing 0, 20, 40, 60, 80 and 100 mol % methanol at 298.15 K. The isentropic compressibilities (K s ) and apparent molal isentropic compressibilities (K s,φ ) of different salts in the solvent mixtures have been calculated. Limiting apparent molal isentropic compressibilities (K o s,φ ) for various salts have been evaluated and split into contributions of individual ions i.e., into (K o s,φ )± values. The variation of (K o s,φ )± values with solvent composition shows that Bu4N+ and Ph4B+ have very large positive (K o s,φ )± values which show strong solvophobic interactions in AN and in AN + MeOH mixtures over the entire solvent composition range. These positive values indicate some special type of interactions with the solvent molecules, which increase with the increase of MeOH composition. ClO4 – shows much weaker solvation in AN + MeOH mixtures by having some interaction with AN in the AN rich region of the mixtures. The negative values of Me4N+ and Et4N+ ions show strong structural effects which arise due to solute-solvent interactions in MeOH rich region of mixtures.