Thermophysical properties of aqueous solutions of tetraalkylphosphonium based ionic liquids at different temperatures and atmospheric pressure

Abstract

Densities, speed of sound and viscosities of the aqueous solutions of three tetraalkylphosphonium based ionic liquids namely Tributylmethylphosphonium methyl sulfate (0 to ∼0.28)mol·kg−1, Triisobutylmehtylphosphonium tosylate (0 to ∼0.52)mol·kg−1 and Tetrabutylphosphonium chloride (0 to ∼0.57)mol·kg−1 have been measured at the temperatures (298.15, 303.15, 308.15 and 313.15)K and at atmospheric pressure. The apparent molar volume, limiting apparent molar volume and coefficient of thermal expansion values have been obtained from the experimental density data whereas relative viscosities and viscosity A and B coefficients have been determined from experimental viscosity data. The density values have been combined with speed of sound values to evaluate the isothermal and isentropic compressibilities. The hydration number has also been calculated from both viscosity and compressibility data. The data obtained are used to interpret the ion-ion and ion-solvent interactions in the studied systems and also we tried to explain the effect of temperature and concentration on it. The volumetric data, negative values of BV (the deviation parameter from Debye–Hückel theory) and the large positive values of viscosity B coefficient for the studied ionic liquids indicate strong solute-solute interaction with water structure making effect of ionic liquids. All these are discussed in terms of cation-cation, cation-anion interactions and probable alterations in water structural effects.