Tetraalkylammonium bromide-water mixtures revisited: Isothermal compressibility and internal pressure variation in limiting concentration range at 298.15 K

Abstract

Abstract The recent literature data of density (ρ) at different temperatures (293.15, 298.15 and 303.15 K) and sound velocity (u) at 298.15 K on aqueous solutions of tetraalkylammonium bromides (R4NBr) in limiting concentration range (0.01–0.15) mol kg−1 have been used to obtain coefficient of thermal expansion (α) and adiabatic compressibility (βs) of solutions at 298.15 K. Using specific heat capacity (Cp) data, the appropriate calculations of isothermal compressibility (βT) and internal pressure (Pi) in solutions have been accomplished. In solutions the Pi and βs were found to increase and decrease with increase in concentration of salt. These have been interpreted in terms of hydrophobic hydration of R4N+ cations in contrast to normal alkali cation hydration. The Tammann-Tait-Gibson (TTG) model was applied to derive the excess volume change for the electrolytes ( V ‾ 2 0 E = V ‾ 2 - V int . ) and intrinsic volume (Vint.) of the studied salts at 298.15 K. The Vint. values are further used to compute ionic radii for tetraalkylammonium cations (R4N+) by subtracting the intrinsic volume of bromide ions. The calculated radii agree satisfactorily with those obtained from model and geometry consideration of cations. The magnitudes of excess volumes are found to be negative and which increase with the extent of methylene group substitution (tetramethyl to tetrabutyl cations) in the salt cations. This is generally known as water structural and caging effect. The analysis of the present work indicates that the thermal expansion coefficient property of the organic cations and water structural alterations (in the form of clathrate like structures), govern the parameters like excess volumes of the ions in contrast to normal electrostrictive alkali cations. The results are in conformity of the Frank and Wen’s model of hydration of cations mainly governed by cation-cation interaction presumably due to the overlapping of extensive hydration structures and not directly caused by columbic hydration.