Thermodynamic studies for molecular interactions of tetraalkylammonium bromide salts in aqueous solutions: A volumetric approach

• V ϕ 0 values are obtained for tetraalkylammonium bromide salts in water. • V ϕ 0 values increase as the chain length increases (Me 4 N + < Et 4 N + < n-Pr 4 N + < n-Bu 4 N + ). • The tetraalkylammonium ions act as water-structure making ions. • The single ion partial molar volume of Br - ion has been calculated. Densities for aqueous solutions of tetraalkylammonium bromide (R 4 NBr) salts, namely, tetramethylammonium bromide (Me 4 NBr), tetraethylammonium bromide (Et 4 NBr), tetra-n-propylammonium bromide (n-Pr 4 NBr) and tetra-n-butylammonium bromide (n-Bu 4 NBr) have been measured at 293.15, 298.15 and 303.15 K. Further, the experimental density data were used for the calculations of apparent molar volumes of solute at finite concentration ( V ϕ ) and apparent molar volumes of solute at infinite dilution ( V ϕ 0 ) at studied temperatures. The V ϕ 0 values increase as the chain length increases and follow the order Me 4 N + < Et 4 N + < n-Pr 4 N + < n-Bu 4 N + . The deviation parameter ( B V ) found to be negative for studied salts in water, indicates that the tetraalkylammonium ions acts as water-structure making ions. The coefficient of thermal expansion (α) of solutions, apparent molar expansibility of solute at finite concentration ( E ϕ ) and apparent molar expansibility of solute at infinite dilution ( E ϕ 0 ) were also calculated at 298.15 K. The E ϕ 0 values are found to be positive for the studied R 4 NBr salts in water at 298.15 K and increases as the chain length increases. The result and analysis lead to the elaboration of hydrophobic interaction effects and concept like cation-cation (pair wise) and cation-anion-cation (triplet) interaction.