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

Abstract

Densities for aqueous solutions of tetraalkylammonium bromide (R4NBr) salts, namely, tetramethylammonium bromide (Me4NBr), tetraethylammonium bromide (Et4NBr), tetra-n-propylammonium bromide (n-Pr4NBr) and tetra-n-butylammonium bromide (n-Bu4NBr) 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 (Vϕ) and apparent molar volumes of solute at infinite dilution (Vϕ0) at studied temperatures. The Vϕ0 values increase as the chain length increases and follow the order Me4N+ < Et4N+ < n-Pr4N+ < n-Bu4N+. The deviation parameter (BV) 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 (Eϕ) and apparent molar expansibility of solute at infinite dilution (Eϕ0) were also calculated at 298.15 K. The Eϕ0 values are found to be positive for the studied R4NBr 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.

Keywords

• Tetraalkylammonium bromide salts
• Water-structure making ions
• Apparent molar volumes
• Apparent molar expansibility