Ultrasonic Velocity and Isentropic Compressibility Studies of Monoalkylammonium Salts in Binary Mixtures of Acetonitrile and N,N-Dimethylacetamide at Variable Temperature and Atmospheric Pressure

Abstract

Ultrasonic velocities and densities of alkyl substituted ammonium perchlorates-RNH₃ClO₄, where R is methyl (Me), ethyl (Et)_,n-propyl (Pr)_,n-butyl (Bu)_,n-hexyl (Hx) and n-octyl (Oc), have been measured in the concentration range 0.03–0.28 mol·kg⁻¹ in binary mixtures of acetonitrile (AN) and N,N-dimethylacetamide (DMA) containing 0, 20, 40, 60, 80 and 100 mol% DMA at temperatures 298–328 K in 10 K intervals, using an Anton Paar density and sound velocity meter (DSA 5000 M). The experimental data of ultrasonic velocities and densities have been used for calculating isentropic compressibilities ($K_{{S}}$) and apparent molal isentropic compressibilities ($K_{{{S,}\phi }}$). Limiting apparent molal isentropic compressibilities ($K_{{{S,}\phi }}^{{\text{o}}}$) for all the electrolytes have been evaluated from $K_{{{S,}\phi }}$ against m^1/2 plots and split into contributions of individual ion ($K_{{{S,}\phi , \pm }}^{{\text{o}}}$). The variation of $K_{{{S,}\phi , \pm }}^{{\text{o}}}$ values with solvent composition shows preferential solvation of alkylammonium (${\text{RNH}}_{3}^{ + }$) ions by DMA in the AN rich region and by AN in the DMA rich region. The extent of solvation of the alkylammonium ions, at all studied temperatures, was found to increase with decreasing alkyl chain length; that is ${\text{OcNH}}_{3}^{ + }$ < ${\text{HxNH}}_{3}^{ + }$ < ${\text{BuNH}}_{3}^{ + }$ < ${\text{PrNH}}_{3}^{ + }$ < ${\text{EtNH}}_{3}^{ + }$ < ${\text{MeNH}}_{3}^{ + }$. $K_{{{S,}\phi , \pm }}^{{\text{o}}}$ values for all ${\text{RNH}}_{3}^{ + }$ ions increase with increasing temperature, indicating a decrease in the solvation of the ions with increasing temperature.