Volume and compressibility changes on mixing solutions of alkali halides/sodium acetate with sodium salt of butyric acid at 298.15 K: Understanding like charge ionic (anion–anion) interactions in water

Abstract

Excess molar volumes of mixing (ΔmVex) and excess adiabatic compression of mixing (Δm(βV)ex) have been calculated by measuring density and sound velocity data for aqueous binary and ternary solution involving sodium acetate-NaBr, sodium acetate-KBr, sodium acetate-KI, sodium butyrate-NaBr, sodium butyrate-KBr, sodium butyrate-KI (in the concentration region 0.0993–1.0929 mol·kg−1 and at ionic strength of 1.0 in ternary solutions) and sodium acetate‑sodium butyrate (in the concentration region 0.1022–1.8480 mol·kg−1 and at ionic strength of 0.5, 1.0 and 2.0 in ternary solutions) at 298.15 K. The density and sound velocity measurements are also reported for the aqueous binary solutions of studied electrolytes at 298.15 K and the data were utilized for the determinations of the apparent molar volume and apparent molar compressibility values at finite concentrations. Friedman's theory has been applied to the data for volume of mixing AX and BX as well for hetero combinations of ions to obtain the coefficients required to assess the contributions to volume changes on mixing due to the structural effects and ion-ion interactions (cation–cation, anion–anion and cation–anion–cation interactions in case of sodium acetate‑sodium butyrate solutions, common cation AX-AY and other heteroionic combinations. Similar calculations were made for adiabatic compressibility data and Δm(βV)ex are obtained. The studies reveal that the ΔmVex and Δm(βV)exvalues for all systems exhibit parabolic behaviours as a function of ‘y’ (fraction of ionic strength of an electrolyte (AX) in the mixture of two electrolytes (AX + BX or other combinations) in water at 298.15 K. The magnitudes of ΔmVex are in the order of CH3(CH2)2COONa + KI ≈ CH3(CH2)2COONa-CH3COONa > CH3(CH2)2COONa + KBr > CH3(CH2)2COONa + NaBr while the magnitudes of Δm(βV)ex are in the order of CH3(CH2)2COONa + KBr > CH3(CH2)2COONa + CH3COONa > CH3(CH2)2COONa + NaBr > CH3(CH2)2COONa + KI at ionic strength of 1.0. Similar trends were obtained for ternary solutions having compounds CH3COONa + NaBr/KBr/KI but having smaller magnitudes of ΔmVex and Δm(βV)ex at ionic strength of 1.0. The observed positive volume changes and negative compressibility changes have been interpreted in terms of hydrophobic interactions of anions, size and polarizability of anions as well the H-bonding with water molecules causing alteration in cosphere water contribution which may be of constructive type cosphere overlap due to anion–anion interactions.