Volume and compressibility changes in aqueous mixed-salt solutions at 25 °C

Abstract

The excess molar volumes of mixing (Δ_mV^ex) and compression of mixing [Δ_m(βV)^ex] have been calculated from the measured density and sound velocity parameters for aqueous binary and ternary solutions of NaCl–KCl, LiBr–KBr, RbBr–KBr, CsBr–KBr, Me₄NBr–KBr, Et₄NBr–Me₄NBr and Bu₄NBr–Me₄NBr at 25 °C (in the concentration range 0.1–1.0 mol kg^–1 and at constant ionic strength of I≈ 0.2, 0.5 and 1.0 mol kg^–1 in ternary solutions). Δ_mV^ex and Δ_m(βV)^ex exhibit parabolic behaviour as a function of y[the fraction of ionic strength due to an electrolyte (AX) in a mixture of two electrolytes (AX + BX)]. Application of the Friedman equation to the Δ_mV^ex and Δ_m(βV)^ex values indicated the presence of cation–cation interactions and specificity of the cosphere overlapping effects, which in turn depend upon the nature of the ion pairs, the water structural effects and hydrophobic interactions in solutions. The results are discussed in terms of the combination of water structure-breaking and structure-making properties of ions. The compressional results show that in alkali-metal halide mixtures, electrostatic interactions affect more water molecules in mutual volumes derived from the ionic cosphere overlapping or interact over greater distances than tetra-alkylammonium salts.