Volume and compressibility changes in aqueous mixed-salt solutions at 25 °C
Abstract
The excess molar volumes of mixing (ΔmVex) and compression of mixing [Δm(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). ΔmVex and Δm(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 ΔmVex and Δm(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.