A thermodynamic study of the preferential interaction of the polyoxometallate electrolyte Na4SiW12O40· 14H2O with ethers in aqueous solution

Abstract

Free-energy and enthalpy changes associated with the interaction of a sodium silico-tungstate salt (Na₄SiW₁₂O₄₀·14H₂O) with diethylether and tetrahydrofuran in dilute aqueous solutions (< 1 mol kg^–1 ether) have been investigated using precise vapour-pressure and microcalorimetry techniques. The standard partial molar volume of the oxometallate salt in pure water at a temperature of 298.15 K has been determined from density measurements in dilute solutions: V_o= 660.5 cm³ mol^–1. It is shown that the solvation process is enthalpy driven in very dilute salt or ether solutions and entropy driven at higher ether concentrations; the enthalpy/entropy compensation leads to a decreasing standard function of transfer of the electrolyte from water to aqueous ether solutions. The available data are compatible with a two-step solvation process: first ether molecules solvate the oxometallate anion at given polarized sites and secondly, with increasing ether concentration, a favourable hydrophobic type of interaction takes place between the solvated anion and free ether molecules. An analysis of the salting constant of tetrahydrofuran in the aqueous sodium silico-tungstate solution using the scaled-particle theory is also presented.