Salting out and coupled diffusion in aqueous dioxane and aqueous propan-2-ol mixed solvents

Abstract

Taylor dispersion and differential refractometry are used to measure ternary interdiffusion coefficients (D_ik) for 13 different water + dioxane(1) + electrolyte(2) systems at 25 °C. The diffusion of the dilute electrolytes in the water + dioxane (50 mass%) mixed solvent is accurately described by the pseudo-binary equation J₂ = −D₂₂∇C₂. Because the gradient in the concentration of each electrolyte drives a measurable coupled flow of dioxane, the dioxane flux is described by the ternary equation J₁ = −D₁₁∇C₁ − D₁₂∇C₂. Vapour pressure data are used to calculate the changes in the dioxane chemical potential (µ₁) with added electrolyte, and hence the thermodynamic driving force for the coupled flux of dioxane produced by the electrolyte gradient. Values of ∂µ₁/∂C₂ range from −0.90 kJ d mol⁻² for sodium tetraphenylboron (dioxane “salted in”) to +1.78 kJ d mol⁻² for NaOH (dioxane “salted out”), in close correlation with D₁₂ values ranging from −0.82 × 10⁻⁵ for NaPh₄B (dioxane diffuses toward sodium tetraphenylboron) to +1.56 × 10⁻⁵ cm² s⁻¹ for NaOH (dioxane diffuses away from NaOH). For comparison with these results, ternary D_ik coefficients are reported for the electrolytes in water + propan-2-ol (50.00 mass%) mixtures.