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

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 degrees C. The diffusion of the dilute electrolytes in the water + dioxane (50 mass%) mixed solvent is accurately described by the pseudo-binary equation J(2) = -D(22)del C-2. 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 J(1) = -D(11)del C-1 - D(12)del C-2. Vapour pressure data are used to calculate the changes in the dioxane chemical potential (mu(1)) with added electrolyte, and hence the thermodynamic driving force for the coupled flux of dioxane produced by the electrolyte gradient. Values of partial derivative mu(1)/partial derivative C-2 range from -0.90 kJ dm(3) mol(-2) for sodium tetraphenylboron (dioxane "salted in") to +1.78 kJ dm(3) mol(-2) for NaOH (dioxane "salted out"), in close correlation with D-12 values ranging from -0.82 x 10(-5) for NaPh4B (dioxane diffuses toward sodium tetraphenylboron) to +1.56 x 10(-5) cm(2) 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.