NaCl-induced phase separation of 1,4-dioxane-water mixtures studied by large-angle X-ray scattering and small-angle neutron scattering techniques

Salt-induced phase separation of 1,4-dioxane-water mixtures with NaCl has b een investigated from the microscopic to mesoscopic scale by large-angle X- ray scattering (LAXS) and small-angle neutron scattering (SANS) methods. A phase diagram of 1,4-dioxane-water-NaCl mixtures has shown that phase separ ation takes place in a range of 1,4-dioxane mole fraction, 0.1 < chi (dio) less than or equal to 0.7. The X-ray radial distribution functions have sho wn that before phase separation the preferential hydration structures of Na + and Cl- are enhanced with increasing NaCl concentration and that after ph ase separation the structures of the organic and aqueous phases are practic ally similar to those of 1,4-dioxane-water mixtures at the corresponding so lvent compositions. The SANS data have been interpreted in terms of the Deb ye correlation length, LD, as a parameter of concentration fluctuation. The LD values were almost constant at similar to9.4 Angstrom in the range of 0 < chi (NaCl) < similar to0.01, but increased quickly to similar to 13 Angs trom at chi (NaCl) = 0.024, which corresponds to 54% of the NaCl concentrat ion required for phase separation. From the present findings, together with the previous results on acetonitrile-water-NaCl mixtures, a possible mecha nism for NaCl-induced phase separation of 1,4-dioxane-water mixtures is dis cussed in terms of hydrogen bonding and dipole-dipole interaction.