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 been 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 separation takes place in a range of 1,4-dioxane mole fraction, 0.1 < x dio ≤ 0.7. The X-ray radial distribution functions have shown that before phase separation the preferential hydration structures of Na+ and Cl- are enhanced with increasing NaCl concentration and that after phase separation the structures of the organic and aqueous phases are practically similar to those of 1,4-dioxane−water mixtures at the corresponding solvent compositions. The SANS data have been interpreted in terms of the Debye correlation length, L D, as a parameter of concentration fluctuation. The L D values were almost constant at ∼9.4 Å in the range of 0 < x NaCl < ∼0.01, but increased quickly to ∼13 Å at x NaCl = 0.024, which corresponds to 54% of the NaCl concentration required for phase separation. From the present findings, together with the previous results on acetonitrile−water−NaCl mixtures, a possible mechanism for NaCl-induced phase separation of 1,4-dioxane−water mixtures is discussed in terms of hydrogen bonding and dipole−dipole interaction.