Large-Angle X-ray Scattering and Small-Angle Neutron Scattering Study on Phase Separation of Acetonitrile−Water Mixtures by Addition of NaCl

Phase separation of acetonitrile−water mixtures by addition of NaCl has been studied on the molecular level by large-angle X-ray scattering (LAXS) and small-angle neutron scattering (SANS) methods. A phase diagram of acetonitrile−water−NaCl mixtures at 298 K has shown that phase separation occurs over a wide range of acetonitrile mole fraction ( x AN ) of ∼0.1 < x AN ≤ ∼0.7, where the microheterogeneity of the mixtures occurs. The radial distribution functions obtained by the LAXS measurements have revealed that before phase separation the amounts of preferentially hydrated Na + and Cl - gradually increase with increasing NaCl concentration and that the number of linear hydrogen bonds among water molecules increases when the concentration of NaCl increases. After phase separation of the acetonitrile−water−NaCl mixtures the structures of the acetonitrile-rich phase are very similar to those of the acetonitrile−water binary mixtures at the corresponding acetonitrile mole fractions. The water-rich phase which contains most of the Na + and Cl - also shows structures similar to those of the acetonitrile−water mixtures at the same solvent compositions, except for the hydration structures of Na + and Cl - . The SANS data have shown a change in size of aggregates formed in acetonitrile−D 2 O and acetonitrile−D 2 O−NaCl mixtures before phase separation. The Debye correlation lengths L D determined have demonstrated that aggregation or microheterogeneity in the acetonitrile−D 2 O mixtures is most enhanced with L D ∼ 20 Å between x AN = 0.3 and 0.4. In the acetonitrile−D 2 O−NaCl mixtures the size of aggregates gradually increases with increasing NaCl concentration and reaches a plateau value L D ∼ 20 Å at x AN = 0.2 at the salt concentration of ∼80% to a value required for phase separation. A possible mechanism for NaCl-induced phase separation is discussed from the present results.