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 similar to0.1 < x(AN) less than or equal to similar to0.7, 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-D2O and acetonitrile-D2O-NaCl mixtures before phase separation. The Debye correlation:lengths L-D determined have demonstrated that aggregation or microheterogeneity in the acetonitrile-D2O mixtures is most enhanced with L-D similar to 20 Angstrom between x(AN) = 0.3 and 0.4. In the acetonitrile-D2O-NaCl mixtures the size of aggregates gradually increases with increasing NaCl concentration and reaches a plateau value L-D similar to 20 Angstrom at x(AN) = 0.2 at the salt concentration of similar to 80% to a value required for phase separation. A possible mechanism for NaCl-induced phase separation is discussed from the present results.