Alkali chlorides-induced phase separation of acetonitrile-water mixtures studied by small-angle neutron scattering

Alkali chlorides (MCl, M = Li+, Na+, and K+) -induced phase separation of acetonitrile-water mixtures at 298 K has been investigated. Phase diagrams of acetonitrile-H2O-MCl systems have shown that phase separation of the mixtures occurs at lower salt concentration in the order of NaCl < KCl < LiCl. Phase separation of acetonitrile-D2O-MCl systems takes place in the same order as the acetonitrile-H2O-MCl ones, but the D2O systems are separated at lower concentration of each salt than the H2O systems. To clarify phase separation of acetonitrile-water-MCl mixtures at a mesoscopic scale, small-angle neutron scattering (SANS) experiments have been made on acetonitrile-D2O-MCl mixtures before phase separation as a function of salt concentration. The SANS results in terms of the Ornstein-Zernike correlation length have suggested that D2O clusters are more easily evolved with increasing salt concentration in the order of NaCl > KCl > LiCl, which is in agreement with the order of phase separation. Based on the present results, mechanisms of LiCl-, NaCl-, and KCl-induced phase separation of acetonitrile-water mixtures have been discussed.