SANS, infrared, and 7Li and 23Na NMR studies on phase separation of alkali halide-acetonitrile-water mixtures by cooling.

Phase separation of alkali halide (MX) (M = Li+, Na+, and K+ and X = Cl– and Br–)–acetonitrile (AN)–water mixtures by cooling has been investigated at the molecular level. The phase diagram obtained for the MX–AN–H2O ternary systems showed that the temperatures of phase separation for the mixtures with MCl are higher than those with MBr. The phase-separation temperatures of the mixtures with MCl and MBr are higher in the sequence of NaX > KX > LiX, although the magnitude of the hydration enthalpies for the alkali metal ions is larger in the sequence of Li+ > Na+ > K+. To elucidate the reasons for the sequence of phase separation on the meso- and microscopic scales, small-angle neutron scattering (SANS), infrared (IR), and 7Li and 23Na NMR measurements have been conducted on MX–AN–water mixtures with lowering temperature. The results of SANS and IR experiments showed that the mechanism of phase separation of the mixtures by cooling is the same among all of the mixtures but did not clearly reveal the reasons for the phase separation sequence. In contrast, the spin–lattice relaxation rates and the chemical shifts of 7Li and 23Na NMR for the mixtures suggested the different solvation structure of Li+ and Na+ in the mixtures. In conclusion, the solvation of acetonitrile molecules for Li+ and the formation of Li+–X– contact ion pairs in the mixtures cause the weakest effect of LiX on phase separation of the mixtures by cooling among the alkali metal ions.