The concentration fluctuation of acetonitrile-water and acetonitrile-water-MCl (M=Li
+, Na
+, and K
+) mixtures has been investigated by using small-angle neutron scattering (SANS) technique. At 298, the concentration fluctuation of acetonitrile-water mixtures is most enhanced at acetonitrile mole fraction of
xAN=0.3-0.4. When the temperature decreases to 279, the concentration fluctuation of the mixtures at
xAN=0.3-0.4 drastically increases. For acetonitrile-water-MCl mixtures, the concentration fluctuation more easily increases in the order of NaCl>KCl>LiCl. This order agrees with that of salt-induced phase separation of acetonitrile-water mixtures.
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