Heterogeneity of acetonitrile-water mixtures in the temperature range 279-307Â K studied by small-angle neutron scattering technique

At 279, 287, 298, and 307 K the heterogeneity of acetonitrile-water mixtures in a range of acetonitrile mole fraction xAN from 0.2 to 0.6 has been evaluated in terms of Ornstein-Zernike correlation length, concentration and density fluctuations, and Kirkwood-Buff parameters obtained from small-angle neutron scattering (SANS) experiments. At 298 K the correlation lengths for CH3CN-D2O (D = 2H) and CD3CN-H2O mixtures have shown that the heterogeneity of acetonitrile-water mixtures at xAN = 0.3 and 0.4 is most enhanced in the mole fraction range examined. With decreasing temperature from 307 to 279 K, the correlation length and the fluctuations for CH3CN-D2O mixtures at xAN = 0.3 and 0.4 most significantly rise in the mole fraction range. The Kirkwood-Buff parameters have revealed that both acetonitrile-acetonitrile and water-water interactions in acetonitrile-water mixtures at the same mole fractions remarkably increase with decreasing temperature, whereas acetonitrile-water interactions decrease. According to all the parameters estimated, a change in mixing state of acetonitrile-water mixtures with lowering temperature was discussed.