Heterogeneity of acetonitrile–water mixtures in the temperature range 279–307 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 x AN 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 CH 3 CN–D 2 O (D = 2 H) and CD 3 CN–H 2 O mixtures have shown that the heterogeneity of acetonitrile–water mixtures at x AN = 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 CH 3 CN–D 2 O mixtures at x AN = 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.