Large-angle X-ray scattering, small-angle neutron scattering, and NMR relaxation studies on mixing states of 1,4-dioxane-water, 1,3-dioxane-water, and tetrahydrofuran-water mixtures

Aqueous mixtures. of cycloethers, 1,4-dioxane (pdio), 1,3-dioxane (mdio), and tetrahydrofuran (THF) have been investigated by using large-angle X-ray scattering (LAXS), small-angle neutron scattering (SANS), and NMR relaxation techniques. The results from the LAXS experiments have revealed that at a microscopic level the structures of the cycloether-water mixtures change with cycloether mole fraction x(C) (C = pdio, mdio and THF) in a similar way: in the range of x(C) < 0.1 the hydrogen-bonded network of water predominates in the mixtures, while in the range of x(C) greater than or equal to 0.3 an ordered structure observed for pure cycloethers is mainly formed in the mixtures. In a narrow range of 0.1 less than or equal to x(C) less than or equal to 0.2, both structures of pure water and pure cycloethers disappear in the mixtures. On the other hand, the SANS results have shown that concentration fluctuations in the cycloether-water mixtures at a mesoscopic level, i.e. the microheterogeneities, where discrete water and cycloether aggregates coexist, at x(C) approximate to 0.3 become enhanced in a sequence THF-water > -1,3-dioxane-water much greater than 1,4-dioxane-water mixtures. The NMR relaxation data have revealed that the rotational motion of water molecules is most restricted at x(C) approximate to 0.3 suggesting that the hydrogen bonds among water molecules are most strengthened at this mole fraction. On the basis of the static structures from the microscopic to mesoscopic levels and the dynamic property of water molecules, the effects of the size of cycloether molecules and the position of oxygen atoms in the molecules on the mixing states of the mixtures are discussed.