A local solvent structure study on 1,4-dioxane-water binary mixtures by total isotropic Rayleigh light scattering method

Rayleigh light scatterings of 1,4-dioxane(D)-water(W) binary mixtures at 546 run have been measured at 25 degreesC by using a conventional fluorometer. Mean-square concentration fluctuation (N <(Deltax)(2)>) and Kirkwood-Buff parameters (KB integrals) of the mixtures at 1,4-dioxane mole fraction range of 0.0 less than or equal to x(dio) less than or equal to 1.0 have been estimated from the scattering results. The N <(Deltax)(2)> of the mixtures increase only slightly with the increase of 1,4-dioxane at x(dio) less than or equal to 0.13; at x(dio) > 0.13, however, the N <(Deltax)(2)-> increase abruptly with increasing x(dio) and show a maximum of N <(Deltax)(2)> = 3.4 at x(dio) = similar to0.7. For mixtures at x(dio) > 0.7 the N <(Deltax)(2)> decrease monotonously with increasing x(dio). KB integrals of the 1,4-dioxane-water mixtures rho (D)G(DD), rho (W)G(WD), rho (W)G(WW), and rho (D)G(DW) show maxima or minima at x(dio) = 0.3 or 0.7, suggesting the changes in local structure of the mixed solvent at these compositions. From the analyses of the N <(Deltax)(2)> and KB integrals of the 1,4-dioxane-water mixtures, it is concluded that local structure of the mixed solvent changes at three specific compositions, x(dio) = 0.13, 0.3, and 0.7, respectively. The mixtures at x(dio) less than or equal to 0.13 are composed of mainly water associates and water-dioxane complexes DWl (l = 6-8). Small clusters of water-dioxane molecules (D1-3W7-21) through hydrogen bonding are formed in mixtures at 0.13 less than or equal to x(dio) less than or equal to 0.3. The hydrogen bonded molecular clusters coexist with 1,4-dioxane aggregates D-n (n = similar to 10) in the mixtures at x(dio) > 0.3. The local structure of the mixtures at x(dio) > 0.3 evolves gradually to the inherent solvent structure of pure 1,4-dioxane solvent with the diminishing of the water-dioxane clusters. The water-dioxane small clusters disappear in the mixtures at x(dio) > 0.7. (C) 2001 Elsevier Science B.V. All rights reserved.