Structure and dynamics of 1,4-dioxane-water binary solutions studied by X-ray diffraction, mass spectrometry, and NMR relaxation

The structure of clusters formed in 1,4-dioxane-water binary solutions has been investigated at ambient temperature as a function of 1,4-dioxane concentration by X-ray diffraction for the corresponding solutions and by mass spectroscopy for liquid droplets formed in vacuum from the liquid mixtures by an adiabatic expansion method The H-2 spin-lattice relaxation times of D2O and 1,4-dioxane-d(8) molecules in 1,4-dioxane-water binary solutions have also been measured at 30 degrees C over a whole range of I,4-dioxane mole fraction. It has been found from the analysis of X-ray radial distribution functions that the number of hydrogen bonds per water and I,4-dioxane oxygen atom decreases with increasing 1,4-dioxane mole fraction X-dio, accompanied by two inflection points at X-dio = similar to 0.1 and similar to 0.3: at X-dio less than or equal to 0.1 the hydrogen-bonded network of water is predominant in the binary solutions, at X-dio greater than or equal to 0.3 the inherent structure of 1,4-dioxane is mostly observed, water molecules probably involved in the structure by hydrogen bonding, and at 0.15 less than or equal to X-dio less than or equal to 0.2 both structures of water and 1,4-dioxane are ruptured to form small binary clusters of one or two dioxane and several water molecules. The mass spectra have revealed that at X-dio = 0.01 water clusters W-n (W = water, n = 6 - 43) are mostly formed, but with increasing X-dio to 0.4 the water clusters are reduced with evolving 1,4-dioxane clusters DmWn (D = 1,4-dioxane, m = 1 - 3, n = 1 - 16). The H-2 spin-lattice relaxation data of D2O molecules in the mixtures showed that the rotation of water molecules is gradually retarded with increasing X-dio to similar to 0.3, where the rotation is the slowest, and is then gradually accelerated with further increase in X-dio. The corresponding data of 1,4-dioxane-d(8) molecules showed a similar tendency, but the slowest motion observed at X-dio = similar to 0.2. The present microscopic cluster structure and dynamic properties of the mixtures are discussed in connection with the heat of mixing. viscosity, hydrophobic hydration, and clathrate hydrate. (C) 1999 Elsevier Science B.V. All rights reserved.