Pressure and concentration effects on the molecular reorientation in water‐dioxane mixtures

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Abstract
The pressure and concentration dependences of the deuteron spin‐lattice relaxation times and viscosity have been measured in D2O‐dioxane and H2O‐dioxane‐d8 mixtures up to 4.5 kbar at 30, 40, and 50°C. The experimental data interpreted in terms of the parameter κ introduced in the theory of the rotational relaxation in liquids by Kivelson, Kivelson, and Oppenheim. The results show that the reorientation of the D2O molecules at 1 bar can be described by the Debye equation, i.e., κ is equal to one and independent of the composition of the mixture. On the other hand, the parameter κ for the reorientation of dioxane‐d8 is of the order of 0.1 and dependent on the composition. Both for D2O and dioxane‐d8 the parameters κ decrease with increasing pressure. The higher the dioxane content in the mixture the larger the change in κ with pressure. This indicates that increasing pressure diminishes the coupling between the reorientational and translational motions in this binary system. Phase transitions were observed under high pressure for the solutions of moderate and high dioxane concentrations. A brief discussion of the pressure, temperature, and concentration behavior of the reorientational correlation times in the solid phase is also given.