Neutron Inelastic Scattering and X-Ray Studies of Aqueous Solutions of Dimethylsulphoxide and Dimethylsulphone

Abstract

Molecular vibrations in solid and liquid dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO2) have been measured by neutron inelastic scattering between 900–8 cm−1. Neutron spectra of the intermolecular frequencies of water molecules in aqueous solutions of perdeuterated DMSO and DMSO2 have also been measured to determine, as a function of concentration and of temperature, the influence of these compounds on the water structure. In addition, the broadening of the incident energy distribution by diffusive motions of H2O molecules has also been investigated. The similarity of intermolecular frequencies in the spectra of both the liquid and solid phases of the pure compounds shows a high degree of dipole association. In dilute aqueous solutions the DMSO and DMSO2 (hydrogenous and perdeuterated) both cause a similar enhancement and sharpening of the intermolecular frequencies of the water structure associated with a small and cooperative perturbation in the orientation of many H2O molecules. This is consistent with the x-ray radial distribution which shows a corresponding small enhancement of the peaks of pure water out to 8 Å. As the concentration of DMSO is raised, the existing water structure is broken down due to the formation of hydrogen-bonded, thermolabile, DMSO–H2O complexes. The angular and temperature dependences of the diffusively broadened incident energy distribution are both in accord with delayed or jump diffusion. At low concentrations, the residence times and the self-diffusion coefficients of H2O molecules in both DMSO and DMSO2 solutions are similar to those for pure water. At higher concentrations of DMSO, the increased formation of the hydrogen-bonded complexes causes a decrease in the self-diffusion coefficient and an increase in the delay or residence time for water molecules. Both neutron spectra and x-ray data indicate that the dipole interactions and hydrogen bonding of DMSO inhibit the formation of hexagonal ice upon freezing; this may account for the cryoprotective properties of DMSO.