Dielectric Relaxation in Dimethyl Sulfoxide/Water Mixtures

Dielectric spectra of dimethyl sulfoxide (DMSO)/water mixtures, over the entire concentration range, were measured using the transmission line method at frequencies from 45 MHz to 26 GHz and at temperatures of 298-318 K. The relaxation times of the mixtures showed a maximum at an intermediate molar fraction of DMSO. A water structure "breaking effect", manifested by a reduced average number of hydrogen bonds per water molecule, was observed in dilute aqueous solutions based on the dielectric relaxation dynamics analysis. The increase in the dielectric relaxation time in DMSO/water mixtures was attributed to the spatial (steric) constraints of DMSO molecules on the hydrogen bond network, rather than being due to hydrophobic hydration of the methyl groups. The interactions between water and DMSO molecules in DMSO-rich mixtures are also discussed.