Mixing schemes for aqueous dimethyl sulfoxide: Support by X-ray diffraction data

Aqueous dimethyl sulfoxide (DMSO) was studied by X-ray diffraction at room temperature. The results indicated that there are three distinct composition regions. In the DMSO-rich region, DMSO molecules retain the same molecular arrangement as in the pure, state, while H2O does not show any structural feature. These findings are in the complete agreement with the suggestion by our earlier thermodynamic study on aqueous DMSO.((1)) In the H2O-rich region, there is an indication that DMSO molecules exist as small clusters bound mainly by S=O dipole attraction. Hence, hydrophobic CH3 groups point outward from such a cluster and DMSO acts in effect as a hydrophobic solute. This is also consistent with the findings of our earlier thermodynamic study. In the intermediate region, a gradual change in the radial distribution function with composition was observed.