DMSO hydration redefined: Unraveling the hydrophobic hydration of solutes with a mixed hydrophilic–hydrophobic characteristic

Abstract

Hydrophobic hydration of solutes with a mixed hydrophilic–hydrophobic characteristics is still poorly understood. This is because both experimental and theoretical methods find it difficult to see the ice-like water structure around the nonpolar solute groups, unlike hydrogen bonds with the hydrophilic groups. In order to unravel this problem, we have investigated DMSO hydration by means of infrared spectroscopy and theoretical methods, namely DFT, ONIOM calculations and AIMD simulations, which allowed us to redefine its hydration. In dilute DMSO solutions the clathrate-like water is formed around the DMSO molecule, supported by interactions of water molecules with the methyl hydrogens (the blue-shifted hydrogen bonds). The cage is constructed by water molecules that form hydrogen bonds of the comparable energy and length with the SO group and between water molecules. When the construction of the cage is completed, DMSO molecule partially regains its rotational freedom inside. Strong hydrogen bonds within the frame are masked by the relatively small population of weakened hydrogen bonds of water molecules in the vicinity of the SO group, due to the improper fit to the bulk water of water molecules hydrogen bonded to the oxygen atom of DMSO. We also propose a new explanation of the highly non-ideal mixing behavior of aqueous DMSO solutions at the eutectic point, as the positive excess entropy of the equimolar amounts of molecular complexes distinguished in the system.