Origins of Heat Evolution in Mixing Water and Dimethyl Sulfoxide

Dimethyl sulfoxide (DMSO) is one of small amphiphiles composed of both hydrophobic and polar groups. DMSO and water mix uniformly accompanied by intense heat evolution but molecular reasons of the negative molar excess enthalpy and entropy remain ambiguous since the 1940s. We present an interpretation on the origins, based on our results of IR and NMR measurements and simulations. Our interpretation can shed new light on the roles of water molecules in biological systems.