Isotope effects on thermodynamic properties in four binary systems: Water (or heavy water) + dimethylsulfoxide (or N,N-Dimethylformamide) at 25‡C

Excess enthalpy, excess isobaric heat capacity, density, and speed of sound in mixtures of heavy water (D2O) + dimethylsulfoxide (DMSO), and D2O + dimethylformamide (DMF) were measured at 25‡C. The same properties of the mixtures of normal water + DMSO, and H2O + DMF were also measured to estimate isotope effects on the thermodynamic excess functions. Both DMSO and DMF are proton acceptors and thus form hydrogen bonds with water. Large negative excess enthalpies and volumes of mixing and excess isentropic compressibilities show that the hydrogen bonding structures of DMSO and DMF with water are stronger and more compact than those in pure water. The excess heat capacity of DMSO-containing mixtures changes sign from negative to positive with increasing water content. The deviations of the excess properties of D2O systems from those of H2O systems indicate that the hydrogen bonding structure with D2O is stronger and more compact.