Heat of dilution of monomethylurea in aqueous solutions at 25°C

The enthalpies of dilution of monomethylurea were determined by a flow microcalorimeter at 25°C. The data were combined with osmotic data to give the thermodynamic excess functions. The behavior of monomethylurea is compared to that of urea and other alkylureas, alkylamides, etc. in water. Enthalpy predominates at low concentrations, and entropy at higher concentrations. Hence monomethylurea falls within the group of substances whose behavior, according to Rowlinson, is "not peculiar in water", for m<1. On the other hand, it must be considered a "peculiar aqueous" solute, for m>1. An analysis of the sign of the virial coefficients of the excess thermodynamic functions shows that the properties of MMU in solution are particular. These properties are briefly discussed on the basis of models proposed by Friedman and Krishnan for the aqueous solutions of predominantly hydrophobic solutes and by Frank and Franks for aqueous solutions of urea. © 1979 Plenum Publishing Corporation.