THE EFFECTS OF UREA AND N-ALKYLUREAS ON THE STRUCTURE OF WATER

Abstract

The partial molal volumes of urea, N-monomethylurea, N, N-dimethylurea and N, N′-dimethylurea at infinite dilution were determined from density measurements at ten-degree intervals from 15°C at 45°C. The partial molal expansibility (at infinite dilution) _??_° was calculated from the partial molal volume data at various temperatures. The _??_° value of urea was high compared with N-alkylureas, and that of N-alkylureas decreased with increasing the number of the methyl groups in the molecule. The _??_° value of N-alkylureas increased by raising the temperature, but that of urea showed no temperature dependency over the range of temperature used in this paper. The low _??_° values of N-alkylureas were attributed to the ability of the methyl group to form an icelike species in solution. The thermal expansion of the structured species in water is low, like that of ice. The positive temperature dependency of _??_° of N-alkylureas would be expected, if the extent of the structurally perturbed layer of water molecules decreased with an increase in temperature. The high _??_° value of urea was interpreted as being due to the structure-breaking ability of urea. The thermal expansion of the destructured species in water is higher than that of the structured species. The packing densities, defined as the ratio of van der Waals volume to the partial molal volume of a solute species, of urea and N-alkylureas have been calculated and discussed.