Apparent molal volumes and heat capacities of urea and methylsubstituted ureas in H2O and D2O at 25 degrees C

The apparent molal volumes and heat capacities of urea, 1,1- and 1,3-dimethylurea, and tetramethylurea were measured in H 2 O and D 2 O at 25 °C. From these data, urea–water interactions seem to cause an overall structure-breaking effect and the substituted ureas, an overall structure-making effect. The effect of the hydrogen-bonding interactions to the volume and heat capacity seems to be small compared with the intrinsic and hydrophobic contributions of a methylene group, as reflected by the isotope effect. Furthermore, transfer values seem to show a significant specificity to the degree and position of methyl substitution.