Effects of hydrophilic and hydrophobic hydration in tetramethylbisurea and N,N'-dimethylpropyleneurea solutions

The thermal effects of dissolving tetramethylbisurea in water at 298-318 K and N,N'-dimethylpropyleneurea at 293-313 K have been measured. It was shown that the standard heat of dissolution of tetramethylbisurea at 298 K was 3.58 +/- 0.04 kJ/mol, and that of N,N'-dimethylpropyleneurea was 22.8 +/- 0.01 kJ/mol. The standard heat capacities of urea derivatives at 298 K differed insignificantly: 167 +/- 10 J/(mol x K) and 149 +/- 5 J/(mol x K) for tetramethylbisurea and N,N'-dimethylpropyleneurea, respectively, indicating the moderately hydrophobic character of hydration of these compounds. It was found that, at temperatures close to the temperature of maximum density of water (277 K), the temperature dependence of Gibbs energy for tetramethylbisurea goes through the maximum.