Apparent molar volumes and apparent molar heat capacities of aqueous urea, 1,1-dimethylurea,and N,N′-dimethylurea at temperatures from (278.15 to 348.15) K and at the pressure 0.35 MPa

Apparent molar volumes V_? and apparent molar heat capacities C_p,? were determined for aqueous solutions of urea, 1,1-dimethylurea, and N,N′-dimethylurea. Measurements were made at molalities m = (0.02 to 6.0) mol · kg^?1 for urea, at m = (0.01 to 1.6) mol · kg^?1 for 1,1-dimethylurea, and at m = (0.01 to 8.0) mol · kg^?1 for N,N′-dimethylurea. Experimental temperatures ranged from (278.15 to 318.15) K for both urea and 1,1-dimethylurea, and from (278.15 to 348.15) K for N,N′-dimethylurea. All measurements were conducted at the pressure p = 0.35 MPa. Density measurements obtained with a vibrating-tube densimeter were used to calculate V_? values. Heat capacity measurements obtained with a twin fixed-cell differential temperature-scanning calorimeter were used to calculate C_p,? values. Functions of m and T were fitted to the results and were compared with the literature values. The “structure making/structure breaking” aspects of urea in water are discussed. Comparisons are made between the different urea compounds, and the effects of the methyl-group additions are outlined.