Mixing Schemes in a Urea−H2O System: A Differential Approach in Solution Thermodynamics

The excess partial molar enthalpies of urea (UR), H U R E , were experimentally determined in UR−H2O at 25 °C. The H U R E data were determined accurately and in small increments in the mole fraction of UR, x U R , up to x U R ≈ 0.22. Hence it was possible to evaluate one more x U R -derivative graphically without resorting to any fitting function, and the model-free UR−UR enthalpic interaction, H U R−U R E , was calculated. Using previous data for the excess chemical potential, μ U R E , the entropy analogue, S U R−U R E , was also calculated. The x U R -dependences of both H U R−U R E and S U R−U R E indicate that there is a boundary at x U R ≈ 0.09 at which the aggregation nature of urea changes. From the results of our earlier works, we suggest that a few UR molecules aggregate at x U R ≈ 0.09, while the integrity of H2O is retained at least up to x U R ≈ 0.20. Together with the findings from our previous studies, we suggest that in the concentration range x U R < 0.22, UR or its aggregate form hydrogen bonds to the H2O network, reducing the degree of fluctuation characteristic to liquid H2O. However, up to at least x U R = 0.20 the hydrogen bond network remains intact. Above x U R ≈ 0.22, the integrity of H2O is likely be lost. Thus, in discussing the effect of urea on H2O and in relating it to the structure and function of biopolymers in aqueous solutions, the concentration region in question must be specified.