title: Mixing schemes in a urea-H2O system: a differential approach in solution thermodynamics.
abstract: The excess partial molar enthalpies of urea (UR), H U R (E ), were experimentally determined in UR-H 2O at 25 degrees 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 approximately 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, mu 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 approximately 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 approximately 0.09, while the integrity of H 2O is retained at least up to x U R approximately 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 H 2O network, reducing the degree of fluctuation characteristic to liquid H 2O. However, up to at least x U R = 0.20 the hydrogen bond network remains intact. Above x U R approximately 0.22, the integrity of H 2O is likely be lost. Thus, in discussing the effect of urea on H 2O and in relating it to the structure and function of biopolymers in aqueous solutions, the concentration region in question must be specified.
authors.affiliation: Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan. koga@chem.ubc.ca
authors.forename: Yoshikata
authors.initials: Y
authors.lastname: Koga
authors.forename: Yuji
authors.initials: Y
authors.lastname: Miyazaki
authors.forename: Yatsuhisa
authors.initials: Y
authors.lastname: Nagano
authors.forename: Akira
authors.initials: A
authors.lastname: Inaba