Quantitative interpretation of isopiestic measurements on aqueous solutions: Urea revisited

• Published in: Biophysical chemistry Vol. 251; p. 106175
• Main Authors: Winzor, Donald J., Wills, Peter R.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01-08-2019
• Subjects: Enthalpy-entropy compensation; Isopiestic measurements; Solvent interactions; Thermodynamic nonideality; Urea dimerization; Urea dimerization; Isopiestic measurements; Thermodynamic nonideality; Enthalpy-entropy compensation; Solvent interactions
• ISSN: 0301-4622; 1873-4200
• DOI: 10.1016/j.bpc.2019.106175

This investigation amends the analysis of isopiestic measurements of solvent thermodynamic activity by taking into account the fact that the solvent activity, traditionally expressed in mole-fraction terms, is a molal parameter because of the constraints (constant temperature and pressure) under which the measurements are made. Application of the revised procedure to published isopiestic measurements on aqueous urea solutions at 25 °C yields a dimerization constant of 0.066 molal−1, which is two-fold larger than an earlier published estimate based on an incorrect definition of the solute activity coefficient. Despite amendments to the quantitative detail, the present study confirms the existence of a large negative entropic contribution that largely counters its enthalpic counterpart arising from the hydrogen bonding responsible for dimer formation. This evidence of enthalpy-entropy compensation is entirely consistent with quantum-mechanical predictions of the adverse effect of water on urea dimerization. Changes in water structure thus contribute significantly to the energetics of urea dimerization in aqueous solution.
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•Expressions for rigorous thermodynamic analysis of isopiestic measurements•Upward revision of the dimerization constant to 0.066 molal–1 for urea in aqueous solution at 25 °C•Identification of the error in an earlier estimate of 0.033 molal–1 from the same isopiestic data•Over-compensation by the standard entropy term that leads to a positive standard free energy change despite a sizeable negative enthalpic contribution•Correlation of the large negative entropic contribution to the energetics of urea dimerization with a quantum-mechanics-based chemical structure showing the adverse effects of changes in water structure on dimer formation.