Properties and structure of aqueous urea up to the singular temperature of overcooled water: Isotopy effects

In equations for the partial volume, dilatability, heat capacity, and adiabatic compressibility of aqueous urea, the properties are “fitted” to the singular temperature T _s of overcooled water T _s = 227.15 K (R. J. Speedy, J. Phys. Chem., 91, 3354 (1987)). The equations for the characteristics of D₂O-(ND₂)₂CO and T₂O-(NT₂)₂CO systems were obtained by scaling the temperature to 6 and 9.4 K, respectively. The properties of infinitely dilute solutions did not reveal the sign reversal of the isotope effect. The isoconcentrates of the partial volumes and dilatability form wagging lines in the region of low temperatures. The temperature dependences of the properties have extrema and inflection points, while the isotope effects in solutions of finite concentrations experience sign reversal. The maximum density temperature (MDT) of the solution decreases as the urea concentration increases, and also on passing from D₂O-(ND₂)₂CO to H₂O-(NH₂)₂CO. Thus, for the 8m solution, the MDT is 259.6 K (protium system) and 266.7 K (deuterium system). The difference is almost equal to the shift of the MDT after the H₂O → D₂O transition (7.2 K).