Thermal coefficients and internal pressure of aqueous urea. Concentration,temperature, and isotopy effects

The thermal expansion α, isothermal compressibility β, and internal pressure coefficients of H₂O-(NH₂)₂CO and D₂O-(ND₂)₂CO fy systems at 278 K, 298 K, and 318 K and aquamolality m ≤ 1.5 were calculated. The changes in the isotope differences Δα, Δβ_T, and Δfy at different solute concentrations and temperature are discussed. In contrast to Δα and Δfy, Δβ_T is almost independent of the urea concentration already at 298 K and independent of m at 318 K. The derivative δfy/δT increased in dilute solution, at lower temperatures, and on passing from protium to deuterium system, which corresponded to increased structuring. The isotope difference for the Grüneisen constant at given temperatures and concentrations is shown to be independent of the urea content.