Effect of temperature on the H/D-isotope effects in the enthalpy of hydration of tetramethyl-bis-carbamide

The heats of dissolution of tetramethyl-bis-carbamide (the pharmaceutical Mebicarum) in H₂O and D₂O were measured at 288.15, 298.15, and 318.15 K using a sealed microcalorimeter with an isothermal shell. The error of measurements did not exceed 0.2%. The limiting molar enthalpies of dissolution Δ_sol H _n ^∞ and the H/D-isotope enthalpy effects of hydration δΔ_hydr H _n ^∞(H₂O → D₂O) were determined. Different effects of temperature on the pattern of variation of δΔ _hydr H _n ^∞ were found: when T ≤ 315 K, this value is positive and decreases with T, while for T ≥ 315 K, hydration of tetramethyl-bis-carbamide upon replacement of H₂O by D₂O progressively becomes less endothermic. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 715–717, April, 2006.