The Phenomenon of Thermally Activated Partial Molar Isobaric Compaction of Urea as a Solute in Ethylene Diamine between 288.15 and 328.15 K at Ambient Pressure

The data on density of dilute solutions of urea (U) in ethylene diamine (EDA) with the molality from ∼0.018 to ∼0.39 mol·(kg solvent)⁻¹ were experimentally obtained between (288.15 and 328.15) K and at ambient pressure. The uncertainty in density measurements did not exceed 0.03 kg m^–3. The standard (partial at infinite dilution) molar volumes, V₂^○, and expansibilities, E_p,2^○, of U as a solute in EDA were computed. The unusual phenomenon of the thermally activated partial molar isobaric compaction of the solute (PMICS) caused by the presence of a negative sign at E_p,2^○ was discovered. The origin of this atypical volumetric effect in both the system under consideration and some other urea-containing amphiprotic media was discussed based on the thermodynamic approach including the Ben-Naim’s concept of the “relative affinity parameter”.