Densities of dilute solutions of urea (U) in tert-butylamine (t-BuNH2) with molalities being up to ∼0.023 mol·kg–1 were measured at T = 278.15, 283.15, 288.15, 293.15, 298.15, and 303.15 K and p = 99.6 kPa. All experiments were carried out using a sealed U-tube vibrating densimeter. The total uncertainty in the density measurements did not exceed 0.03 kg·m–3. The standard (apparent at infinite dilution) molar volumes and expansibilities of U as a solute in t-BuNH2 were computed. Similar to infinitely dilute solutions of U in methanol (MeOH) and tert-butanol (t-BuOH), the phenomenon of the so-called “negative partial molar expansibility” or “partial molar isobaric compaction of a solute” was found in the studied binary system. The mixed structural aggregate or “solvation complex” formed by a U molecule in the t-BuNH2 medium becomes increasingly compact as the temperature rises because the solute-to-solvent affinity more and more noticeably prevails over the affinity of the solvent molecules for each other. In doing so, the stronger heterocomponent H-bonding leads to the marked increase in the compactness of solvation complexes formed in U solutions going from t-BuOH to t-BuNH2.
