Heat capacities of urea, N-methylurea, N-ethylurea, N-(n)propylurea, and N-(n)butylurea in the range 200 to 360K

The heat capacities at constant pressure of urea, N-methylurea, N-ethylurea, N-(n)propylurea and N-(n)butylurea were measured by differential scanning calorimetry over the range 200-360 K, in gas-flow conditions. A He + Ne mixture and N-2 only were selected as the purge gases below and above room temperature, respectively. Benzoic acid was used as the reference material and the accuracy of the measurements was estimated to be within 2% across the temperature range. The heat capacities at 298.15 K were obtained by interpolating the smoothed fitting equations. No anomalies were detected for urea or N-methylurea. One fully reversible solid-solid transition was observed for N-ethylurea at T = (294.5 +/- 2.0) K, with Delta(trs)H(m) = (1.0 +/- 0.3) kJ mol(-1) and for N-(n)propylurea at T = (289.6 +/- 1.0) K, with Delta(trs)H(m) = (3.0 +/- 0.2) kJ mol(-1). Two solid-solid transitions were found for N-(n)butylurea at T = (315.0 +/- 2.0) K, with Delta(trs)H(m) (7.0 +/- 2.0) kJ mol(-1) and at T = (346.0 +/- 1.2) K, with Delta(trs)H(m) = (1.0 +/- 0.1) kJ mol(-1), the first one being recoverable at temperatures lower than 240 K only.