Phase transitions in the urea/n-nonadecane system by calorimetric techniques

Abstract

A calorimetric study of urea/n-nonadecane, CO(NH(2))(2)/C(19)H(40), and the deuterated derivatives, CO(ND(2))(2)/C(19)D(40) and CO(NH(2))(2)/C(19)D(40), around the structural phase transition temperature is presented. For this purpose differential scanning (DSC), temperature-modulated (AC) and adiabatic calorimetry have been used and the obtained results are compared. Leaving apart the noticeable peak associated with the main phase transition at 158.5, 149.4 and 154 K respectively, small anomalies of the specific heat are found at lower temperatures and their corresponding entropic and enthalpic changes are reported. Heating and cooling experiments show the influence of the temperature rate and the thermal history on the detailed profile of the specific heat traces. The presence of thermal hysteresis and latent heat as a way to characterize the order of the phase transitions is discussed. Finally, a tentative approach to the urea and the alkyl chain contributions to the specific heat and their influence on the phase transition mechanisms is presented.

A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds

“…In §3, we have proposed instead that the ‘transition’ at T c2 actually corresponds to a ‘crossover’ between two competing order parameters belonging to the same symmetry. Now, the observed phase transition at T c1 is clearly first order, given the abrupt changes in lattice parameters (and hence in the spontaneous strain e 1 – e 2 ) and the thermal hysteresis observed at this transition [ 12 , 29 , 57 ], which implies that (and C 1 > 0) in equation (3.5). We also recall that two zone boundary order parameters Q 1 and Q 2 both contribute to the new reflections ( h , k , l′ , m′ ) that appear in the low-temperature phase in positions with h + k odd, including main reflections for the host substructure (with m′ = 0), main reflections for the guest substructure (with l′ = 0) and satellite reflections (with l′ ≠ 0 and m′ ≠ 0).…”

“…For n- nonadecane/urea, the I ↔ II phase transition occurs at T c1 ≈ 157 K (on cooling). However, the ‘classical’ description of this phase transition, as discussed above, was questioned recently [ 28 ], together with the report of another phase transition at a lower temperature T c2 ≈ 140 K (on cooling), corresponding to a weak thermal event in DSC data [ 29 ]. The ‘new phase’ below T c2 was denoted as ‘phase III’ [ 28 ].…”

The true structural periodicities and superspace group descriptions of the prototypical incommensurate composite materials: Alkane/urea inclusion compounds

“…These simple relationships suggest that, for the lowtemperature phases of both n-hexadecane/urea and nnonadecane/urea, the (3 + 2)-dimensional superspace group descriptions are not necessary. In our single-crystal X-ray diffraction study of n-nonadecane/urea, data were recorded for phase II at 147 K (i.e., just below the I↔II transition temperature T 1 = 157 K [59]) and for phase III at 100 K (i.e., well below the II↔III transition temperature T 2 = 140 K [59]). Representative one-dimensional l-scans through reciprocal space, parallel to the c * host and c * guest axes, are shown for phase II in fig.…”

Comment on Couzi et al . (2018): a phenomenological model for structural transitions in incommensurate alkane/urea inclusion compounds

“…These results have been discussed extensively in the literature [3,4,6–9] and were obtained using excellent spatial resolution and temperature calibration, including measurements using cold neutron scattering on triple axis spectrometers [3,4], and on a synchrotron X-ray diffractometer [7]. For n -nonadecane-h 40 /urea-h 4 , T c1 = (158.8 ± 0.1) K and T c2 = (147.0 ± 0.1) K, according to adiabatic measurements [10].…”