Variation d'enthalpie subie de 260 K à 340 K par les n-paraffines, comprises entre l'octadécane (n-C₁₈) et l'hexacosane (n-C₂₆)

Temperatures and enthalpies of (solid + solid) and (solid + liquid) transitions of n-alkanes

“…(26,73) The o-d transition corresponds to an equilibrium between the ordered phase at low temperatures and the following disordered phase: (i) orthorhombic β(Fmmm) (15,17,37,39,40,49) for the odd-numbered C 9 to C 25 : (7) in this structure, the stacking mode of the molecular layers along the long crystalline axis c corresponds to the orthorhombic (or hexagonal) ABAB sequence of crystalline planes (00 ), but the molecules orientate themselves in a disorderly manner around their long axis according to the two positions defined in the ordered structures, so that all occupancies by the molecules become equivalent in the A and B crystallographic planes by this "disorder in the molecular orientation" and lead to the centered-face space group denoted Fmmm in the Hermann-Mauguin symbols; (80) (ii) rhombohedral α-RII (R3m) for the even-numbered C 22 to C 26 : (15,17,37,39,40,49) in this structure which has the rhombohedral space group denoted R3m in the Hermann-Mauguin notation, (80) the stacking mode of the molecular layers (00 ) corresponds to the ABC rhombohedral sequence in which the molecules are probably in rotation around their axis. (49,77) In addition to this movement, defects appear in the molecular chains that lead to non-planar conformers which can be observed by infrared and Raman spectrometries.…”

“…Above the o-d transition temperature, the normal alkanes can undergo other transitions of disorder-disorder type: (15,17,37,39,40,49) (i) β(Fmmm) ⇒ β-RI(Fmmm): the o-d transition is characterized by X-ray diffraction as follows: disappearance of all the diffraction peaks corresponding to crystalline planes (hk ) whose Miller indices h, k, do not have the same parity and which thus lead to the face-centered space group Fmmm in the Hermann-Mauguin notation, (80) an important shift in the diffraction (0 2 0) towards small Bragg angles that corresponds to an increase in the crystallographic parameter b along the crystallographic axis O y and in the unit-cell base area (O x, O y) in relation to the orientation disorder appearance of the molecules around their axes.…”

Phase diagram ofn-hexacosane andn-octacosane: experimental determination and calculation

“…According to Clydesdale and Roberts (13) and Barbillon, (42) it is triclinic. For Heyding et al (2) and Barbezat-Debreuil, (15) it is triclinic and monoclinic.…”

“…In this paper, n-alkanes C n H 2n+2 are denoted by C n where n is the number of carbon atoms in the chain. In our laboratory, we are currently engaged in thermodynamic and structural studies of the liquid and solid phases of pure n-alkanes, (42) the (solid + liquid), and (solid + solid) equilibria in binary, (43)(44)(45)(46)(47)(48) ternary, (49) and multicomponent mixtures (50)(51)(52) of n-alkanes. Using the results already obtained, Dirand et al (53) highlighted a general behaviour of the thermodynamic and structural properties of consecutive binary n-alkanes (19 < n < 27), and deduced the following conclusion: existence of orthorhombic intermediate solid solutions, phenomenological laws for the succession of these intermediate phases when the composition varies, evolution of an intermediate solid solution with increase in temperature, the appearance of another orthorhombic phase, and rotator phases.…”

Nanolink-based thermal devices : integration of ALD tin thin films

“…(5.6)). Between the solid and the liquid phase, tricosane has an extra crystalline phase, so two phase transitions are considered [127,128]: crystalline 1 → crystalline 2 (at 40.5 ºC) and crystalline 2 → liquid (at 47.7 ºC), with the phase transition enthalpies of 21.500 kJ/mol and 52.250 kJ/mol, respectively. Phase change 1 and 2 require energies of 13 and 32 mJ, as calculated for the given amount of tricosane.…”