Mixtures of numerous different n-alkanes: 2. Studies by X-ray diffraction and differential thermal analyses with increasing temperature

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

“…(54)(55)(56)(57)(58)(59)(60)(61)(62)(63)(64)(65)(66) Using the experimental data it is possible to validate the different models, in which the solid phase precipitation is explained by one of the following hypotheses: crystallization of one multi-n-alkane solid solution, (67,68) or successive crystallizations of pure nalkanes. (32,69,70) Our experimental observations, (50)(51)(52)71) those of Craig et al, (12) and those of Flöter et al (60) agree with the first hypothesis, but the difficulty lies in finding an excess thermodynamic function for the representation of the solid behaviour. In this work, we investigate the phase diagram of n-hexacosane (C 26 ) and n-octacosane (C 28 ), in a range of compositions represented by the mean carbon number of paraffinic crude oil (n ≈ 27).…”

“…This solid solution, hereafter denoted β , has an orthorhombic structure, and is identical to the one already observed in many binary and ternary n-alkanes systems, (43)(44)(45)(46)(47)(48)(49)53) or in paraffinic crude oil. (50)(51)(52) .…”

“…The β(Fmmm) phase has already been observed in oddnumbered pure n-alkanes (1,2,16,17,22,72) and in many n-alkanes systems. (43)(44)(45)(46)(47)(48)(49)51) When the characteristic peaks of the β phase disappear completely at T = 321.5 K, the rotator state RI of the β phase is detected by a small shift in the β phase (020) line. At T = 324.2 K, the line (020) of the β-RI phase disappears, and the major line characterizes the α-RII rotator phase.…”

“…This phase is isostructural with those found in real multicomponent mixtures. (12,(50)(51)(52) The structure evolution against temperature shows the presence of a second orthorhombic phase β(Fmmm) that shows a β-RI rotator state, and an α-RII rhombohedral rotator phase. The representation of the phase diagram has been made on the basis of the following assumptions: ideality of the liquid and the high temperature α-RII Rotator phase, and non-ideality of the mixture in the β-RI, and β phases.…”

“…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.…”