Phase transitions among the rotator phases of the normal alkanes

Abstract

We present the first calorimetric study of the normal alkanes CH3–(CH2)n−2–CH3 (21≤n≤30) covering the temperature range of the five rotator phases (whose structures were previously identified using x-ray scattering) with sufficient resolution to observe the various rotator to rotator transitions. We find first-order hexagonal–orthorhombic distortion transitions; second-order azimuthal tilt-angle rotation transitions, and two types of second-order tilting transitions, one of which has the higher symmetry phase at low temperature. These transitions appear to be mean field in character, in that they are without significant pretransitional fluctuations. We discuss the calorimetric signatures for the transitions in terms of the order parameters obtained from x-ray scattering data. In addition to the transitions, we find strong temperature variation of the heat capacity in the rotator phases not associated with the transitions.

A new calorimetric technique for phase change materials and its application to alkane-based PCMs

“…In terms of DSC analysis, it is the integral of the heat flux above the transition's baseline. We have compared Dh values from our experiments with those available in the literature for RT42 [47] and C23 [34,44] and obtained always good agreement with these values, both for ASC and DSC.…”

“…The effect is a bit larger here. Earlier ASC measurements on a series of alkanes showed that the Liq-Rot transition hardly supercools, the authors gave an upper limit of 0.03 K [44]. Our value here is a bit higher, but this may be a consequence of samples of different size, geometry and purity used here, as well as a difference in measuring speed.…”

“…For the Rot-Cry transition, the situation is different for odd and even alkanes. While both are susceptible to supercooling, the effect is limited for odd ones, but much larger for the even ones [44], an observation that we already earlier made from ASC data for C24 [46].…”

“…This is not a consequence of a failure of the measurement method, like in DSC, but an indication that a substantial temperature gradient is generated inside the sample. Figure 5 shows that C23 has no less than five transitions between 35 and 50 C. We refer the reader to References [34,[43][44][45] for a detailed description of the different phases of this compound. The exact nature of the phases is not relevant for the discussion here, but we introduce the names of the phases for easy referral to the different transitions.…”

Molecular dynamics insight to phase transition in n-alkanes with carbon nanofillers

“…This further complicates any attempts at theoretical modelling of such solid-solid phase transitions. 33,34 Molecular dynamics facilitates investigations and prediction of thermo-physical attributes through displacement of particles, change in energies and radial distribution function. The present study aims to predict mass, thermal and transport properties to compare and analyze the phase change behaviours of various systems under consideration.…”