Calculation of the phase diagram of n-alkanes (CnH2n+2) by the Landau mean field theory

Abstract

We study the temperature T and concentration n (number of carbon atoms) dependence on the transitions of the liquid, rotator (RI, RII,RIII,RIVand RV) and crystal phases in n-alkanes (CnH2n+2). The Landau phenomenological model is used by expanding the free energy in terms of the lattice distortion parameter (D) and the tilt angle (θ) of the rotator (and crystal) phases with their quadratic coupling (D2θ2).The phase line equations are obtained with some approximations by considering the first order transitions between the phases in n-alkanes. By assuming the temperature and concentration dependence of some parameters in the Landau mean field expansion, the T-n phase diagram is established by using the observed data from the literature.Our results are satisfactory to explain the first order transition between the phases, mainly for the rotator and crystal phases in n-alkanes (CnH2n+2). The T-n phase diagrams can also be obtained for some other compounds of similar structures by the Landau phenomenological model as given in this study.