Solid-Liquid equilibrium of n-alkanes using the Chain Delta Lattice Parameter model

The formation of a solid phase in liquid mixtures with large paraffinic molecules is a phenomenon of interest in the petroleum, pharmaceutical, and biotechnological industries among others. Efforts to model the solid-liquid equilibrium in these systems have been mainly empirical and with different degrees of success. An attempt to describe the equilibrium between the high temperature form of a paraffinic solid solution, commonly known as rotator phase, and the liquid phase is performed. The Chain Delta Lattice Parameter model (CDLP) is developed allowing a successful description of the solid-liquid equilibrium of n-alkanes ranging from n-C₂₀ to n-C₄₀. The model is further modified to achieve a more correct temperature dependence because it severely underestimates the excess enthalpy. It is shown that the ratio of excess enthalpy and entropy for n-alkane solid solutions, as happens for other solid mixtures, is related with the values of the melting temperatures by a function common to the entire homologous series. When applied to systems with a symmetric behavior, this yields a correct description of both the enthalpic and entropic parts of the excess Gibbs free energy with the CDLP model. Further considerations about the pressure dependence of the model are also made showing that the model presented can be applied to pressures as high as 100 bar.