Pressure Effect on Thermodynamic Quantities for the Solid-Liquid Phase Transition in n-tridecane, n-hexadecane and n-octadecane

The pressure effect is investigated regarding the solid – liquid equilibria (SLE) in n-alkanes. Using the Landau phenomenological model, the pressure dependences of the thermodynamic functions are predicted and the phase diagrams are constructed for the solid – liquid transitions in the binary mixtures of n-alkanes. The experimental data from the literature are used for the phase diagrams in the mixtures. Our fits for the phase diagrams are reasonably good. Regarding the cubic dependence of the concentration (T-X, P-X) and the linear dependence of the pressure (P-T) on the temperature, our results show that the n-tridecane is distinguished from the other mixtures due to its lowest freezing temperature (T_1=291.08 K) and correspondingly higher concentration (x_1=0.1982). It is found that the divergence behaviour of the heat capacity (C) with the critical exponent 1⁄2 from the extended mean field model is in particular more apparent at the room temperature (293.15 K) at various pressures for the solid – liquid transition. This is accompanied with the pressure dependences of the order parameter, susceptibility, entropy and enthalpy for those mixtures as studied here.