Perfect families of mixed crystals: the "ordered'' crystalline forms of n-alkanes

The experimental systems considered in this paper are isobaric binary mixed crystals of n-alkanes in the crystalline forms O-p, M-dci and M-dep, which, on the temperature scale, precede mixed crystals in the rotator forms. The properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. For 14 binary systems in the range C-17 to C-28, quantitative thermodynamic data have been obtained by studying the transition to the rotator forms, for which the thermodynamic mixing properties had been determined earlier. The most important outcome of the investigation is that the complete set of systems is characterised by (i) a uniform ratio between excess enthalpy and excess entropy, having the value of 335 K, and (ii) a uniform dependence of the excess enthalpy on relative difference between number of carbon atoms in the molecules of the components of the system.