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. Th e 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 th e transition to the rotator forms, for which the thermodynamic mixing prope rties had been determined earlier. The most important outcome of the invest igation is that the complete set of systems is characterised by (i) a unifo rm ratio between excess enthalpy and excess entropy, having the value of 33 5 K, and (ii) a uniform dependence of the excess enthalpy on relative diffe rence between number of carbon atoms in the molecules of the components of the system.