Uniform thermodynamic description of the orientationally disordered mixed crystals of a group of neopentane derivatives

The experimental systems considered in this paper are isobaric binary plastic-crystalline mixed crystals of neopentane derivatives, and their properties studied are (i) solid-liquid phase behavior and (ii) the thermodynamic mixing properties, actually the excess enthalpy, entropy, and volume. More in particular, 10 systems are considered, nine being face-centered cubic and one body-centered cubic, and their components are derived from neopentane, C(CH3)(4), by substituting one or more -CH3 groups by one or more -CH2OH groups and/or one -NO2 or -NH2 groups. The (mean) number of -CH2OH groups per molecule appears to have a dominating influence with regard to melting temperature and degree of compactness. The values of the excess properties vary from system to system, whereas their quotients are system-independent. For the group of fee systems, the quotient of excess enthalpy and excess entropy (compensation temperature) has the uniform value of 630 K, and the quotient of excess enthalpy and excess volume the value of about 0.5 GPa.