Phase diagram calculations of organic “plastic crystal” binaries: (NH2)(CH3)C(CH2OH)2–(CH3)2C(CH2OH)2 system

Thermodynamic analysis of a binary, 2-amino-2-methyl-1,3-propanediol (AMPL, (NH2)(CH3)C(CH2OH)2)–2,2-dimethyl-1,3-propanediol (NPG, (CH3)2C(CH2OH)2), “plastic crystal” system has been performed and the AMPL–NPG phase diagram has been calculated. The low temperature phases generally have layered or chain structures (α or β phases), and when heated transform to the high temperature plastic crystal (γ′ or γ) phase. The AMPL-rich γ′ phases have a BCC and the NPG-rich γ phases have FCC structure. The low temperature (α and β) and liquid phases are assumed to be ideal and the plastic crystal phases (γ′ and γ) are described using subregular solution models. The Gibbs energies of pure components were determined with the inclusion of heat capacity data. The Gibbs energies of metastable modifications were estimated by assuming ideal solutions of the phases. The excess Gibbs energy parameters were optimized using Thermo-Calc (TCC) software to fit the experimental phase diagram data. The magnitudes of the interaction parameters are very small and thus the solution phases are almost ideal.