Molecular mixed crystals of neopentane derivatives. A comparative analysis of three binary systems showing crossed isodimorphism

A thermodynamic analysis is presented for three binary systems out of the four neopentane derivatives: neopentylglycol (NPG), pentaglycerin (PG), pentaerythritol (PE) and tris(hydroxymethyl) aminomethane (TRIS). These substances give rise to a plastic crystalline state, which is bcc for TRIS and fcc for NPG, PG and PE. In each of the three systems considered TRIS is one of the components: they are examples of crossed isodimorphism and the estimation of metastable melting points is an essential part of the analysis. Unlike the plastic-crystalline state, miscibility in the ordered solid state is poor, with the exception of the PE side in the system PE/TRIS. analysis, the liquid mixtures were taken as ideal and for the plastic crystalline solutions the excess Gibbs energy was expressed in terms of the (A,B,⊖) model.