Structural accommodation as the physical basis of molecular crystal phase behavior.

Understanding why some molecular crystals form continuous solid solutions whereas others separate into compositionally distinct crystalline phases remains a longstanding challenge. Conventional thermodynamic descriptions reproduce phase equilibria accurately, but fitted interaction parameters provide limited insight into the structural origins of compatibility. Here, we re-examine published liquidus data for eight binary triacylglycerol systems by comparing pure-solid precipitation, ideal-solid-solution, asymmetric Margules, and piecewise descriptions. Compatibility emerges at two levels. Phase topology establishes whether unlike molecules share a crystalline phase: SSS-SOS and SSS-OOO exhibit exclusion into distinct solids, irrespective of liquid-phase ideality. Within shared solid solutions, thermodynamic nonideality characterizes the magnitude and directionality of accommodation. SSS-SEE, SSS-SSE, and SSS-SES span ideal, near-ideal, and weakly asymmetric nonideal accommodation, whereas SOS-POS and POS-POP remain highly accommodating despite substantial asymmetric interaction terms. SOS-POP exhibits limited solid-state accommodation and a broad two-solid miscibility gap; its liquidus is described satisfactorily by an asymmetric Margules model, while structured residuals are consistent with a composition-dependent accommodation response. Boundary mismatch free energy, Δgb, therefore provides a physical interpretation linking phase topology, crystal packing, directional nonideality, and composition-dependent compatibility in polymorphic molecular solids.