The aim of the present study was to evaluate the thermodynamic properties of in-situ formation of cocrystal formulations by the melt-mixing method. Specifically, the thermodynamic mixing behaviour of carbamazepine-nicotinamide and ibuprofen-nicotinamide cocrystals prepared with the aid of Soluplus® (SOL) were evaluated using thermodynamic lattice-based solution theories. Thermodynamic miscibility of both cocrystals with SOL was predicted by calculating Gibb's free energy based on the Flory-Huggins (FH) interaction parameter (χ), while the activity coefficient of cocrystals estimated with the aid of solid-liquid equilibrium equation and FH lattice theory, showed good thermodynamic miscibility of the components at elevated temperatures used normally during melt-mixing based processes. Complete phase transition diagrams constructed with the aid of DSC measurements and FH solution theory, suggested the existence of two transition zones: (1) a stable cocrystal zone, located at the right-hand-side of the spinodal phase separation curve, where stable cocrystals are prepared and (2) an unstable cocrystal zone, located at the left-hand-side of the spinodal curve up to liquidus, where the matrixforming polymer sets a kinetic barrier to recrystallization and hence, a barrier to the formation of cocrystals. The validity of the suggested thermodynamic phase transition zones was experimentally verified by ATR-FTIR and hot-stage polarized light microscopy.
Keywords: Cocrystals; Flory Huggins theory; Melt mixing; Polymer matrix; Thermodynamics.
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