Modeling and predictions of solid–liquid equilibria for citalopram oxalate as a representative of a solid solution forming system

A thermodynamic framework has been suggested to describe solid liquid equilibria (SLE) of enantiomeric mixtures involving solid solution in the crystalline phase. As a representative of solid solution forming systems the pharmaceutically active compound, citalopram oxalate has been selected. Differential scanning calorimetry (DSC) and solubility measurements have been used to quantify the solid–liquid equilibria (SLE) of enantiomeric mixtures of R- and S-citalopram oxalate. The melt phase diagram obtained by means of DSC experiments has been exploited to determine activities of enantiomers in the solid phase while solubility data of the single S enantiomer have been used to calculate activities in the liquid phase, i.e., in a solvent, which comprised mixture of methanol–water 1:1 (v/v). The activity coefficients and their concentration and temperature dependence in both phases have been quantified by fitting parameters of the NRTL model to the obtained equilibrium data. Finally, the SLE model has been used to simulate the phase behavior under different process conditions.