The elusive form II of aspirin has been obtained during co-crystallization experiments with levetiracetam or acetamide, and it has been characterized by IR, DSC, HPLC, and single-crystal X-ray diffraction.

Enthalpy of formation values for 1:1 carbamazepine:saccharin (CBZ:SAC), cyheptamide:saccharin (CYH:SAC), and 10,11-dihydrocarbmazepine:saccharin (HCBZ:SAC) co-crystals and relative enthalpy differences between polymorphs of the co-crystal formers ; CBZ(P2 1 /c, P1), CYH (P2 1 /c, P1), and HCBZ (P2 1 /c, Pbca) ; were calculated from enthalpy of solution measurements. The results showed that the co-crystals were enthalpically stabilized and the more stable polymorphs of the co-formers were CBZ-P2 1 /c, CYH-P2 1 /c, and HCBZ-Pbca. The differences in enthalpies between polymorphs were related to the differences in the packing of the molecules. Likewise, the heats of formation of the co-crystals were related to differences in molecular and crystal structures. The transfer enthalpies for the components were assessed and included in the enthalpy of formation calculation. The free energy of formation of CBZ:SAC calculated from the ternary CBZ-SAC-methanol phase diagram showed that the co-crystal was the stable thermodynamic form, and the kinetics of crystallization followed the thermodynamics of crystallization.

A new polymorph of aripiprazole has been discovered, making it the most polymorphic drug to date with twelve reported anhydrous forms, and a record-breaking ninth solved crystal structure. The new form was induced by seeding with crystals of the active metabolite dehydro-aripiprazole.

Solid solutions of isonicotinamide 2 :[fumaric acid (x) succinic acid (1-x) ] (IN 2 :[FA x SA (1-x) ] 1 ) co-crystals were prepared. The system is anchored by two isostructural bimolecular co-crystals with compositions IN 2 SA 1 and IN 2 FA 1 , respectively. The IN 2 :[FA x SA (1-x) ] 1 solid solutions of the co-crystals were formed using both solid-state grinding and solution crystallization. The solid solutions were found to be distinct from physical mixtures of IN 2 SA 1 and IN 2 FA 1 by analysis using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). DSC analysis of the solid solutions showed a linear interpolation of the melting point between the binary co-crystals whereas mixtures of the two separate co-crystals showed melting of the individual phases. PXRD was consistent with linear volume expansion of the lattice parameters without a concomitant change in the full width at half-maximum for the diffraction peaks for the solid solutions, but peak broadening and bimodality were observed for physical mixtures. Both techniques provided independent evidence that the solid solutions were a single phase and not physical mixtures of the two binary co-crystals.