Understanding the Influence of Surface Solvation and Structure on Polymorph Stability: A Combined Mechanochemical and Theoretical Approach

We explore the effect of solvent concentration on the thermodynamic stability of two polymorphs of a 1:1 cocrystal of theophylline and benzamide subjected to ball-mill liquid assisted grinding (LAG) and we investigate how this can be related to surface solvent solvation phenomena. In this system, most stable bulk polymorph form II converts to metastable bulk polymorph form I upon neat grinding (NG), while form I can fully or partially transform into form II under LAG conditions, depending on the amount of solvent used. Careful and strict experimental procedures were designed to achieve polymorph equilibrium under ball-mill LAG conditions for 16 different solvents. This allowed us to determine 16 equilibrium polymorph concentration curves as a function of solvent concentration. Ex-situ powder X-ray diffraction (PXRD) was used to monitor the polymorph concentration and crystallite size. The surface site interactions point (SSIP) description of noncovalent interactions was used in conjunction with the SSIMPLE method for calculating solvation energies to determine which functional groups are more or less exposed on the polymorph crystal surfaces. Our results demonstrate that (i) ball-mill LAG equilibrium curves can be successfully achieved experimentally for a cocrystal system; (ii) the equilibrium curves vary from solvent to solvent in onset values and slopes, thus confirming the generality of the interconversion phenomenon that we interpret here in terms of cooperativity; (iii) the concentration required for a switch in polymorphic outcome is dependent on the nature of the solvent; (iv) the SSIP results indicate that the theophylline Jr-system face is more exposed on the surface of form I while the theophylline N-methyl groups are more exposed in form II; and (v) for some solvents, form II has a significantly smaller crystal size at equilibrium than form I in the investigated solvent concentration range. Therefore, the free energy of the 1:1 cocrystal of theophylline and benzamide polymorphs studied here must be affected by surface solvation under ball-mill LAG conditions.