The lattice energy and crystal morphologies of the α and β forms of p-aminobenzoic acid are correlated to experimentally grown crystals by calculating their strength, directionality and chemical state of their bulk (intrinsic) and surface (extrinsic) synthons.

The molecular assembly and subsequent nucleation of para-amino benzoic acid (PABA) from ethanolic solutions is probed using a multi-scale and multi-technique approach. This is applied by examining and interrelating information regarding the molecular, solution-state, cluster, solid-state and surface structures to understand why the alpha form of PABA is crystallised in preference to its low temperature beta form. Calculations suggest that conformational changes within the solute molecule play little or no role in directing the nucleation of either the alpha or beta crystal forms. Combined ab initio and molecular dynamics calculations of the stability of small clusters in solution suggests that the hydrogen-bonded carboxylic acid dimers, present in the alpha structure, are the most stable in solution and play a major role in the self-assembly and polymorphic expression of the alpha form in ethanol in preference to the beta form. These calculations are in good agreement with X-ray small-angle scattering analysis which reveals the presence of PABA clusters in ethanol which are consistent with the size and shape of a carboxylic acid dimer. SAXS studies also reveal the presence of larger cluster structures in a size range 10-40 nm which appear to grow, perhaps reflecting a change in the balance between monomers and dimers within the solution during the nucleation process. The results of crystallisation-kinetics experiments indicate an instantaneous nucleation mechanism where the number of instantaneously nucleated crystallites is calculated to be 1360-660 nuclei per ml and the subsequent growth is found to be only rate limited by diffusion of the growth unit to the crystallite surface. A linear dependence of growth rate with respect to supersaturation is observed for the (0 1 -1) capping face, which is associated with strong π-π stacking interactions. This is consistent with a solid-on-solid mechanism associated with surface roughened growth and concomitant poor lattice-perfection. Conversely, the side (1 0 -1) surface has a growth mechanism consistent with a 2D nucleation birth and spread mechanism. Hence, these mechanisms result in very fast growth along the b-axis and the needle-like morphology that is observed for alpha-PABA.

The crystallization of α-p-aminobenzoic acid (pABA) from mixed solutions in ethanol (EtOH) and nitromethane (NMe) is reported. From solutions with compositions >60 wt % NMe, the known α-polymorph of pABA appears. In contrast, crystals prepared from mixed solvent with <60 wt % NMe reveal the presence of a previously unknown NMe solvate, which crystallizes concomitantly with the α-form. The crystal structure of this new form has been determined and is compared with the previously known structure of the α-polymorph. The crystal structure of the NMe solvate has similar synthonic interactions with respect to α-pABA, in particular, the OH•••O H-bonded dimers and the NH•••O Hbonds between the pABA molecules. However, the π−π stacking interactions between the phenyl ring groups are found to be much more offset and do not form a continuous chain through the structure, as found in α-pABA. The synthonic interactions in the NMe solvate structure are generally weaker than those found in α-pABA, and the lattice energy is calculated to be significantly lower, suggesting the solvate structure is metastable with respect to α-pABA. The impact of NMe on the morphology of α-pABA crystals, together with molecular modelling results suggest that this solvent is able to disrupt the π−π stacking interactions that dominate growth along the needle (b-axis) direction of α-pABA, and are intimately linked to the ultimate formation of the solvate.

The key intermolecular (synthonic) interactions, crystal morphology and surface interfacial stability of the anti-inflammatory drug RS-ibuprofen are examined in relation to its bulk crystal and surface chemistry, and to rationalise its growth behaviour as a function of the crystallisation environment. The OH…O H-bonding dimers between adjacent carboxylic acid groups are calculated to be the strongest bulk (intrinsic) synthons, with other important synthons arising due to interactions between the less-polar phenyl ring and aliphatic chain. Morphological prediction, using the attachment energy model predicts a prismatic facetted shape, in good agreement with the shape of the experimentally grown crystals from the vapour phase. Crystals grown from solution are found to have higher aspect ratios, with t hose prepared in polar protic solvents (EtOH) producing less needle-like crystals, than those prepared in less polar and aprotic solvents (toluene, acetonitrile and ethyl acetate). Though the anisotropy factors of the {011} and {002} forms are relatively similar (39.5% and 43.4% respectively), examination of the surface chemistry reveals that the most important extrinsic (surface-terminated) synthons on the capping {011} surface involve H-bonding interactions, whilst those on the side {002} surfaces mostly involve van der Waal's (vdW) interactions. This suggests that a polar, protic solvent is more likely to bind to the capping {011} surface and inhibit growth of the long axis of the needle, compared to apolar and/or aprotic solvents. A previously unreported re-entrant face is found to appear in the external crystal morphology at higher supersaturations (in the range of = 0.66-0.79), not due to twinning, which is provisionally identified as being consistent with the {112} or {012} form. Analysis of the calculated surface entropy-factors suggest that the capping {011} faces would be expected to be least smooth on the molecular level, with a higher degree of unsaturated extrinsic synthons, in comparison to the