The term “co-crystal” is failing as a clear and consistent scientific descriptor. If it is to be retained, it should be used only as a synonym for “multi-component molecular crystal”.

Nanoindentation is applied to the two polymorphs of aspirin to examine and differentiate their interaction anisotropy and shear instability. Aspirin provides an excellent test system for the technique because: (i) polymorphs I and II exhibit structural similarity in two dimensions, thereby facilitating clear examination of the differences in mechanical response in relation to well-defined differences between the two crystal structures; (ii) single crystals of the metastable polymorph II have only recently become accessible; (iii) shear instability has been proposed for II. Different elastic moduli and hardness values determined for the two polymorphs are correlated with their crystal structures, and the interpretation is supported by measured thermal expansion coefficients. The stress-induced transformation of the metastable polymorph II to the stable polymorph I can be brought about rapidly by mechanical milling, and proceeds via a slip mechanism. This work establishes that nanoindentation provides “signature” responses for the two aspirin polymorphs, despite their very similar crystal structures. It also demonstrates the value of the technique to quantify stability relationships and phase transformations in molecular crystals, enabling a deeper understanding of polymorphism in the context of crystal engineering.

The crystal structures of the complete series of n-alkyl carboxylic acids from hexanoic to pentadecanoic acid have been determined following in situ crystallisation. The structures reveal that the melting point alternation across the series is correlated with alternating crystal density. All even acids crystallise from the liquid as the C modification, while the odd acids exhibit a transition from the C′ to the C″ modification between undecanoic and tridecanoic acid. In each structure, molecules form hydrogen-bonded dimers arranged into bilayers, with a rectangular packing arrangement in the plane perpendicular to the dimer long axes. The packing density within bilayers is comparable in each case and the alternating crystal density can be attributed solely to alternating packing density between bilayers. The carboxyl groups are identically disposed in all structures, but adjacent dimers are offset to differing degrees parallel to their long axes to reduce in-plane O⋯O repulsions, giving rise to angles between n-alkyl chains. The extent of the lateral offset diminishes in general across the series as the n-alkyl chains exert an increasing driving force towards parallel alignment. The systematic alternation in the packing density between bilayers arises as a result of different orientations adopted by the terminal C–C bonds with respect to the methyl group interface. Several of the key structural features may be rationalised using a simple two-dimensional model that describes the packing of modified parallelograms and trapezoids, representing the even and odd acids, respectively. Extension of the model into three dimensions permits complete rationalisation of the structures.