Abstract
ABSTRACT Cocrystallisation of butanoic acid (BA), pentanoic acid (PA), and eicosanoic acid (EA) with 4,4’-azopyridine (azp) and trans-1,2-bis(4-pyridyl)ethylene (bpe) results in 2:1 lamellar cocrystals. All cocrystals display the OH···Npyridyl seven-membered ring supramolecular synthon however, each cocrystal responds differently upon heating. Positive thermal expansion (PTE) occurs along the c-axis due to π···π stacking in the isostructural BA cocrystals, while the PA cocrystals are not isostructural and display a mixture of anti and gauche conformations of the acid chains because of the length mismatch with the coformer. PA2·azp displays negative thermal expansion (NTE) of the intrasynthon C···O hydrogen bond. The two EA cocrystals both display c-axis elongation on warming due to the expansion of the distance between the alkyl chains. The expansion by more than 1 Å in EA2·azp is highly anisotropic, however, there is much less c-axis expansion in EA2·bpe.
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