The importance of chain-length on the wettability and solubility of organic homologs

The solubilities of an homologous series of compounds (alkyl p-hydroxybenzoates) were measured in hexane and water at 298 K. The hexane solubilities followed a trend that could be predicted by the melting points, but the aqueous solubilities showed a linear decrease with increasing alkyl chain length, until the chain was over 4 carbon units long, then the effect of adding extra methylene groups was significantly lower. With both solvents the effect on solubility was small after the chain exceeded 5 (+/- 1) carbon units in length. This phenomenon is explained on the basis that the side chain has greater rotational freedom about C-C bonds as it extends beyond five carbons. The freedom of movement will potentially influence the crystal packing (due to the increased possibility for different orientations and conformations) and hence the hexane solubility results, and the orientation of the molecules that are to be dispersed in the solvent (thus the aqueous solubility). Chain lengths of 2-5 showed no significant variation in aqueous contact angle for the powders. The contact angle did increase slightly above a chain length of 6 carbons. The results indicate that the crystal packing is influenced by chain length and that for chain lengths up to five carbons, the chain is not all present in the crystal surface (no increase in contact angles). For wettability and solubility there is a critical point at a chain length of 5 (+/- 1) carbons where the physical properties of the alkyl p-hydroxybenzoates change. Although molecular structure affects both solubility and wettability, there is no direct correlation between these two properties, wettability of the powder being influenced only by the proportions of chemical groupings that are present at the surface of the crystal.