How alkali-metal cations affect the inclusion of decanoic acid in β-cyclodextrin

An equimolar mixture of decanoic acid (Dec) with a concentration approximately 18 times above its critical micellar concentration (cmc) and beta-cyclodextrin (betaCD) in deuterated water have been studied by H-1 NMR, and the betaCD CH protons (H3, H4, H5, H6) have been used as probes for assessing the effects of varying the concentration of various alkali-metal chlorides (LiCl, NaCl, KCl, CsCl) on guest inclusion and aggregation processes. The observed chemical shift variations are consistent with the progressive aggregation of decanoic acid induced by an increase in [NaCl] which, in turn, caused a progressive decrease in the amount of included decanoic acid and an increase in the deshielding of betaCD H5 and H3 protons, as decanoic acid was replaced by water in the betaCD cavity. When the different salts were considered and their concentrations varied, Na+, K+, and Cs+ displayed a similar and common slope while Li+ showed a smaller gradient. On the whole, the observed chemical shift variations point to a competition between the aggregation of decanoic acid and its inclusion in betaCD. In particular, the self-association of decanoic acid induced by an increase in electrolyte concentration produces a variation in the observed chemical shift value. Apart from a global medium effect, the electrolyte is found to produce betaCD-site and ion specific effects in the observed chemical shift variations.