DOSY-NMR and Raman Investigations on the Self-Aggregation and Cyclodextrin Complexation of Vanillin

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is a phenolic aldehyde with limited solubility in water; in this work, we investigate its self-aggregation, as well as its complexation equilibria with beta-cyclodextrin by using nuclear magnetic resonance (NMR) and vibrational spectroscopy. In particular, diffusion-ordered NMR (DOSY) measurements allowing to detect diffusional changes caused by aggregation/inclusion phenomena lead to a reliable estimate of the equilibrium constants of these processes, while Raman spectroscopy was used to further characterize some structural details of vanillin self-aggregates and inclusion complexes. Although the self-association binding constant of vanillin in water was found to be low (K-a similar to 10), dimeric species are not negligible within the investigated range of concentration (3-65 mM); on the other hand, formation of beta-cyclodextrin self-aggregates was not detected by DOSY measurements on aqueous solutions of beta-cyclodextrin at different concentrations (2-12 mM). Finally, the binding of vanillin with beta-cyclodextrin, as measured by the DOSY technique within a narrow range of concentrations (2-15 mM) by assuming the existence of only the monomeric 1:1 vanillin/beta-CD complex, was about an order of magnitude higher (K-c similar to 90) than self-aggregation. However, the value of the equilibrium constant for this complexation was found to be significantly affected by the analytical concentrations of the host and guest system, thus indicating that K-c is an "apparent" equilibrium constant.