Solid Inclusion Complexes of Vanillin with Cyclodextrins: Their Formation, Characterization, and High-Temperature Stability

This study reports the formation of solid vanillin/cyclodextrin inclusion complexes (vanillin/CD ICs) with the aim to enhance the thermal stability and sustained release of vanillin by inclusion complexation. The solid vanillin/CD ICs with three types of CDs (alpha-CD, beta-CD, and gamma-CD) were prepared using the freeze-drying method; in addition, a coprecipitation method was also used in the case of gamma-CD. The presence of vanillin in CD ICs was confirmed by FTIR and H-1 NMR studies. Moreover, H-1 NMR study elucidated that the complexation stoichiometry for both vanillin/beta-CD IC and vanillin/gamma-CD IC was a 1:1 molar ratio, whereas it was 0.625:1 for vanillin/alpha-CD IC. XRD studies have shown channel-type arrangement for CD molecules, and no diffraction peak for free vanillin was observed for vanillin/beta-CD IC and vanillin/gamma-CD IC, indicating that complete inclusion complexation was successfully achieved for these CD ICs. In the case of vanillin/alpha-CD IC, the sample was mostly amorphous and some uncomplexed vanillin was present, suggesting that alpha CD was not very effective for complexation with vanillin compared to beta-CD and gamma-CD. Furthermore, DSC studies for vanillin/beta-CD IC and vanillin/gamma-CD IC have shown no melting point for vanillin, elucidating the true complex formation, whereas a melting point for vanillin was recorded for vanillin/alpha-CD IC, confirming the presence of some uncomplexed vanillin in this sample. TGA thermograms indicated that thermal evaporation/degradation of vanillin occurred over a much higher temperature range (150-300 degrees C) for vanillin/CD ICs samples when compared to pure vanillin (80-200 degrees C) or vanillin/CD physical mixtures, signifying that the thermal stability of vanillin was increased due to the inclusion complexation with CDs. Moreover, headspace GC-MS analyses indicated that the release of vanillin was sustained at higher temperatures in the case of vanillin/CD ICs due to the inclusion complexation when compared to vanillin/CD physical mixtures. The amount of vanillin released with increasing temperature was lowest for vanillin/gamma-CD IC and highest for vanillin/alpha-CD IC, suggesting that the strength of interaction between vanillin and the CD cavity was in the order gamma-CD > beta-CD > alpha-CD for solid vanillin/CD ICs.