Adsorption of an active molecule on the surface of halloysite for controlled release application: Interaction, orientation, consequences

The goal of the study was to check the possible use of halloysite (Hal) nanotubes as a controlled release natural antioxidant device with quercetin as the active component. The mineral was thoroughly characterized by various techniques including the determination of particle and tube morphology, specific surface area, pore size and volume, and surface energy. The high surface energy of Hal predicted strong adsorption of active molecules on its surface and consequently difficult release. FTIR spectroscopy confirmed the existence of strong interactions, energetically heterogeneous Hal surface and multilayer coverage at large loadings. FTIR and XRD experiments proved the complete lack of intercalation and showed that below 3.5 wt% quercetin loading, most of the molecules are located within the Hal tubes. Molecular modeling indicated the parallel orientation of quercetin molecules with the surface. Critical concentrations derived from various measurements agreed well with each other further confirming that up to about 4.0 wt% loading, quercetin is bonded very strongly to the Hal surface. As a consequence, the dissolution of active molecules is very difficult or impossible, especially into apolar media; thus, neither stabilization nor controlled release effect can be expected below that concentration