Encapsulation of Caffeic Acid in Ionic Liquid-Modified Kaolinite and Halloysite Nanostructures for Slow Release of Antioxidants

Encapsulation of chemical compounds in reservoirs to prevent degradation and for controlled release is of capital importance in various domains, such as healthcare and food industry. In this work, kaolinite and tubular halloysite clay minerals functionalized by interlayer grafting of an ionic liquid (1-methylnaphthyl-3-(2-hydroxyethyl) imidazolium chloride) were used for the encapsulation of caffeic acid (CA). As shown by the characterization methods (XRD, solid-state 13C NMR, FTIR, TGA), the initially neutral layers of these clay minerals turned cationic following the grafting of the organic cation, and the resulting structural charge was compensated by exchangeable chloride anions. The anion exchange capacity of these nanohybrid materials was successfully exploited for the encapsulation of CA in its anionic form. Functionalized halloysite showed CA adsorption capacity (230.9 μmol/g) 2-fold higher than functionalized kaolinite (123.3 μmol/g), certainly due to the accumulation of the phenolic compound both in the interlayer space and inside the halloysite tubes. When studying the release of CA in phosphate buffer medium, a slow release was observed, assigned to the high retention of phenolic compounds within the modified clay minerals through hydrogen-bonding and van der Waals interactions. The study of the effect of the phosphate buffer concentration highlighted the release mechanism following ionic exchange between anionic CA and phosphate anions. Moreover, the antioxidant properties of the encapsulated CA (evaluated by the DPPH assay) were effective. The kinetic curve displayed slow antioxidant properties that can be exploited for the long-term protection of a medium against oxidation.