Solvent-free synthesis of halloysite-layered double hydroxide composites containing salicylate as novel, active fillers

Solvent-free syntheses provide an alternative path that reduces the use of hazardous substances. In this work, a sustainable synthesis of nanometric layered double hydroxide particles grafted over halloysite nanotubes (HNT@LDH) is reported using a mechanochemical technique. Samples loaded with different LDH percentage (100%, 50%, 40%, 30%, 20%, 10% with respect to the HNT content) were prepared and characterized. Spectroscopic and morphological characterizations allowed to select the sample with 20% by weight of LDH as the best composite (HNT@LDH20), wherein no LDH phase segregation was observed and the nanosheets were homogeneously grafted throughout the surface of the halloysite nanotubes. The combination of cationic and anionic clays in one filler enables the preparation of a composite which can be easily dispersed in a polymer and release active molecules. To this end, salicylate ions were incorporated into the layered double hydroxide nanoflakes of HNT@LDH20 and three intercalation strategies, comprising both solvent-free and traditional intercalating routes, were tested. Results of salicylate release tests indicate that the intercalating route used affects the amount of species hosted, the strength of the salicylate-LDH interactions and the drug release time, thus offering the possibility of selecting the most suitable method of drug incorporation to tune the salicylate content depending on a specific aim.