Experimental and DFT Characterization of Halloysite Nanotubes Loaded with Salicylic Acid

Halloysite nanotubes (HNTs) and salicylic acid (SA) are natural substances widely used in different fields. HNTs are very promising as nanocarriers because of their biocompatibility, atoxicity, anti-inflammatory properties and capacity to maintain the biological activity of immobilized enzymes. Due to its bactericidal and antiseptic properties, salicylic acid (SA) is used in pharmaceutical formulations, and as an additive for preserving foods and cosmetics. In this study, we set up a procedure for the loading of HNTs with SA for their possible application in active food packaging. Pristine HNTs were studied together with acidic etched HNTs with enlarged internal lumen, and various pH values for the loading solutions were tested in order to obtain the maximum loading. The HNTs - empty and loaded with SA - were characterized by TG-FTIR, FTIR SEM, STEM and nitrogen adsorption/desorption isotherms measurements. We obtained a maximum loading of 10.5% (w/w), using HNTs pretreated with H2SO4 2 M at 25°C for 48h and a solution of sodium salicylate at pH 8. We also characterized the interaction of SA-HNTs at a molecular level by combining ATR-FTIR measurements and periodic density functional theory (DFT) calculations. We believe that the information on the SA/HNT complexes derived from our research should help to improve the current knowledge of SA-clay interactions. In addition, it should be of interest for environmental and earth sciences since SA is used to model natural organic matter (NOM) in both experimental and theoretical studies of NOM adsorption on different kinds of mineral surfaces