Equilibrium and kinetics of 5-aminosalicylic acid adsorption by halloysite

Abstract

This paper investigates equilibrium, kinetics and thermodynamic aspects of the adsorption of 5-amino salicylic acid [5-ASA], an anti-inflammatory agent, onto halloysite, a mesoporous polymorph of kaolin appearing as hollow nanotubes with diameters typically smaller than 100 nm and lengths ranging from 500 nm to over 1 μm. Kinetic and equilibrium studies were performed in aqueous medium at different temperatures and drug concentrations. The amounts of 5-ASA retained by the clay was evaluated by UV spectroscopy. Mathematical treatment of the data was performed following Valenzuela et al. [C. Valenzuela-Calahorro, A. Navarrete-Guijosa, M. Stitou, E. Cuerda-Correa, Colloids Surf. A: Physicochem. Eng. Aspects 224 (2003) 135–147], and thermodynamic parameters were then calculated by using the Eyring equation. The overall adsorption process of 5-ASA onto halloysite was explained as the result of two separate processes. Initial rapid adsorption of 5-ASA (specific adsorption rates between 10.0 and 12.3 s −1) on the external halloysite surface (ΔH = 7.59 kJ/g) was followed by slow adsorption (specific adsorption rates between 0.2 and 0.4 s −1) of the drug inside the phyllosilicate pores (ΔH = 42.28 kJ/g). Adsorption experiments showed that promising delivery systems may be designed by supporting 5-ASA onto halloysite.