Absorption isotherm of caffeine and release kinetics from swollen NIPAAm hydrogels: Experiments and modeling

A series of NIPAAm-based hydrogels, including nonionic, cationic, and anionic types, was prepared by the free radical polymerization method, and the absorption equilibriums and release kinetics of caffeine in the prepared hydrogels were experimentally measured at 25 degreesC. The experimental absorption data were successfully correlated by the Langmuir isotherm model that results in the same absorption equilibrium constant for the same type of hydrogels and relative affinity sequence: anionic types > cationic types > nonionic types. The maximum caffeine absorption capacity significantly increases with the swelling ratio of the nonionic and cationic hydrogels, while it slightly decreases with the swelling ratio for the anionic hydrogels. The caffeine release kinetic data were analyzed, and the effective diffusion coefficients in different hydrogels were estimated by the traditional and a modified mass transfer model, respectively. The results show that the traditional model underestimates the effective diffusion coefficients, while the modified model provides more accurate estimation of the effective diffusion coefficients. For the same type of hydrogels, the diffusion coefficients estimated by the modified model increase with the swelling ratio of the hydrogels.