Isothermal kinetics of exchange of water absorbed in calcium alginate hydrogel with ethanol

Abstract

The isothermal kinetics of the exchange of absorbed water in calcium alginate hydrogels with ethanol was investigated. The isothermal kinetic curves were measured within the temperature range from 297 to 316 K. By the application of differential isoconversional methods, it was found that the process of exchange of water absorbed in calcium alginate hydrogel with ethanol has a unique rate determining step (a single step process). Using the model fitting method, it was determined that the kinetics of the exchange can be mathematically described with the kinetic model of the first-order chemical reaction. The values of the kinetic parameters of water exchange: the rate constants (km), energy of activation (Ea = 18.0 kJ?mol?1), and the pre-exponential factor (ln[A/min?1] = 4.0) were calculated. By using Eyring's equation, the values of thermodynamics parameters: standard enthalpy of activation (?H*), standard entropy of activation (?S*), and Gibbs energy of activation (?G*) were calculated. It was found that the rate of rearrangements of the bound water around hydrophilic groups of a polymeric network has a dominant effect on the kinetics of the exchange of the absorbed water with ethanol. The mechanism of the exchange of adsorbed water with ethanol was suggested.