Kinetics of the Release of Nicotinamide Absorbed on Partially Neutralized Poly(acrylic-co-methacrylic acid) Xerogel under the Conditions of Simultaneous Microwave Heating and Cooling

The kinetics of release of nicotinamide (NIAM) that was absorbed on partially neutralized poly(acrylic-co-methacrylic) (PAM) xerogel/hydrogel, under the conditions of simultaneous microwave heating and cooling (SMHC) were examined. The kinetics curves of NIAM release into an aqueous solution at temperatures of 308-323 K were recorded. By applying the model-fitting method (MFM), it was found that the kinetics of NIAM release can be modeled by a kinetic model of a first-order chemical reaction. The values of the release rate constants (k(M)) at different temperatures were calculated, and their values were found to be within the range 8.4 10(-3) s (-1)-15.7 10(-3) s(-1). It has been established that the Arrhenius equation was valid even in the conditions of SMHC. The values of the kinetic parameters (activation energy (Ea) and pre-exponential factor (A) of the NIAM release process adsorbed on PAM xerogel/hydrogel were calculated as follows: Ea = 25.6 kJ/mol and ln (A/s(-1)) = 5.21. It has been proven that the higher value of the rate constant at SMHC in relation to CH is not a consequence of the overheating of the reaction system or the appearance of hot-points. The values of change of the enthalpy of activation (Delta H*) and the change of entropy of activation (Delta S*) were calculated as follows: Delta H* = +23.82 kJ/mol and Delta S* = -201.4 J/mol K. The calculated higher values of the kinetic parameters and thermodynamic parameters of activation are explained by the formation of a specific activated complex under SMHC, whose structure and degree of order are different than in the one formed under CH.

Automated summary

In this study, the kinetics of NIAM release from PAM xerogel/hydrogel under SMHC conditions were investigated, and it was found to follow a first-order chemical reaction kinetic model. The release rate constants ranged from 8.4 10(-3) s(-1) to 15.7 10(-3) s(-1) at different temperatures. The validity of the Arrhenius equation under SMHC conditions was confirmed, and higher kinetic and thermodynamic parameters were attributed to the formation of a specific activated complex.