Water adsorption in rosemary essential oil microparticles: Kinetics, thermodynamics and storage conditions

The water adsorption kinetics of rosemary essential oil microparticles encapsulated with gum arabic by spray drying was determined by the static method at 15 °C, 25 °C, 35 °C and 45 °C. The isotherm parameters adjusted by the GAB mathematical model were used to calculate the differential and integral thermodynamic properties, and prediction of the most stable microparticle storage conditions. The mechanisms that direct water vapor adsorption by the theoretical study of enthalpy-entropy compensation were also evaluated. The product reached the equilibrium phases from 144 h of storage and the temperature increase from 15 °C to 45 °C reduced the adsorptive capacity. The Gibbs free energy was negative under all conditions, indicating that the adsorption phenomenon is a spontaneous process. The minimum values for the net integral entropy were indicative of maximum product stability conditions, corresponding to water activities of 0.358-0.119 for the temperature range of 15-45 °C. Isokinetic compensation was confirmed for the adsorption phenomenon and the approach resulted in two zones. At low moisture content the process was controlled by entropy, that is, barriers which limit the water molecules in the product microstructure are more important than the energy of the interactions related to the chemical composition of the microparticles, and at the intermediate-high moisture content, the process was controlled by enthalpy.