Modeling the controlled release of essential oils from a polymer matrix—A special case

• A particular case study of solid-state diffusion of essential oils through polymers.
• Evidence of a Kirkendall-type vacancy diffusion mechanism.
• A Dusty Gas model is proposed, consistent with the Kirkendall effect.
• Prediction of interdiffusivity is in good agreement with gravimetric data.
• Simulation of the controlled release kinetic behavior is qualitatively good.

Experimental evidence of a vacancy diffusion mechanism has been found in menthol/LDPE and thymol/LDPE solid-state diffusing systems, when samples are prepared by melting, mixing, and casting. The void pattern found in the cross-section of the samples after volatile release suggests the occurrence of a phenomenon similar to the Kirkendall effect. A Dusty Gas model, consistent with a Kirkendall system where no macroscopic volume change occurs, is proposed to describe the kinetics of the controlled release of the essential oils from the polymer matrix. The interdiffusion coefficient is predicted via a Free-Volume theory. The prediction of the diffusion coefficient and the simulation results are in good agreement with experimental gravimetric data.