Modeling of essential oils adsorption onto clays towards a better understanding of their interactions

Abstract

A large number of aromatic plants have been used against diverse insect pests. Unlike conventional pesticides, usually these natural products present less risk to human health and the environment. To enhance the insecticidal activity, essential oils (Eos) can be adsorbed on a solid support such as clay since it is abundant in nature. Clays were characterized by several physicochemical techniques such as X-ray diffraction (XRD), thermal gravimetric and differential analysis (TGA, TDA), Fourier transformed infrared spectroscopy (FTIR) and nitrogen adsorption-desorption isotherms. The measured adsorption isotherms of essential oil component (sabinene) onto Cameroonian clays (montmorillonite and kaolinite) were modeled using empirical and theoretical models. The analytical expressions of this theoretical model(so-called finite multilayer with multisite occupancy model (FMM)) have been established from an application of the grand canonical ensemble in statistical physics. The finite multilayer with multisite occupancy model was identified to be the most appropriate option for modeling the adsorption isotherms in terms of physical significance, goodness of model fit and error analysis. The results of the theoretical investigations lead us to point out that a number of agglomerated sabinene molecules are oriented perpendicular to the surface. The main mechanisms interaction for adsorption of sabinene onto clays were electrostatic, hydrophobic, π-π interactions and hydrogen bonds. Furthermore, the chemical modification of clays by basic solution has to reduce the number of adsorbed layers at the surface. However, the treatment of montmorillonite with low concentration of acid solution promotes the formation of several layers. The highest adsorption capacity of sabinene was observed in the case of montmorillonite activated with acid solution. The evolution of the calculated of some thermodynamic functions such as the configurational entropy, Helmholtz and Gibbs free enthalpies indicate that the sabinene adsorption is exothermic and spontaneous in nature.