Determination of the adsorption mechanism of imidazolium-type ionic liquids onto kaolinite: implications for their fate and transport in the soil environment

In the present study, the mechanism of sorption of ionic liquids onto kaolinite clay mineral has been investigated in detail. Results obtained indicate a multilayer type of adsorption. At final saturations, the highest distribution coefficients were found for the long alkyl chain molecules. The mean free energy values obtained are below values of a typical ion-exchange process, which thus suggests that the adsorption mechanism is a combination of electrostatic interaction and physical sorption. At the beginning of the binding process (formation of a monolayer), ion-exchange and van der Waals interactions are predominantly responsible for the process, whereas at higher concentrations of ionic liquids, dispersive interactions become dominant. Thermodynamic parameters were also calculated from graphical interpretation of experimental data. Negative values of Delta G indicate a spontaneous sorption process for ionic liquids. Standard heats of adsorption were found to be exothermic and entropy contributions were negative in all cases studied.