Kinetics Of Desorption Of Water, Ethanol, Ethyl Acetate, And Toluene From A Montmorillonite

Abstract

Desorption processes of low-molecular-weight compounds from the surface of smectites into the gas phase determine a number of processes, e.g. those involved in drug delivery and the release of herbicides. The desorption has not been investigated thoroughly and is not well understood. The present study was undertaken in order to understand better the factors influencing these desorption mechanisms. Starting with a very pure standard (Na+-rich) montmorillonite (Kunipia-F), which was exchanged against cations with different hydration properties (Ca2+, Li+, phenyltrimethylammonium, hexyltrimethylammonium), the experiments explored the rate of desorption of volatiles with different chemical functionalities (water, ethanol, ethyl acetate, and toluene). The desorption was monitored by thermogravimetry and differential scanning calorimetry under isothermal conditions, and by ramping the temperature at a constant rate. The experiments were compared with numerical calculations based on finite-element methods and with analytical models. These data point to a two-step mechanism where the desorption follows the curve of the equilibrium desorption isotherms of those molecules on the montmorillonite. The bulk-like volatiles (i.e. volatiles with release kinetics close to that of the bulk liquids) were desorbed in a first step. With a decrease in the degree of coverage of the volatile on the montmorillonite, the desorption was increasingly dominated by the strength of interaction between the volatile and the interlayer cations of the montmorillonite