The non-reversibility of adsorption and desorption, i.e. the fact that desorption isotherms do not conform to the adsorption isotherm is commonly observed. This hysteresis has been variously attributed to kinetic effects, i.e. the hysteresis would disappear if the desorption time were long enough, or to the existence of a metastable equilibrium where the hysteresis persists long enough and is stable enough so that it can be modeled as though it were at equilibrium. The site transformation model of sorption hysteresis is based on the idea that at adsorption equilibrium a fraction of the sites with adsorbed chemical are transformed into sites that reversibly bind the chemical more strongly than the untransformed sites. Therefore both the untransformed and transformed sites are reversible. The model equations makes explicit the transformation step and predict the form of the consecutive desorption isotherm. Depending on the model used for the adsorption isotherm, the model reproduces the Reversible-Resistant and the Linear � Langmuir models that have been fit to experimental adsorption-desorption data. For the site transformation model the form of the consecutive desorption isotherm is not chosen to fit the data but rather follows from the adsorption isotherm and the equation that describes the site transformation. This isolates the hypothesis that produces the hysteresis so that experiments can be designed to test various site transformation mechanisms.

Site transformation model of adsorption-desorption hysteresis

245th National Meeting (2013)

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