Treatment of the Langmuir adsorption isotherm as introduction to

Treatment of the Langmuir Adsorption Isotherm as Introduction to Adsorption Dynamics 

Langmuir s basic treatment for an adsorption isotherm was set out in his paper The condensation and evaporation of gas molecules (1917). Langmuir (1917, 1918) and Volmer (1925) derived a similar formula for the interpretation of the kinetics of adsorption/desorption. In these papers the process of adsorption was proposed as a dynamic situation for the first time. The basic relationship describing the adsorption as a kinetic mechanism results from the balance of the adsorption flux and desorption flux 

The simplest case results when a non-localised adsorption is assumed (Baret 1968a, b), so that jjd C(, and r. As the result we obtain Eq. (4.31), where are k j and kj are the rate constants of adsorption and desorption, and c is the bulk concentration of the adsorbing species. On the basis of a localised adsorption the Langmuir mechanism Eq. (4.32) results. Further transfer mechanisms used to describe the kinetics of adsorption are given in Section 4.4, Eqs (4.31) - (4.34). To use these so-called transfer mechanisms for model of dynamic adsorption layers they have to be coupled with the transport process in the bulk. Baret (1969) suggested replacing c by the so-called subsurface or sublayer concentration. This is per definition the bulk concentration adjacent to the adsorption layer c(0,t) localised at x = 0. The following two flux balance equations for the molecular transfer results. 

In adsorption equilibrium, when the adsorption flux equals the desorption flux, the Langmuir and Henry isotherms results. 

In this book, adsorption and desorption processes play a key role. Thus it is important to discuss the physical background of adsorption isotherms and their use in describing the mechanism of surfactants in relaxation processes of adsorption layers at liquid interfaces. 

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