General idea of adsorption kinetics

There are two general ideas to describe the dynamics of adsorption at liquid interfaces. The diffusion controlled model assumes the diffusional transport of interfacially active molecules from the bulk to the interface to be the rate-controlling process, while the so-called kinetic controlled model is based on transfer mechanisms of molecules from the solution to the adsorbed state and vice versa. A schematic picture of the interfacial region is shown in Fig. 4.1. showing the different contributions, transport in the bulk and the transfer process. 

Transport in the solution bulk is controlled by diffusion of adsorbing molecules if any liquid flow is absent. The transfer of molecules from the liquid layer adjacent to the interface, the so-called subsurface, to the interface itself is assumed to happen without transport. This process is determined by molecular movements, such as rotations or flip-flops. As pointed out in Chapter 2 adsorption of surface active molecules at an interface is a dynamic process. In equilibrium the two fluxes, the adsorption and desorption fluxes, are in balance. If the actual surface concentration is smaller than the equilibrium one, T F, the adsorption flux to the interface predominates, if F F , the actual amount adsorbed at the interface is higher than the equilibrium value F , and the desorption flux prevails. 

Models which consider diffusion in the bulk as the only rate-controlling process are called diffusion controlled. If the diffusion is assumed to be fast in comparison to the transfer of molecules between the subsurface and the interface the model is called kinetic-controlled or barrier-controlled. Both steps are taken into account in mixed diffusion kinetic controlled models. 

In technological applications as well as in scientific experiments specific boundary conditions are often given, such as definite changes of the interfacial area. A schematic representation is given in Fig. 4.2. which shows various bulk and interfacial transport processes of surface active molecules diffusion in the bulk, interfacial diffusion, bulk flow of different origin, interfacial compression and dilation. 

Theoretical models for the interpretation of experimental data and quantitative descriptions of processes in particular technologies have to consider these specific conditions which usually lead to very complex theoretical models. 

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