The dynamics of adsorption at liquid interfaces

Surface active substances are able to modify significantly the properties of interfaces by adsorption. This fact is used in many processes and many new technologies are based on adsorption effects. In general, these technologies work under dynamic conditions and improvement of their efficiency is possible by a controlled use of interfacially active material. The interfaces involved are freshly formed and have only a small effective age of some seconds or sometimes even less than a millisecond. 

To optimise the use of surfactants, polymers and mixtures of them, specific knowledge of their dynamic adsorption behaviour rather than of equilibrium properties is of great interest (Kretzschmar Miller 1991). The importance of dynamics of adsorption in different applications has been recently discussed froth flotation (Malysa 1992), foam generation (Fainerman et al. 1991), demulsification (Krawczyk et al. 1991), or emulsification (Lucassen-Reynders Kuijpers 1992). 

The most frequently used parameter to characterise the dynamic properties of liquid adsorption layers is the dynamic interfacial tension. Many techniques exist to measure dynamic tensions of liquid interfaces having different time windows from milliseconds to hours and days. 

The aim of this chapter is to present the fundamentals of adsorption at liquid interfaces and a selection of techniques, for their experimental investigation. The chapter will summarise the theoretical models that describe the dynamics of adsorption of surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures. Besides analytical solutions, which are in part very complex and difficult to apply, approximate and asymptotic solutions are given and their range of application is demonstrated. For methods like the dynamic drop volume method, the maximum bubble pressure method, and harmonic or transient relaxation methods, specific initial and boundary conditions have to be considered in the theories. The chapter will end with the description of the background of several experimental technique and the discussion of data obtained with different methods. 

The adsorption kinetics of interfacial active molecules at liquid interfaces, for example surfactants at the aqueous solution/air or solution/organic solvent interface, can be described by quantitative models. The first physically founded model for interfaces with time invariant area was derived by Ward Tordai (1946). It is based on the assumption that the time dependence of interfacial tension, which is directly correlated to the interfacial concentration T of the adsorbing molecules, is caused by a transport of molecules to the interface. In the absence of any external influences this transport is controlled by diffusion and the result, the so-called diffusion controlled adsorption kinetics model, has the following form 

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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. 

There are many other experimental method for studying the dynamics of adsorption at liquid interfaces. First of all, many other techniques exist to measure dynamic surface and interfacial tensions. Only a subjective selection of some more experimental developments are given in the following section. Moreover, other than surface and interfacial techniques are discussed in this chapter too, such as radiotracer, ellipsometer, electric potential, and spectroscopic methods. 

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