ABSTRACT

In this chapter the phenomenon of surface adsorption of particles is discussed. The general problem of adsorption of neutral particles is stated. The rates of adsorption and desorption are introduced permitting the presentation of some known isotherms (Langmuir, BET and Temkin). Simple kinetic equations at the interfaces are constructed in order to investigate dynamical aspects of the adsorption phenomena (as will be done in Chapter 10). A statistical approach for the adsorption problem-based on the two-level approximation-is discussed in details, underlining the main limitations of the Langmuir approximation. An application of the two-level formalism is proposed in connection with the adsorption of magnetic grains in magnetic fluids, in order to illustrate the relevance of the adsorption phenomena on the nematic ordering, in general, followed by a careful analysis of the meaning of the two-level system approximation. The subsequent part of the chapter is dedicated to the discussion of a model for the adsorption of molecules at the surface, responsible for the anchoring transition in liquid crystalline systems containing azobenzene materials. In these systems, reversible transformations in the molecular conformation from cis to trans forms occur upon illumination. According to the concentration of cis-isomers, a transition from planar to homeotropic alignment can take place at the surface. This phenomenon-connected to the adsorption phenomenon-can be the basis for the photo-manipulation of the anchoring energy and this point is discussed in details, in connection with experimental data.