Abstract
This chapter discusses thermodynamics of the solid/liquid interface and its application to adsorption and colloid stability. Charging of solid surfaces in liquid medium is a consequence of interactions of ions from the bulk of the solution with surface groups. Surface charge determines the distribution of ions in the liquid layer near the surface. Electrostatic interactions between charged particles are responsible for the stability of colloid dispersions—that is, the rate of particle aggregation depends on the equilibrium in the electrical interfacial layer (EIL) located between solid and liquid medium. It is shown that the colloid stability caused by electrostatic repulsion among particles is determined by the electrostatic potential at the onset of the diffuse layer and the distribution of ions within the diffuse layer. To understand the colloid stability phenomena, one should consider the equilibrium at solid/liquid interface. Interfacial reactions are considered by surface complexation model (SCM) and corresponding interfacial equilibrium constants. There are different theoretical models describing the structure of EIL differing in the quantitative relationships between electrostatic potentials and surface charge densities. Theoretical interpretation of experimental data depends on the choice of the employed model. The most frequently investigated aqueous dispersions are those the surface charge of which depends on pH, such as metal oxides and hydroxides. In the derivation of the activity coefficients of interfacial species, it will be assumed that the major contribution to the non-ideality is because of the electrostatic effects. The reason for such a simplification is that electrostatics plays a major role.
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