Adsorption Phenomena at Interfaces

Adsorption as a physical-chemical parameter can be defined in a number of ways. We will focus here on the same principle as we used in defining the surface energy, that is, the principle of surface excess, originally introduced by Gibbs [1-4], 

Physical-Chemical Mechanics of Disperse Systems and Materials 

FIGURE 2.1 (a) To the definition of adsorption according to Gibbs. The figure is schematic, not drawn to 

The meaning of Gibbs law (Equation 2.2) can be formally stated as follows the snrface tension of a liquid phase (or in the general case the interfacial free energy), o(c), decreases with the increase in surfactant concentration c of the adsorbing component, and the rate of this (relative) decrease at constant temperature is described by the value of adsorption, E 

The common universal approximation frequently utilized in thermodynamics is that of ideal solutions, that is, a restriction to the limit of very low concentrations. When the parameters of linear thermodynamic equations have very small values, they are usually proportional to each other. This proportionality provides the necessary additional universal relationship. 

---

When we described the adsorption phenomena at interfaces between a two-component liquid and its vapor (or air), major attention was given to how the liquid phase composition affected the properties of its surface and the structure of adsorption layers. We did not take into account changes in the vapor pressure (i.e. the total pressure in the system) that take place when the composition of a two-component solution is changed. This simplified approach to the solution - vapor interface is justified, if change in the vapor pressure is small and does not affect interfacial properties in any noticeable way. The presence of air or other non-adsorbing gas does not influence interfacial properties either. Under these conditions one can compare properties of different solutions at the same outer pressure by viewing the ternary solute-solvent-air system as a binary one. 

Let us briefly address the basic laws governing adsorption phenomena at interfaces separating two condensed phases when the third component, surface active with respect to the said interface, is introduced into the system. According to the polarity equalization rule, originally formulated by Rehbinder [2], the surface activity of the introduced component is determined by its ability to compensate for the striking difference in polarities of two unlike substances with low mutual solubility. 

Section 4.1.3 illustrates the consequences of adsorption phenomena at Interfaces. 

---