Liquid Solution Surfaces

In Chapter 4, we dealt with the thermodynamic, physical and chemical properties of pure liquids. However, in most instances solutions of liquids are used in chemistry and biology instead of pure liquids. In Chapter 5, we will examine the surfaces of mainly nonelectrolyte (ion-free) liquid solutions where a solid, liquid or gas solute is dissolved in a liquid solvent. A solution is a one-phase homogeneous mixture with more than one component. For a two-component solution, which is the subject of many practical applications, the major component of the solution is called the solvent and the dissolved minor component is called the solute. Liquid solutions are important in the chemical industry because every chemical reaction involves at least one reactant and one product, mostly forming a single phase, a solution. In addition, the understanding of liquid solutions is useful in separation and purification of substances. 

The presence of a solute in a solution affects the entropy of the solution by introducing a degree of disorder that is not present in the pure solvent, so that many physical properties of the solution become different from that of its pure solvent. Furthermore, solute-solvent molecular interactions affect the total internal energy of the solution. The change in the vapor pressure or the surface tension of a solution from those of its pure solvent are examples of these solute effects. The concentration of the solute in the surface layer is usually different (or very rarely the same) from that in the bulk solution, and the determination of this concentration difference is very important in surface science. 

The investigation of solution and surface film properties of two- or three-component liquid solutions is the subject of this chapter. In one extreme, the components in the liquid solution are completely miscible giving a one-phase solution, and in the other extreme, the components are almost completely immiscible, and an insoluble monomolecular film of one component forms on the surface of the other giving a two-phase solution. Between these two extremes, different kinds of films form on the solution surfaces depending on the extent of molecular interactions between the components. The theoretical approaches and experimental techniques that are applied to these solution types will be described in Chapters 5 and 6 respectively. 

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Equation (608) is exactly equal to Equation (433), given in Section 5.5.3 for the two-dimensional perfect gas for liquid solution surfaces. Equation (608) relates 7Tto the surface excess and is called the surface equation of state. Similarly to Equation (436), we can write [ Amoiecuie = kT] for gas-solid adsorption, where A oWe is the area available per adsorbate molecule in the monolayer, and k is the Boltzmann constant (R = kNA). The adsorption isotherm given by Equations (607) and (608) corresponds to the so-called Henry s law limit, in analogy with the Henry s law equations that describe the vapor pressures of dilute solutions. Equation (606) predicts a linear relation between m (or fractional surface coverage, 0f, and adsorbate gas pressure, P2, as shown in the linear plot in Figure 8.1. 

Foam is a gas/liquid dispersion system, with gas bubbles forming an inner non-continuous phase and liquid forming a continuous phase. As bubbles pass through a liquid solution, surface-active compounds preferentially adsorb onto a bubble surface. The surface-active compounds can be carried out from the liquid phase by these bubbles into a foam phase, which can be formed when these bubbles accumulate above the gas/liquid pool interface. The most strongly surface active component or component with the largest bubble net adsorption rate in the liquid solution will have the highest relative adsorption in the foam phase. When the foam phase collapses to form a new liquid phase, a liquid solution can be produced with a... 

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