Adsorption and Nature of the Adsorbent Surface

The nature of the solid surface involved in the adsorption process is a major factor in determining the mode and extent of solute adsorption. When one considers the possible nature of an adsorbent surface, three principal groups readily come to mind (1) surfaces that are essentially nonpolar and hydrophobic, such as polyethylene (2) those that are polar but do not possess significant discrete surface charges, such as polyesters and natural fibers such as cotton and (3) those that possess strongly charged surface sites. Each of these surface types will be discussed, beginning with what is probably the simplest, type 1. 

The adsorption of surface-active agents onto nonpolar surfaces from non-aqueous solvents has been much less intensively studied than aqueous systems. Such studies have generally been limited to carbon black and crosslinked polymer dispersions in hydrocarbon solvents. The orientation of the adsorbed molecules in those cases appears to remain more or less parallel to the surface, although the exact details will depend on the history of the carbon surface (e.g., the presence of oxide layers, or charges) and the mode of preparation of the polymer dispersion (i.e., polar or ionic catalyst residues in the surface). 

Uncharged Surfaces. Polar, uncharged surfaces include many of the synthetic polymeric materials such as polyesters, polyamides, and polyacrylates, as well as many natural materials such as cotton and silk. As a result of their surface makeup, the mechanism and extent of adsorption onto such materials is of great potential technological importance, particularly in terms of dyeing processes, waterproofing, and detergency. The mechanism of adsorption onto these surfaces can be much more complex than that of the nonpolar case discussed above, since such factors as orientation will be determined by a balance of several forces. 

The potential forces operating at a polar surface include the ever-present dispersion forces, dipolar interactions, and hydrogen bonding and other acid-base interactions (see Chapter 4). The relative balance between the dispersion forces and the uniquely polar interactions is of supreme importance in determining the mode of adsorption. If dispersion forces predominate, adsorption 

FIGURE 9.10. When a surfactant adsorbs on a solid surface, it may assume one (or more) of several orientations, depending on the natures of the surface and the molecule. For nonpolar surfaces the options will include (a) trains lying more or less flat on the surface (fe) L s, in which significant portions of the molecule remain adsorbed parallel to the surface and (c) vertical or perpendicular in which the major portion of the chain has no direct contact with the sohd surface. 

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