Chemistry Reference
In-Depth Information
10
Solid Surfaces and Dispersions
The technological, environmental, and biological importance of adsorption from
solution onto a solid surface can hardly be overestimated. The impact of such
phenomena on our everyday lives is evident in such areas as foods and food science,
agriculture, cosmetics, pharmaceuticals, mineral ore froth flotation, cleaning and
detergency, the extraction of petroleum resources, lubrication, surface protection,
and the use of paints and inks. Each of these applications, and many more,
would be difficult if not impossible in the absence of the effects of adsorbed surfac-
tants and stabilizers at the solid-liquid interface.
The presence of amphiphilic materials adsorbed at a colloidal interface may
reduce interfacial energies to promote the formation of small particle sizes, enhance
the wetting of the solid by a liquid phase, and provide a stabilizing surface layer to
prevent or retard particle coagulation or flocculation. In addition, the action of
surfactants in conjunction with dispersed drugs can greatly enhance the efficacy
of drug delivery.
At the macroscopic level, the modification of a solid surface by adsorption can
greatly affect its subsequent rewetting characteristics (waterproofing), its electrical
properties (antistatic agents), its physical ''feel'' against human skin (fabric soft-
eners), its interaction with contacting surfaces (lubrication), or its ability to adsorb
other solutes (dyeing modifiers), just to name only a few commonly encountered
examples. Comprehensive discussions of some of the theoretical and practical
aspects of surfactant adsorption at solid interfaces can be found in the references
cited in the Bibliography. In practically all the above mentioned examples of sur-
face modification by adsorption, and many more, the primary physical phenomena
involved are pretty much the same. The following discussion introduces the basic
concepts involved in terms of the natures of the adsorbing species and the surface
being affected.
10.1. THE NATURE OF SOLID SURFACES
While the fluid interfaces discussed so far are relatively easy to treat from a thermo-
dynamic standpoint because of the assumptions of molecular smoothness and
