Chapter 9

### LEARNING OBJECTIVES

After completing this chapter, the reader should be able to:

• ► Discuss the significance of surface and interfacial phenomena in biological and pharmaceutical systems.

• ► Measure surface and interfacial tensions of liquids using the appropriate method.

• ► Estimate the ability of a liquid to spread over a surface.

• ► Propose ways to enhance the wettability of pharmaceutical powders in liquid media.

• ► Organize surfactants in different categories.

• ► Classify surfactants on the basis of their hydrophile-lipophile balance values.

• ► Evaluate the process of adsorption at the liquid-air interface, with emphasis on the adsorption of surface active agents (surfactants).

• ► Determine the critical micelle concentration of a surfactant.

• ► Calculate the surface excess concentrations and molecular areas of adsorbing species by using the Gibbs adsorption equation.

• ► Discuss the adsorption of gases on solid surfaces and the use of the Langmuir, Freundlich, and BET adsorption isotherms.

• ► Analyze adsorption isotherm data and calculate the amount of adsorbate required to form a monolayer, and the specific surface area of the adsorbent.

• ► Assess the adsorption at the solid-liquid interface, and discuss the adsorption of proteins onto solid surfaces.

### SURFACE TENSION

In this chapter we discuss the behavior of the molecules at the boundaries of phases. The behavior of molecules at those boundaries is different from their behavior in the bulk of the phases, which has implications for the physiology of the human body as well as for pharmacy. Interfacial phenomena affect drug delivery systems. For example, solubilization and dispersion of drugs, suspension or emulsion stability, and adsorption of drugs on different substrates are all affected by the interfacial properties of drugs and their environment.

The boundaries of solids, liquids, or gases with other solids, liquids, or gases are called interfaces. The boundaries of solids or liquids with air are called surfaces (Table 9-1.

TABLE 9-1

Surfaces and Interfaces

#### Definition of Surface Tension

Molecules that reside in the bulk of phases are surrounded by other molecules of the same kind; thus, they experience equal forces from all directions (Fig. 9-1). In contrast, molecules at the interfaces are subjected to differing forces because only some of their neighboring molecules are of the same kind. They experience high attractive forces from molecules of the same kind and lower attractive forces from molecules of the neighboring phase. (If the molecules of the neighboring phase were attracting the molecules of the other phase with stronger attractive forces than their own, the two phases would be miscible and mix together like ...

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