ABSTRACT

Surfactant molecules tend to arrange themselves at the interface in a thin surface film, which is the site where they affect surface tension. This surface film is typically composed of a single layer of surfactant molecules and is thus called a monomolecular film or monolayer (Figure 2-6). The liquid phase supporting the surfactant film is often referred to as the liquid subphase. A surfactant film always lowers surface tension below values found without surfactant present. To rationalize this, recall that surface tension is generated by the unbalanced attraction between interfacial molecules and those in the bulk liquid (Figure 2-3). The presence of surfactant molecules at the interface reduces this imbalance because the attractive forces between surfactant molecules and liquid molecules are less than the attractive forces of liquid molecules for each other. If this were not so, and surfactant molecules had greater attraction for the liquid than for each

Surfactant molecules can be complex in structure, but they are often drawn in simplified form with a single hydrophobic region ( )

and polar head group ( )

Figure 2-5 Schematic of an idealized surfactant molecule. All compounds that are surface active at the air-water interface are amphipathic: they have polar and nonpolar regions of structure. An idealized surfactant molecule is often drawn with a polar headgroup represented by a circle and a nonpolar tail or chain represented by a wavy line. This idealization is widely used for convenience, although many surfactants including proteins, polymers, and even lipids have multiple or complex polar and nonpolar regions of structure.