The gas—liquid interface

Certain organic molecules will orient themselves at the interface between a gaseous and a liquid phase (or between two liquid phases) to minimize their free energy. The resulting surface film is one molecule in thickness and is commonly called a monomolecular layer or simply a monolayer. In the previous chapter the individual properties of bulk phases were outlined. The interface region will now be examined.

The boundary between a liquid and a gas (e.g., the air/water interface) marks a transition between the composition and properties of the two bulk phases. A surface layer will exist with different properties from those of either bulk phase (Adamson, 1982; Gaines, 1966). The thickness of this region is very important. If the molecules are electrically neutral, then the forces between them will be short-range and the surface layer will be no more than one or two molecular diameters. In contrast, the Coulombic forces associated with charged species can extend the transition region over considerable distances.

The microscopic model of a real interface is one of dynamic molecular motion as molecules move in and out of it. However, for the interface to be in equilibrium, as many molecules must diffuse from the bulk of the liquid to its surface per unit time as leave the surface for the bulk.