Surface pressure
The principal apparatus used for the study of surface monolayers is a surface film balance (Fig. 19.44). This device consists of a shallow trough and a barrier that can be moved along the surface of the liquid in the trough, and hence compress any monolayer on the surface. The surface pressure, π, the difference between the surface tension of the pure solvent and the solution (π = γ * − γ) is measured by using a torsion wire attached to a strip of mica that rests on the surface and pressing against one edge of the monolayer. The parts of the apparatus that are in touch with liquids are coated in polytetrafluoroethylene to eliminate effects arising from the liquid–solid interface. In an actual experiment, a small amount (about 0.01 mg) of the surfactant under investigation is dissolved in a volatile solvent and then poured on to the surface of the water; the compression barrier is then moved across the surface and the surface pressure exerted on the mica bar is monitored.
Some typical results are shown in Fig. 19.45. One parameter obtained from the isotherms is the area occupied by the molecules when the monolayer is closely packed. This quantity is obtained from the extrapolation of the steepest part of the isotherm to the horizontal axis. As can be seen from the illustration, even though stearic acid (12) and isostearic acid (13) are chemically very similar (they differ only in the location of a methyl group at the end of a long hydrocarbon chain), they occupy significantly different areas in the monolayer. Neither, though, occupies as much area as the tri p-cresyl phosphate molecule (14), which is like a wide bush rather than a lanky tree. The second feature to note from Fig. 19.45 is that the tri-p-cresyl phosphate isotherm is much less steep than the stearic acid isotherms. This difference indicates that the tri-p-cresyl phosphate film is more compressible than the stearic acid films, which is consistent with their different molecular structures. A third feature of the isotherms is the collapse pressure, the highest surface pressure. When the monolayer is compressed beyond the point represented by the collapse pressure, the monolayer buckles and collapses into a film several molecules thick. As can be seen from the isotherms in Fig. 19.45, stearic acid has a high collapse pressure, but that of tri-p-cresyl phosphate is significantly smaller, indicating a much weaker film.
Fig. 19.44 A schematic diagram of the apparatus used to measure the surface pressure and other characteristics of a surface film. The surfactant is spread on the surface of the liquid in the trough, and then compressed horizontally by moving the compression barrier towards the mica float. The latter is connected to a torsion wire, so the difference in force on either side of the float can be monitored.
Fig. 19.45 The variation of surface pressure with the area occupied by each surfactant molecule. The collapse pressures are indicated by the horizontal dotted lines.
