The last equation indicates a relationship similar to ideal gas law. However, it should be noted that the concentration-dependence of surface tension is valid only when the solutions are dilute and concentrations are low. Hence, at very low concentrations of the surfactant, the molecules behave like ideal gas molecules.
Experimentally, the surface pressure is usually measured using the Wilhelmy plate. A pressure sensor/electrobalance arrangement detects the pressure exerted by the monolayer. Also monitored is the area to the side of the barrier which the monolayer resides.
Figure 2. A Wilhelmy plate
only when .
Here, and are the dimensions of the plate, and is the difference in forces. The Wilhelmy plate measurements give pressure – area isotherms that show phase transition-like behaviour of the LB films, as mentioned before (see figure below). In the gaseous phase, there is minimal pressure increase for a decrease in area. This continues until the first transition occurs and there is a proportional increase in pressure with decreasing area. Moving into the solid region is accompanied by another sharp transition to a more severe area dependent pressure. This trend continues up to a point where the molecules are relatively close packed and have very little room to move. Applying an increasing pressure at this point causes the monolayer to become unstable and destroy the monolayer.
Figure 3. (i) Surface pressure – Area isotherms. (ii) Molecular configuration in the three regions marked in the -A curve; (a) gaseous phase, (b) liquid-expanded phase, and (c) condensed phase. (Adapted from Osvaldo N. Oliveira Jr., Brazilian Journal of Physics, vol. 22, no. 2, June 1992)
Read more about this topic: Langmuir–Blodgett Film