Expanded and condensed Langmuir monolayers

The surface pressure, n, is measured as a function of the area of the water surface available to each molecule at constant temperature, and the resulting plot is known as a surface pressure-area isotherm. The barriers compress the surface area of the film preferably at a constant rate. Depending on the film material being studied, repeated compressions and expansions are necessary to achieve reproducible results. 

If we further compress the surface area, the head-groups become dehydrated and the 7r-As isotherm is also linear with a steeper slope. The close-packed and dehydrated molecular area, A , can be obtained by extrapolating the slope of this (S) phase to zero pressure. However, in reality, it is not possible to obtain both A and Ah values simultaneously for many 7r-As isotherms, because the upper part of the isotherm curve is frequently not so linear. The reason is that the phase transition between S and (L-Con.) is usually not sharp, but smooth. In these circumstances, only one of these molecular areas can be determined. If we further compress the area very slowly, the surface pressure increase will stop at a specific surface pressure value, the collapse of the monolayer (C) occurs, and % 

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