What are Langmuir Monolayers?

A Langmuir monolayer (LM) is a monomolecular amphiphilic film at the air-water interface.  Each amphiphilic molecule consists of a polar head group (hydrophilic moiety) and a nonpolar tail typically a hydrocarbon (hydrophobic) chain. Typical examples are fatty acids, lipids, alcohols and others.
In general the monolayer is spread from volatile solution onto the surface of a Langmuir trough.  After evaporation of the solvent, the density of the monolayer can be manipulated by a controlled movable barrier.  The surface pressure versus molecular area for arcachidic acid is measured with a Wilhelmy film balance as is shown below.