The Langmuir trough is commonly used to analyse surfactants (surface-active agents) trapped at the interface between two phases. For example, a polymer containing hydrophobic and hydrophilic regions may be examined at the air-water interface. Automated trough systems are composed of a PTFE (PolyTetraFluoroEthylene) trough, a barrier, and a pressure sensor (Figure 1). Experiments commonly involve the deposition of the surfactant (dissolved in a volatile solvent) onto the surface of the subphase. On closing the barrier, the surfactant is compressed causing a change in the surface pressure. Plotting the surface pressure (p) as a function of trough area (A) produces what is known as a p-A isotherm. Analysis of these isotherms provides information on the arrangement of the surfactant at the interface. An example is when a short polystyrene chain, terminated by a hydroxyl group, is spread. In that case as the barrier is closed a monolayer is formed which on further application of pressure collapses to a trilayer. This can be followed on the isotherm.
Figure 1. Model 601S NIMA ¨ Langmuir trough