Substances that reduce the surface tension of a solvent are called surface-active compounds or surfactants. They are also known as amphiphiles, as their molecular structure contains both lipophilic and hydrophilic (non-polar and polar) parts. Due to this structure, amphiphilic molecules prefer to accumulate at the water/air interface in aqueous solutions. The hydrophilic head group is in the aqueous phase, while the lipophilic side chains protrude into the gas phase. This disrupts the interaction of the water molecules at the interface and reduces the surface tension as the surfactant concentration increases.

In the first part of this experiment, the surface tension of an aqueous surfactant solution is measured as a function of the surfactant concentration using a ring tensiometer. From this, conclusions are drawn about the aggregation of the surfactants in the bulk (volume phase) and their surface area requirement at the interface.

In the second part of the experiment, monomolecular films of fatty acids that are insoluble in water are prepared at the water/air interface. The lateral pressure of these films is increased by compressing them in a Langmuir trough. In analogy to a pressure-volume isotherm in 3D, a lateral pressure-surface isotherm can be recorded and phase transitions of monomolecular layers can be investigated.