Langmuir Film (monolayer), L-film

(Study Langmuir films with KSV LB Troughs)

Definition:

An insoluble spread monolayer of atoms or molecules floating at the gas/liquid interface.

Summary/Description:

There exists a wide range of lipids, polymers or other organic molecules with an amphiphilic nature, which drastically lower the surface tension of water. Many of these amphiphilic substances insoluble in water can with the help of a volatile and water insoluble solvent easily be spread on a water surface to form an insoluble monolayer at the air/water interface. These monolayers, also called Langmuir (L) films, represent the extreme case when considering adsorption to interfaces because all molecules are concentrated in a one molecule thick layer at the interface. The amphiphilic nature of the molecules dictates the orientation at the interface (air/water or oil/water) in such a way that the polar head group is immersed in the water and that the long hydrocarbon chain is pointing towards air, gas or oil. The name Langmuir film origins from the name of the pioneer of these films Irving Langmuir, who extensively studied these insoluble monolayers in the early 20th century.

The hydrocarbon chain of the substance used for monolayer studies has to be long enough in order to be able to form a insoluble monolayer. A rule of thumb is that there should be more than 12 hydrocarbons or groups in the chain ((CH₂)_n, n > 12). If the chain is shorter, though still insoluble in water, the amphiphile on the water surface tend to form micelles. These micelles are water soluble, which prevents the build-up of a monolayer at the interface. On the other hand if the length of the chain is too long the amphiphile tends to crystallize on the water surface and consequently does not form a monolayer. It is difficult to determine the optimal length for the hydrocarbon chain because its film forming ability also depends on the polar part of the amphiphile. Furthermore, the amphiphile has to be soluble in some organic solvent, which is highly volatile and water insoluble (chloroform or hexane is commonly used).

During the last decade the Langmuir film concept has also been used for the preparation and study of highly organized monolayers of colloidal and nanoparticles at the air/water interface.