Original Entry by Xu Zhang
An insoluble spread monolayer of atoms or molecules floating at the gas/liquid interface.
The name Langmuir film origins from the name of the pioneer of these films Irving Langmuir, who extensively studied
insoluble monolayers in the early 20th century.
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.
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.