Langmuir monolayers

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Peter Foster, Fall 2011

A Langmuir monolayer of surfactant. Image taken from Wikipedia

Definition

A Langmuir monolayer is a layer of amphiphilic molecules that forms at the interface between air and water the has the thickness of a single molecular length. To form this monolayer, the surfactant molecules must be insoluble in water. In 1932, Irvine Langmuir won the Nobel Prize in Chemistry for his investigations on the monolayers that bear his name (Reference 1). The general idea is that amphiphilic molecules have a hydrophilic (polar) head group and a hydrophilic (nonpolar) tail group. The lowest energy conformation for surfactant concentrations less than the critical micelle concentration has the hydrophilic section sitting just under the water surface with the hydrophobic section sitting just above the water surface into the air.


Applications

Through the creation of a Langmuir monolayer it was first discerned that cells have a lipid bilayer (Reference 2). In 1925 E. Gorter and F. Grendel took red blood cells (which lack internal membranes) and extracted the lipid. The extracted lipid was then spread on the surface of a Langmuir trough, which allowed the total area of lipid to be measured. They knew the approximate surface area of a cell and the approximate number of cells in their sample and found that the monolayer created from the extracted lipids had about twice the area they would expect from assuming cells having a lipid monolayer. Thus, the idea of a lipid bilayer was proposed.


The formation of a Langmuir monolayer can also be used to measure molecular dimensions (Reference 3). If one takes a known volume of oil and places it on a Langmuir trough, a Langmuir monolayer will form. Thus, the oil will spread out and cover a certain area. If this area is measured, then the measured area multiplied by the molecular thickness must equal the volume of oil placed in the trough.


References

[1] The Nobel Prize in Chemistry 1932

[2] Karp, Gerald. Cell And Molecular Biology, Concepts And Experiments. 6th ed. Hoboken, NJ: Wiley, 2010. 120. Print.

[3] Estimating the size of a molecule using an oil film


Keyword in references:

Single-particle Brownian dynamics for characterizing the rheology of fluid Langmuir monolayers