Description of insoluble monolayers
There exists a wide range of surfactants 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 surfactants dictates the orientation of the molecules 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 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).
The history of Langmuir-Blodgett (LB) film started in 1774 when Benjamin Franklin reported to British Royal society:
"At length at Clapman where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropped a little of it on the water. I saw it spread itself with surprising swiftness upon the surface. the oil, though not more than a teaspoonful, produced an instant calm over a space several yards square, which spread amazingly and extended itself gradually until it reached the leeside, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass."
What Franklin didn’t realize, is how the thickness of the oil on the water surface is on the order of nanometers!
Irwing Langmuir, however, was the first to perform systematic studies on floating monolayers on water in the 1910s and 1920s. The first detailed description of sequential monolayer transfer was done later by Katherine Blodgett. These monolayer assemblies are called Langmuir-Blodgett films. Langmuir was awarded the Nobel Prize in Chemistry much later in 1932.
Interest in LB films have grown significantly only half a century later as a result of Hans Kuhn and colleagues’ work on energy transfer in multilayer system. This started the field of molecular engineering, i.e. Using LB techniques to position certain molecular groups at precise distances to others.
One of the current researches in LB films is the construction of LB-film memory chip in which each data bit is represented by a single molecule.
Self Assembled Monolayers
Self assembled monolayers (SAMs) are an alternative to the Langmuir Blodgett films. They have played a large role in research areas like biosensors, bioelectronics, and specifically the creation of nano-transistors. Instead of using that technique to add molecules to the surface SAMs are prepared simply by adding a solution of the desired molecule to the substrate surface and washing off the extra. This creates a thermodynamically stable monolayer. The ease of creation offers a huge advantage to SAMs as they can easily be prepared in the lab.
One example would be alkane thiol on gold. Van der waals forces will cause the alkane thiol head group will stick to the gold surface and form an ordered assembly with alkyl chains packing together. The image below (from []) shows the preparation of such a SAM. This example is gold on silicon and it demonstrates the steps required.
Some commonly used SAMs include:
Interest in SAMs has started to increase drastically over the last few years as you can see even in the older graph below. Around 1991, the number of publications about them started to increase rapidly.
The films created can be as small as only a few nanometers thick and serve as ideal building blocks for nano-devices due to their unique electrical and optical properties. SAM surfaces can be used to create interesting nano-level architectures and play a huge role in the development of nano-electronics such as sensor arrays. The application of SAMs in material chemistry is pretty wide ranging. They have been leveraged for work ranging from devices for nanoelectronics to biomimetric material synthesis. Artifically designed SAM surfaces have been used for selected catalytic reactions and molecular recognition.