Langmuir–Blodgett film

A Langmuir–Blodgett film contains one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. A monolayer is adsorbed homogeneously with each immersion or emersion step, thus films with very accurate thickness can be formed. This thickness is accurate because the thickness of each monolayer is known and can therefore be added to find the total thickness of a Langmuir-Blodgett Film. The monolayers are assembled vertically and are usually composed of amphiphilic molecules (see Chemical polarity) with a hydrophilic head and a hydrophobic tail (example: fatty acids). Langmuir–Blodgett films are named after Irving Langmuir and Katharine B. Blodgett, who invented this technique while working in Research and Development for General Electric Co. An alternative technique of creating single monolayers on surfaces is that of self-assembled monolayers. Langmuir-Blodgett Films should not be confused with Langmuir films, which tends to describe an organic monolayer submersed in an aqueous solution . LB films are formed when amphiphilic molecules like surfactants interact with air at an air-water interface. Surfactants (or Surface acting agents) are molecules with hydrophobic 'tails' and hydrophilic 'heads'. When surfactant concentration is less than critical micellar concentration (CMC), the surfactant molecules arrange themselves as shown in Figure 1 below. This tendency can be explained by surface-energy considerations. Since the tails are hydrophobic, their exposure to air is favoured over that to water. Similarly, since the heads are hydrophilic, the head-water interaction is more favourable than air-water interaction. The overall effect is reduction in the surface energy (or equivalently, surface tension of water).For very small concentrations, far less than critical micellar concentration (CMC), the surfactant molecules execute a random motion on the water-air interface. This motion can be thought to be similar to the motion of ideal gas molecules enclosed in a container. The corresponding thermodynamic variables for the surfactant system are, surface pressure (Π), surface area (A) and number of surfactant molecules (N). This system behaves similarly to a gas in a container. The density of surfactant molecules as well as the surface pressure increase upon reducing the surface area A ('compression' of the 'gas'). Further compression of the surfactant molecules on the surface shows behavior similar to phase transitions. The ‘gas’ gets compressed into ‘liquid’ and ultimately into a perfectly closed packed array of the surfactant molecules on the surface corresponding to a ‘solid’ state. Instruments like Langmuir-Blodgett trough can be used to quantify such phenomena.

Further information:

1. Chen X.; Lenhert S.; Hirtz M.; Lu N.; Fuchs H.; Chi L. (2007) "Langmuir–Blodgett Patterning: A Bottom-Up Way To Build Mesostructures over Large Areas" Acc. Chem. Res.; 40(6); 393–40
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15. Article Langmuir–Blodgett film from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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