Langmuir–Blodgett trough

The idea of a Langmuir–Blodgett (LB) film was first proven feasible in 1917 when Irving Langmuir (Langmuir, 1917) showed that single water-surface monolayers could be transferred to solid substrates. 18 years later, Katharine Blodgett made an important scientific advance when she discovered that several of these single monolayer films could be stacked on top of one another to make multilayer films (Blodgett 1935). Since then, LB films (and subsequently the troughs to make them) have been used for a wide variety of scientific experimentation, ranging from 2D crystallization of proteins to Brewster angle microscopy. The LB trough's general objective is to study the properties of monolayers of amphiphilic molecules. An amphiphilic molecule is one that contains both a hydrophobic and hydrophilic domain (e.g. soaps and detergents). The LB trough allows investigators to prepare a monolayer of amphiphilic molecules on the surface of a liquid, and then compress or expand these molecules on the surface, thereby modifying the molecular density, or area per molecule. This is accomplished by placing a subphase (usually water) in a trough, spreading a given amphiphile over the surface, and then compressing the surface with barriers (see illustration). The monolayer's effect on the surface pressure of the liquid is measured through use of a Wilhelmy plate, electronic wire probes, or other types of detectors. An LB film can then be transferred to a solid substrate by dipping the substrate through the monolayer.

Langmuir-Blodgett is also used extensively to prepare the lipid membrane and investigate the interactions with surface molecules. Cell membrane are complicated with various proteins embedded in them. The cell membrane is therefore mimicked in model membranes to investigate a particular function in simplified system. For example, a lipid monolayer can be deposited on air-water interface and protein can be injected into the water. The change in the surface pressure can provide the direct information to the adsorption kinetics of proteins into the membrane.

In addition to amphiphilic materials, Langmuir-Blodgett Troughs are commonly used nowadays to create nanoparticle coatings with controlled packing density.[1][2]

Monolayer transfer onto a substrate after film compression. The substrate is moving from bottom to top and is hydrophilically coated since the polar head groups are adhering to the surface

In early experiments, the trough was first constructed from metals such as brass. However difficulties arose with contamination of the sub-phase by metal ions. To combat this, glass troughs were used for a time, with a wax coating to prevent contamination from glass pores. This was eventually abandoned in favor of plastics that were insoluble in ordinary solvents, such as Teflon (polytetrafluoroethylene). Teflon is hydrophobic and chemically inert, making it a highly suitable material, and the most commonly used for troughs today. Occasionally metal or glass troughs coated with a thin layer of Teflon are used; however they are not as enduring as solid PTFE troughs.[3]

In the case of liquid-liquid experiments where the compression is performed at the interface of a polar liquid such as water and a dispersive liquid such as oil, the trough is commonly manufactured from POM (polyoxymethylene). POM is more hydrophilic and aids in keeping the liquid-liquid interface stable.

Different mechanisms have been used to compress or expand the monolayers throughout the development of the LB trough. In their first experiments, Langmuir and Blodgett used flexible silk threads rubbed with wax to enclose and compress the monolayer film. Most commonly used systems are made of movable barriers that slide parallel to the walls of the trough and are in contact with the top of the fluid. These barriers are typically made from hydrophilic POM to form a meniscus on them that will aid in keeping the molecules inside even in high packing densities. PTFE barriers are also available for instances when additional chemical resistance is needed.[4]

Another version with a variable perimeter working zone is the circular trough in which the monolayer is located between two radial barriers. A constant perimeter trough was developed later in which the barrier is a flexible Teflon tape wrapped around three pairs of rollers. One of the pairs is fixed and the other two are movable on trolleys, so that the length of the tape remains constant as the area of the working zone is changed.

Special Alternate troughs allow for preparation and deposition of alternating monolayers by having two separate working zones that can be compressed independently or synchronously by the barriers.[3]

An important property of the system is its surface pressure (the surface tension of the pure subphase minus the surface tension of the subphase with amphiphiles floating on surface) which varies with the molecular area. The surface pressure – molecular area isotherm is one of the important indicators of monolayer properties. Additionally, it is important to maintain constant surface pressure during deposition in order to obtain uniform LB films. Measurement of surface pressure can be done by means of a Wilhelmy plate or Langmuir balance.[3]

The Wilhelmy method consists of a plate partially immersed in the liquid connected to an electronic linear-displacement sensor, or electrobalance. The plate can be made of platinum or filter paper which has been presoaked in the liquid to maintain constant mass. The plate detects the downward force exerted by the liquid meniscus which wets the plate. The surface tension can then be calculated by the following equation:

${\displaystyle {\begin{aligned}\mathrm {[Force\ on\ Plate]} &=\mathrm {[Weight\ of\ plate]} &+\mathrm {[Surface\ Tension\ Force]} &-\mathrm {[Buoyant\ Force]} \\F&=\left(m_{p}g\right)&+2(t_{p}+w_{p})\gamma _{LV}\cos(\theta )&-\rho _{l}V_{p}g\end{aligned}}}$

where

${\displaystyle {\begin{aligned}m_{p}&=\mathrm {mass\ of\ plate} \\g&=\mathrm {gravitational\ acceleration} \\t_{p}&=\mathrm {thickness\ of\ plate} \\w_{p}&=\mathrm {width\ of\ plate} \\\gamma _{LV}&=\mathrm {surface\ tension\ of\ liquid} \\\theta &=\mathrm {contact\ angle} \\\rho _{l}&=\mathrm {density\ of\ liquid} \\V_{p}&=\mathrm {Volume\ of\ the\ proportion\ of\ plate\ immersed\ in\ liquid} \end{aligned}}}$

The weight of the plate can be determined beforehand and set to zero on the electrobalance, while the effect of buoyancy can be removed by extrapolating the force back to the zero depth of immersion. Then the remaining component force is only the wetting force. Assuming that perfect wetting of the plate occurs (θ = 0, cos(θ) = 1), then the surface tension can be calculated.[5]

${\displaystyle F=2(t_{p}+w_{p})\gamma _{LV}\cos(\theta )\,}$

The surface pressure is then the change in surface tension due to the addition of the monolayer [6]

${\displaystyle \Pi =\gamma _{o}-\gamma \,}$

Where

${\displaystyle {\begin{aligned}\Pi &=\mathrm {surface\ pressure} \\\gamma &=\mathrm {surface\ tension\ of\ subphase\ with\ monolayer} \\\gamma _{o}&=\mathrm {surface\ tension\ of\ pure\ subphase} \end{aligned}}}$

In Langmuir's method, the surface pressure is measured as the force exerted directly on a moveable barrier.[3]