The Langmuir Trough

Because of the importance of surfactants in controlling and modifying the surface tension of fluids, it is appropriate here to describe methods for the measurement of surface tension and surfactant film characterization using the Langmuir trough apparatus. 

FIGURE 3.20 Schematic diagram of a Langmuir-Blodgett trough. Surfactants on the water surface can be compressed by a moveable barrier while surface tension is measured. 

The LB trough is commonly used to measure surface pressure/area isotherms for a particular surfactant film. To carry out this measurement, the thin surfactant film is compressed by moving a barrier across the fluid surface at a constant rate while monitoring the surface tension. At a constant temperature the surface pressure Tt is measured as a function of the interfacial area available to each molecule as 

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Hysteresis was generally observed in the compression-expansion cycles of the force-area isotherms, indicating that the timescale for relaxation of the fully compressed film back to its expanded state was slower than the movement of the barrier of the Langmuir trough. Our studies, like many others, imply that monolayers are metastable and that reversible thermodynamics can only be applied to their analysis with caution. 

An HM-HEC monolayer at the air/aqueous interface was formed by adsorption from an aqueous solution of the polymer placed in the Langmuir trough overnight. In "stress-jump" experiments, HM-HEC monolayers were placed under rapid compression to a large degree and surface pressure was measured as a function of time after compression was stopped. (The compressional "jumps" required a minute or two to complete, and in some cases were on the order of the polymer monolayer relaxation times. See later section for discussion). In hysteresis experiments, the adsorbed monolayers were subjected to continuous compression-expansion cycles at a specific speed, while surface pressure was determined as a function of surface area. 

Bruno R, Akkara JA, Samuelson LA, Kaplan DL, Marx KA, Kumar J et al (1995) Enzymatic mediated synthesis of conjugated polymers at the Langmuir trough air-water interface. Langmuir 11 (3) 889-892... 

Thin-liquid-film stability. The effect of surfactants on film and foam stability. Surface elasticity. Froth flotation. The Langmuir trough and monolayer deposition. Laboratory project on the flotation of powdered silica. 

The force per nnit length (P/l) generated by the snrfactant film is called the snrface film pressure flp. In the Langmuir trough device, illustrated in Figure 8.11, the density and hence the pressure in the film can be varied via a movable barrier. 

Figure 8.11 Schematic sectional diagram of the Langmuir trough showing a trapped film of insoluble surfactant molecules.

Since the total number of molecules of surfactant added to the Langmuir trough is known (from the molecular weight) the area per molecule is also known and can be varied simply by moving the boom. The relation between the film pressure and the area per molecule can, therefore, be measured. This is in fact a very elegant method for the study of molecular films. The precise isotherm is characteristic of the surfactant but the general features often observed are shown in Figure 8.15. 

Fig. 11. (A) Design of the Langmuir trough used in the current studies. The trough is shuttled under computer control for IR illumination of the background surface and monolayer covered surface. (B) Schematic of the IRRAS instrument displaying the sections of the accessory that rotate in concert under computer control to provide the desired angle of incidence. Taken from Ref. [81] with permission from American Chemical Society.

You may have done the Langmuir trough experiment in a physical chemistry practical class. This involves measuring the size of a molecule by allowing an oil to spread on the surface of water in a uni molecular layer. 

A third example of the usefulness of the Langmuir-trough technique to... 

Only two years after Langmuir published the design of his surface film balance ), now known as the Langmuir trough, his co-worker Miss Katharine Blodgett developed a technique for transferring monolayers from the alr/water interface to... 

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