Langmuir 43, isotherm trough

Key words Langmuir trough -isotherms - fluorescence microscopy - Brewster angle microscopy... 

Monolayers of distearoylphosphatidylcholine spread on the water-1,2-dichloro-ethane interface were studied by Grandell et al. [52] in a novel type of Langmuir trough [53]. Isotherms of the lipid were measured at controlled potential difference across the interface. Electrocapillary curves derived from the isotherms agreed with those measured under the true thermodynamic equilibrium. Weak adsorption or a stable monolayer was found to be formed, when the potential of the aqueous phase was positive or negative respectively, with respect to the potential of the 1,2-dichloroethane phase [52]. This result... 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

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. 

This way it has been possible to observe the anticipated conformational transition by experiment. Cylindrical brushes of polybutylacrylate-graft polymethacrylate were adsorbed on the surface of water from a CHCI3 solution on a Langmuir trough and studied by recording the tt-A (surface pressure vs molecular area) isotherm. At different stages of compression, the monolayer was... 

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. 

To test this model, the surface properties of the 22-kDa fragment at an air—water interface have been examined. The air—water interface system has been used extensively to model the interaction of apoli-poproteins with lipid (Phillips and Sparks, 1980 Shen and Scanu, 1980 Camejo and Munoz, 1981 Phillips and Krebs, 1986). When the 22-kDa fragment was spread as monomolecular film in a Langmuir trough, the surface pressure-molecular area isotherm was calculated to be —16 A /... 

Another method applicable to interfaces is the determination of the partial moleeular area a of a biopolymer partitioning into a lipid monolayer at the water-air interfaee using the Langmuir trough [28]. The first step is to record a series of pressure Ti-area (A) isotherms with different amounts of an amphiphilic biopolymer spread at the interface. 

An automated Langmuir trough was used for measurements of surface pressure-area per molecule (n-A) isotherms and film deposition. The isotherms were recorded at a compression speed of 0.2-0.3 A /(moleculexmin). AFM images were obtained with a Nanoscope Ilia (Digital Instruments, USA) operated in the constant force mode (1-10 nN). 

Figure 10. Pressure-area (IT — A) isotherm for a hypothetical amphiphile organized as a monomolecular layer at the water-air interface showing a phase transition from a fluid-expanded to a solid-condensed state upon compression. The insets show cross-sectional diagrams of the Langmuir trough with the compressed monolayer in different phase states.

Figure 12 Schematic showing transitions of a typical sur ce pres-sure/area(n/A) isotherm for a compressed mono-layer of surfactant myristic acid spread on 0.1 M HCl in a Langmuir-trough apparams. (From Ref. 141.)...

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