Langmuir collapse area

Molecule 11 forms amphiphilic Pockels-Langmuir monolayers at the air-water interface, with a collapse pressure of 34 mN m and collapse areas of 50 A at 20 °C these monolayers transfer on the upstroke only, with transfer ratios around 100% onto hydrophilic glass, quartz, or aluminum,or onto fresh hydrophilic Au, but transfer poorly on the downstroke onto graphite, with a transfer ratio of only abont 50%. The LB monolayer thickness of 11 was 23-25 A by X-ray diffraction, spectroscopic ellip-sometry, surface plasmon resonance, and XPS. With... 

Figure 8.20 Sequence of the preparation of a Langmuir-Btoggett film on water (a) before compression, (b) during compression, (c) at limiting pressure, (d) film collapse at higher pressure. Right Pres sure-surface diagram showing the phase changes L (gas-liquid) and S (liquid-solid). it, pressure and A surface area...

Different molecular areas of Langmuir monolayers can be determined. They can be defined in three ways Ao is the area per molecule extrapolated to zero differential surface tension, Ac is the minimum area per molecule at the collapse point, at the point in the tt - A isotherms where the pressure is the maximum reversible pressure (or collapse pressure ttc) and Am is the area at the midpoint pressure rrm = 0.5 TTC. 

The monomolecular area of the Langmuir films of DODA/HPA obtained at mN/m are 28.18 49.07iun mol. The monomolecular areas have been observed to decrease in the order of Cu>Mn>Ni>Co>Zn. The collapse pressures are 28,75-43.85 mN m, indicating that the monolayer and multilayer prepared have good stability. 

Mohammed et al. (144, 145), in a series of studies, examined several aspects of emulsion films with and without demulsifiers as well as their chemistries. Using the Langmuir balance for studying the air/ crude/water interface, they examined the sinface pressme n-area isotherm for monolayers of Buchan crude s asphaltenes and resins and their mixtures, spread on distilled water at pH 6.2 and 25°C. They found that the asphaltenes upon compression formed solid films, that could withstand pressures up to 45 mN m" in contrast to the resin films at 7 mN m which thereafter collapsed. The asphaltenes formed highly stable emulsions in contrast to the resins alone, which formed the least stable emulsions. They found that film compressibility and emulsion stability decreased as resin content increased. Temperature increases caused no significant effects on asphaltene monolayer compressibility as was observed earlier by Reis-berg and Doscher (126) for natural crude oil films. 

The compressed solid-analogous Langmuir film of Bixin is extremely sensitive to mechanical perturbations. At a surface pressure of 32 dyn/cm the film collapses under the influence of surface waves. We managed this problem by reducing the water level in the trough. The ratio of transferred film area for up- to downstroke motion was almost 1. By comparison of the IR spectra of a multilayer sample on Si (20 LB) to a KBr pellet we could not yet determine if the molecules are stacked in head-to-tail or head-to-head orientation. 

Table 2. Pressure-Area isotherm data for Pockels-Langmuir films. IT and A. are the pressure and molecular area, respectively, at the collapse point.

Fig. 3 Langmuir film balance schematic, a - monolayer of lipid in a quasi gas state, b -state of maximal compaction indicative of molecular area, c - monolayer collapse point reflecting monolayer rigidity. Tested monolayer was palmitic acid over H2O to produce a typical surface pressure/molecular area isotherm.

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