Monolayer compression

Yoshida et al. reported that LB films of 48 and 92-97 showed piezochromism, observing a reversible UV spectroscopic change during monolayer compression on a water surface using the setup shown in Figure 33 which permitted both compression-induced and flow-induced orientation of the molecules.66 67 273... 

The Motomura analysis in Figure 2 shows the effects of monolayer compression. As expected, compression causes some ejection of surfactant, particularly at low surfactant concentrations. There even appears to be negative adsorption at 0.6 mmol kg" but while this result is plausible in a qualitative sense, the depth from which surfactant would need to be excluded is unacceptably great. 

Variations on the vertical dipping technique have been utilized to construct films containing divalent metal ions. For example, the quartz crystal microbalance (QCM) has been used to evaluate the horizontal lifting method of CdSt LB Film construction (26). In this method, the QCM quartz plate was touched to monolayers compressed on a subphase and lifted horizontally. Y-type transfer (transfer ratio of 1) was demonstrated with two centrosymmetric monolayers deposited for each cycle. A combination of the vertical and horizontal dipping techniques has been utilized to prepare multilayer films from an amphiphilic porphyrin compound (27). 

Monolayer casting experiments made use of similar experimental conditions to form organized and highly compressed membranes. Such films were deposited on polyacrylamide gel by dipping the polymer through the air-water interface of the trough, with monolayer compression held constant at pressures of 30 to 40 mN.m". ... 

The interfacial properties of HM - PNIPAM, including the formation and the compression - expansion reversibility of the monolayers, at different subphase temperatures were more recently studied by using the Langmuir film balance technique [90], The stability and dynamic nature of the HM - PNIPAM monolayers were also further studied by the time - dependent surface pressure measurements. All results have suggested a compression - promoted temperature - and rate - dependent conformational rearrangement of the polymer on the water surface. Increasing the level of hydrophobic modifications progressively improved the monolayer compressibility and stability, and reduced the hysteresis. 

Figure 3. Dependence of surface pressure on area per particle in the monolayer for monodisperse magnetic particles 7.5 nm in diameter,39 thin dashed line - first monolayer compression thin solid line - third monolayer compression cycle thick solid line - theoretical calculation.

The monolayer of amphiphile spread on water is then compressed into a well-packed state. The profile of monolayer compression is recorded as surface pressure-molecular area (n-A) isotherm. Typical examples of n-A isotherms are shown in Fig. 4.33. The transverse axis of the isotherm represents the molecular area, which can be obtained by dividing the total surface area by the number of amphiphile molecules. The surface pressure is derived by subtracting the surface tension of the monolayer-covered water surface from that of pure water. This has dimensions corresponding to two-dimensional pressure. Surface pressure cannot be regarded as a normal three-dimensional pressure. For convenience, when discussing the phase of two-dimensional monolayer here, the surface pressure is treated as a two-dimensional pressure. 

W. Nitsch, C. Kurthen, Convective Monolayer Compression in Channel Flow Behavior and Transfer of Soluble and Insoluble FUms , Thin Solid Films, 178, 145 (1989)... 

A novel method for detennination of the particle contact angle at the fluid/fluid interface based on the excluded area concept revealed some serious difliculties connected with the exact quantitative particle deposition at the interface and with changes in the particulate contact angle upon binary monolayer compression. The comprehensive theoretical consideration of the contact angle behavior made for such system allowed considerable improvement of the proposed method that was then successfully proven by experimental data. 

Vibrational modes of the phospholipid polar head groups (in particular the symmetric and anti-symmetric PO stretching vibration) reflect their ionization and hydration state. The hydration state of the head group of DPPC was found to change under monolayer compression or by addition of cations such as Ca + There are indications that the transition at Ttg (to the solid state S, see fig. 3.6) involves ordering and dehydration of the head groups. 

Figure 10. Molecular models showing possible effects of monolayer compression on the configuration of (top to bottom) A4-octadecynoic acid, A8-octadecynoic acid, and A14-octadecynoic acid...

Attempts to fit these and other compression isotherms between 400 A2 and the area at which phase transition (9,10,11,12) begins with a variety of two-dimensional equations of state indicates that a constant value for the co-area or the partial molecular area term of these equations cannot suffice in describing the behavior of these monolayers. Such unsuccessful attempts were actually the motivating factor for these investigations of the influence of the hydrocarbon solvent on the monolayer compression isotherms. 

The isotherm for cholesterol (I) is surprisingly similar to that for lecithin (Table II) both are important components of many cell membranes. The extrapolated area of 39.0 A2/molecule approximates that of a double-chain system, and the cohesive and adhesive forces give a collapse pressure of 43 dynes/cm, close to the 40 dynes/cm for lecithin. In Table II cholesterol is also compared with stearic acid, the classic compound in monolayer research. Collapse pressures and compressibilities are similar. The rigid packing of the complex cholesterol molecules is clearly demonstrated by its monolayer compressibility—0.0012, the smallest value in these studies. 

The incommensurate, hexagonal monolayers are compressed compared to the bulk metal and they are rotated from the substrate by several degrees. From the results, the monolayer compressibility could be calculated. The restructuring (i.e. surface reconstruction) of top layers of single crystal metal surfaces as a function of solution composition and electrode potential has been studied [73]. The induced charge density was found to be the critical parameter [74]. Structural changes during... 

The completed Pb UPD is metallic, and represents an incommensurate, hexagonal ML that is compressed compared with the bulk metal by 0.1-3.2%, and rotated from the substrate (Oil[-directionby 4.5° [426, 427, 429-431]. The rotation of the adlayer with respect to the substrate lattice gives rise to a characteristic Moire pattern as observed in several in situ STM studies [360, 426, 427] (Fig. 28). The interaction between solvent molecules and the Pb adatoms does not influence the structure of the complete ML deposited in C104 or acetate-containing electrolyte, since the UPD phase is essentially identical to that of vapor-deposited Pb on Ag(lll) at full coverage [420, 435]. The monolayer compression in the vacuum experiment (1-2%) is slightly less than for... 

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. 

Resistance to collapse of mixed PVPC6 PI monolayers. Compression-expansion cycles of mixed films are carried out under conditions comparable to those involved in the surface pressure and surface potential studies (see above). 

However, though interfacial shear viscosity (ri ) can influence the kinetics of droplet coagulation, it is not possible to rely solely on this measurement to predict emulsion behavior. Specific effects can occur which affect monolayer compressibility and film elasticity and these are not registered by shear viscosity measurements. ... 

Unwin and coworkers studied oxygen transfer across a monolayer dispersed at an air-water interface [74]. They have constructed an SECM measurement system in a Langmuir trough. A monolayer of 1-octadecanol was formed on the aqueous subphase containing potassium nitrate and the surface pressure of the mono-layer was controlled with a compression barrier. A U-shaped microelectrode probe was placed in the aqueous solution and moved up to the air-water interface. When the probe approaches an air-water interface in the absence of the monolayer, the reduction current for O2 is enhanced by positive feedback, due to fast translocation of O2 from the gas phase to the solution. They recorded a set of approach curves while controlling the monolayer compression. A plot of the oxygen transfer rate as a function of the molecular area has been shown. 

Fig. 15 A set of 2D cyclic voltammograms of Os(DPP)3 monolayer at the air/water interface recorded concurrently with the monolayer compression. The CVs were recorded with a single 400-pm line microelectrode at a scan rate of 0.2 V s . Increasing current corresponds to decreasing mean molecular area of Os(DPP)3 from ca. 120 to 90 A /molecule (from Ref 75).

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