Compression of a monolayer

The Marangoni effect has been observed on the rapid compression of a monolayer [54] and on application of an electric held, as in Ref. [55] it occurs on evaporation [56]. 

Fig. 30. Left Lateral compression of a monolayer of PBA brushes on water. Right Corresponding surface pressure-area Isotherm measured during compression...

FIG. 6.9 Wavelength at the peak maximum of the poly(phenylene sulfonate) 9 A band upon compression of a monolayer (complex with dioctdecyidimethylammonium bromide) and the corresponding isotherm (reproduced with permission from reference 32). 

BAM has been widely used for studying the formation and morphological features of condensed domains in the LE-LC coexistence region and to observe and analyze the phase diagrams of Langmuir monolayers ). Upon rapid compression of a monolayer across a phase transition, non-equilibrium structures like dense-... 

Proteins are found either not to desorb or to desorb only with great difficulty from quiescent interfaces. Langmuir and Schaefer (1939) calculated, on the basis of the Gibbs adsorption equation, that compression of a monolayer of protein of molecular weight 35,000 by 15 mN m-1 should increase its solubility by a factor of 1095. This results from the large area occupied by the molecule at the interface and the resultant large pressure increment of solubility. The failure of protein monolayers to desorb readily on compression was thus taken as an indication of irreversibility. 

Figure 22. Langmuir isotherm for the compression of a monolayer of poly(/ -butyl acrylate) brush molecules with the degree of polymerization of the side chains n = 46.168...

A factor which has not been considered is the entropy change associated with the compression of a monolayer. In a manner analogous to micelle formation in solution, we would expect both Tift off and the association of oriented chains to involve a negative entropy change the removal of the alkyl chain from contact with water should contribute... 

The time ti / G is denoted as retardation time, applicable to bodies other than a Maxwell body. The relaxation curve according to Eq. (3.2) or its spectrum (the process may be complex and can have more than one relaxation time), was applied for fitting the pressure decay obtained after a sudden stop of the compressing of a monolayer at a certain surface pressure. A typical experimental result is shown in Fig. 3.2 (Tabak et al. 1977). 

Fig. 3.2. Typical pressure relaxation after the continuous compression of a monolayer has been stopped suddenly at a certain molecular area according to Tabak et al. (1977)...

The formation of the liquid-expanded phase at compression of a monolayer means that condensation from a gaseous phase to a coherent film in the liquid state takes place. Further compression can give phases with extended hydrocarbon chains tilted in relation to the surface. Such phases are called liquid-condensed (L2) phases and a consequence of this misleading term is that the formation from a liquid-expanded phase is described as a condensation. 

FIG. 17 H-A isotherms for C60 on a pure aqueous subphase at 21°C (a) Monolayer prepared by spreading 50 p.L of a 0.1 mM solution in benzene (b) monolayer prepared by spreading 200 p,L and (c) a compression-expansion cycle of a monolayer prepared by spreading 100 p,L of a 0.5 mM solution. (Reproduced with permission from Ref. 232. Copyright 1993 American Chemical Society.)... 

The results of this study demonstrated that the rate of oxygen transfer across a clean air-water interface was diffusion-controlled on the time scale of SECM measurements. The rate of this transfer process was, however, significantly reduced with increasing compression of a 1-octadecanol monolayer. Figure 28 illustrates this point, showing approach curves for O2 reduction recorded with the monolayer at different surface pressures. The transfer rate was found to depend on the accessible free area of the interface, as described by the following equation ... 

The A F-A isotherm of PS II core complex is rather different from that of PS II membrane (Fig. 4). The surface potential of a monolayer of PS II core complex increases slightly as the molecular area is compressed from 600 to about 150nm, while surface pressure changes from 5 to 35mN/m. Further compression results in a sharper increase in surface potential. The surface potential starts to decrease only after the surface area is compressed to about 80 nm or surface pressure becomes larger than 40mN/m. This is consistent with the previous discussion that PS II core complexes form a more ordered monolayer structure at relatively high surface potential and will not form multilayered... 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

As has been indicated recently [27], the relaxation process during the compression of the monolayers of saturated fatty acids is rather slow and usually incomplete. Thus, the experimental Jt-A curves obtained under the usual continuous compression include the nonequilibrium effects. 

The interpretation of the Langmuir experiments with the carbosilane den-drimers is supported by the results of molecular dynamics simulation. Figure 14 shows snapshots of a dumbbell-like conformation of carbosilane dendximers observed during lateral compression of a dendrimer monolayer on a polar sub-... 

Fig. 14. Snapshot of MD computer simulation of a monolayer of carbosUane dendrimers with OH end groups upon lateral compression on a polar surface [148]...

Further, the equilibrium elasticity of a monolayer film is related to the compressibility of the monolayer (analogous to bulk compressibility) by... 

This natural circulation occurs by a direct transfer of momentum across the interface, and the presence of a monolayer at the interface will affect it in two ways. Firstly, the surface viscosity of the monolayer may cause a dissipation of energy and momentum at the surface, so that the drop behaves rather more as a solid than as a liquid, i.e., the internal circulation is reduced. Secondly, momentum transfer across the surface is reduced by the incompressibility of the film, which the moving stream of gas will tend to sweep to the rear of the drop (Fig. 14b) whence, by its back-spreading pressure n, it resists further compression and so damps the movement of the surface and hence the transfer of momentum into the drop. This is discussed quantitatively below, where Eq. (32) should apply equally well to drops of liquid in a gas. 

Cardiolipin Monolayers. Among various phospholipids studied by monolayer techniques, only cardiolipin (41) and phosphatidylserine (36) monolayers show significant condensation of their surface pressure-area curves in the presence of divalent as compared with monovalent cations in the subsolution. The condensation of cardiolipin is explained by the decrease in molecular area caused by the attraction between a divalent cation and the two phosphate groups in the molecule. This condensation is eliminated when the ratio of monovalent to divalent cations is greater than 5 to 1. At high surface pressures, the difference in the compressibility of cardiolipin monolayers correlates with the ionic radii of the metal ions (Mg2+ < Ca2+ < Sr2+ < Ba2+). 

The apparent condensation of mixed monolayers of lecithin in the presence of cholesterol is explained by a consideration of molecular cavities or vacancies caused by thermal motion of fatty acyl chains, the height of these cavities being influenced by the length, inclination, and degree of unsaturation (especially the proportion of monounsaturation) of the fatty acyl chains and the extent of compression of the monolayer. Mono-layers are liquefied by the presence of unsaturated fatty acyl chains or by the addition of cholesterol. 

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

The collapse or critical pressure frequently is a highly controversial subject, but in this collapse area, the compression of the monolayer is not possible without destroying the monolayer. 

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