Monolayers single-component

The simplest situation is the symmetrical one (NA = NB), with the solvent equally good for both blocks. We imagine that the excluded volume interactions of A and B are stronger than the A-B repulsive interactions so that the overall structure of the layer is like that of a single component in other words, both components are equally stretched. The issue is whether or not they are homogeneously mixed with one another in the monolayer. This is essentially a two-dimensional random mixing process. In that spirit, we write the free energy... 

The Fowler-Guggenheim-Jovanovic model [3] assumes (as it was the earlier case also) the occurrence of intermolecular interactions among the molecules adsorbed as a monolayer but is based on the Jovanovic isotherm. The single-component isotherm is represented by the equation ... 

It has been shown by Harvey et al. (1989) that incorporation of palmitic acid into a monolayer spread from stearoylserine methyl ester (SSME) breaks up intermolecular SSME interactions. The palmitic acid acts as a two-dimensional diluent. Figures 52(A-C) give the Yl/A isotherms for mixtures of FE and SE C-15 6,6 -A with palmitic acid. Dilution of the monolayer cast from the second eluting isomer with 15 mol% palmitic acid separates the diacid molecules from one another on the water surface and perhaps allows for the expression of their stereochemically dependent conformations. The mixed film (15% palmitic acid/85% C-15 6,6 -A) expands at low II and behaves in much the same manner as the single-component monolayer (C-15 6,6 -A) behaves at 30°C. Addition of 15 mole% palmitic acid into a monolayer cast from the FE C-15 diacid has little effect on its energetics of compression, indicating a stronger intermolecular interaction afforded by its stereochemically dependent conformation at the air-water interface. 

The values of the envelopes of SH intensity at 45 ° incidence in the mixed monolayers are plotted against the fraction of C180AZ0N02 in Fig.ll. The molecular tilt angle <(> evaluated by the above-mentioned procedure is also shown in this figure. The SH intensity of the single-component monolayer of C180AZ0N02 is very small. This result means that the orientation of amphiphile... 

Further increase of fraction C180AZ0N02, however, brought about rapid decrease of the SH intensity. The fraction of the single-component domains of C180AZ0N02 which possess minimal SHG activity increased with increasing fraction of C180AZ0N02, as demonstrated by the absorption spectra of the mixed monolayers. 

In the present study, the monolayer technique is used to investigate the surface properties of octadecyldimethylamine oxide and sodium octadecyl sulfate, as single component films and in combination. The interpretation of the results provides a direct understanding of the mechanism of interaction between these two surface active agents. 

An alternative way of looking at monolayers is to consider them as two-dimensional binary solutions rather than two-dimensional phases of a single component. The advantage of this approach is that it does acknowledge the presence of the substrate and the fact that it plays a role in the overall properties of the monolayer. Although quite an extensive body of thermodynamics applied to two-dimensional solutions has been developed, we consider only one aspect of this. We examine the film pressure as the two-dimensional equivalent of osmotic pressure. It will be recalled that, at least for low osmotic pressures, the relationship among uosm, V, n, and Tis identical to the ideal gas law (Equation (3.25)). Perhaps the interpretation of film pressure in these terms is not too farfetched after all ... 

Phase rule of Defay-Crisp describing the number of degrees of freedom for a system having one single plane surface (monolayer) Multi-component monolayers consisting of immiscible amphiphiles exhibit the same surface pressure for phase transitions and collapse points as the corresponding one-component monolayers, while these surface-pressures are different for mixtures of miscible amphiphiles. 

Figure 5. Self-assembled monolayers as rigid electrochemical spacers. Close packing of the SAM enforces a precise distance between the redox probes and the electrode. (A) In a tethered redox probe system, the probes are covalently attached to the outer surface of the monolayer. The probes may be undiluted (a probe attached to each chain in the monolayer) or mixed (probe-modified chains are diluted with nonelectroactive chains). (B) In a soluble redox probe system, the monolayer typically contains a single component that provides a well-defined separation between the soluble probe and the electrode surface.

The next model, which assumes single-component localized monolayer adsorption with specific lateral interactions among all the adsorbed molecules, is the Kiselev model. The final equation of this model is... 

Monolayers consisting of a single component have been subjected to uniaxial compression (expansion). Particles were initially spread at random at the interface and equilibrated over (f time steps. Then, the monolayers were compressed (expanded) in the x-direction. The velocity of compression (expansion) has been set at = 0.132aT. ... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Monolayers are more expanded at the O/W interface than at the A/W interface this has been generally confirmed since the early work of Davies (19). The condensing effect on fully ionized monolayers from increasing the electrolyte concentration (19) is shown in Figures 3-5. The equimolar Ci8 suIfate-Ci8 TAB monolayers are more condensed than the single-component charged monolayers shown by Phillips and Rideal (17, 18) and Brooks and Pethica (14) although in the present... 

Figure 7. Surface pressure-area curve for mixture of dimethylsiloxane and methyl methacrylate homopolymers, 42.2% siloxane. Solid curve observed for mixed film. Dashed curve calculated for additivity of single component monolayers.

In an elegant experiment, Niwa and coworkers prepared a mixed monolayer by codepositing asymmetrical disulphides 14 and 15 in the ratio 200 1. The xanthate groups in disulphide 15 serves as a photo-initiator. Irradiation of the film results in the polymerization of the styrene moieties as seen by cyclic voltammetry. Virtually no polymerization occurs in the single-component monolayer of 14, which proves the importance of having a photo-initiator in the film346,347. 

Configurational-bias Monte Carlo in the Gibbs ensemble has been successfully applied to the calculations of single-component vapor-liquid phase equilibria of linear and branched alkanes [61,63-67], alcohols [68,69], and a fatty-acid Langmuir monolayer [70]. The extension to multicomponent mixtures introduces a case in which the smaller molecules (members of a homologous series) have a considerably higher acceptance rate in the swap move than do larger molecules. In recent simulations for alkane mixtures... 

Single-component monolayers of CyDs are conveniently deposited on electrode surfaces using the self-assembly method, provided that the CyD derivative has one or more anchoring units, such as the thiol group, responsible for the attachment of the molecule to the metal electrode surface [16, 38, 45, 46, 60, 61]. 

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