Multicomponent monolayers

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

FORMATION OF MULTICOMPONENT MONOLAYERS OF A GIVEN COMPOSITION AND STRUCTURE ON SILICA SURFACE... 

From the ML principles and analysis of experimental data on stability of Si-O-E bonds to effect hydrogen chloride it was possible to distinguish the following ways of multicomponent monolayers synthesis ... 

The most widely used approach for the description of multicomponent monolayers consisting of molecules of different size is the integration of the Gihhs equation... 

The main problem in the thermodynamic theory of penetration is to determine the dependence of the adsorption of a soluble surfactant on its bulk concentration for any given (constant) adsorption of the insoluble surfactant (surface concentration), and the onset of the surface pressure jump in mixed monolayers, caused by the adsorption of a soluble surfactant in the presence of the insoluble component. There exist several main theoretical approaches to the description of the penetration thermodynamics. One is based on the Gibbs adsorption equation for multicomponent monolayers [143-146], Another approach, initially proposed by Pethica... 

Very interesting behavior of incorporating anions can be observed when a multicomponent electrolyte is used for oxide formation. Here, anion antagonism or synergism can be observed, depending on the types of anions used. The antagonism of hydroxyl ions and acid anions has been observed in a number of cases. Konno et a/.181 have observed, in experiments on anodic alumina deterioration and hydration, that small amounts of phosphates and chromates inhibit oxide hydration by forming monolayer or two-layer films of adsorbed anions at the oxide surface. Abd-Rabbo et al.162 have observed preferential incorporation of phosphate anions from a mixture of phosphates and chromates. 

For constant-separation factor systems, the /(-I rails formal ion of Helfferich and Klein (gen. refs.) or the method of Rhee et al. [AlChE J., 28, 423 (1982)] can be used [see also Helfferich, Chem. Eng. Sci., 46, 3320 (1991)]. The equations that follow are adapted from Frenz and Horvath [AlChE ]., 31, 400 (1985)] and are based on the h I ransiomialion. They refer to the separation of a mixture of M — 1 components with a displacer (component 1) that is more strongly adsorbed than any of the feed solutes. The multicomponent Langmuir isotherm [Eq. (16-39)] is assumed valid with equal monolayer capacities, and components are ranked numerically in order of decreasing affinity for the stationary phase (i.e., Ki > K2 > Km). 

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

The pseudo-phase separation approach has been successfully applied in developing a generalized nonideal multicomponent mixed micelle model (see I4) and it is Interesting to consider whether this same approach can be used to develop a generalized treatment for adsorbed nonideal mixed surfactant monolayers. The preferred form for suoh a model is that it be suitable (at least in principle) for treating multiple components and be extendable to other interfaoes and properties of interest suoh as oontaot angles. Earlier models (5, 18, 27) based on the pseudo-phase separation approach and... 

Complete monolayers adsorbed on surfaces can also be observed by STM.139 The separation of monolayer components in self-assembled monolayers on gold was unknown and unexpected before multicomponent films were examined using STM.140 Having the ability to resolve components with molecular resolution,107 this field has since advanced rapidly to exploit intermolecular interactions to produce desired patterns.141 Such advances in other important materials could easily be driven by this ability to observe their structures and functions with atomic resolution. Other environments—liquid, gas, elevated and reduced temperature—... 

Firstly it can be used for obtaining layers with a thickness of several mono-layers to introduce and to distribute uniformly very low amounts of admixtures. This may be important for the surface of sorption and catalytic, polymeric, metal, composition and other materials. Secondly, the production of relatively thick layers, on the order of tens of nm. In this case a thickness of nanolayers is controlled with an accuracy of one monolayer. This can be important in the optimization of layer composition and thickness (for example when kernel pigments and fillers are produced). Thirdly the ML method can be used to influence the matrix surface and nanolayer phase transformation in core-shell systems. It can be used for example for intensification of chemical solid reactions, and in sintering of ceramic powders. Fourthly, the ML method can be used for the formation of multicomponent mono- and nanolayers to create surface nanostructures with uniformly varied thicknesses (for example optical applications), or with synergistic properties (for example flame retardants), or with a combination of various functions (polyfunctional coatings). Nanoelectronics can also utilize multicomponent mono- and nanolayers. 

The preliminary results just reported for DPPC/OA monolayers illustrate the way in which neutron reflectometry can be used to study the interaction of components in model membrane systems. In particular, the technique has been shown to be useful in the study of the water associated with lipid headgroups. Data analysis by the partial stmcture factor method offers the potential to study complex multicomponent membrane systems, which have more relevance to the behavior of biological membranes in vivo. 

In electrochemical multicomponent systems", i.e., using electrolytes containing different Me , mixed UPD and OPD deposition of different metals can be used for a sequential deposition of different Me, monolayers forming sandwich-structured ultrathin metal films, S/Mei/Me2/...Me . The formation of 2D Me-S and Me,-Mey surface alloys and 3D Me-S bulk alloys can be utilized to form ultrathin surface alloy films such as S/Me-S, S/Me,-Mey, and S/Me, -S/Me/-Mey. Ultrathin sandwich-structured films and surface alloys will be denoted as heterostructures. 

As typical examples, formation of ultrathin sandwich-structured films can be demonstrated in the multicomponent systems Ag(M/)/Pb, and Aa(fik )/Ag, Tl. The first system offers the possibility to form ultrathin sandwich films consisting of two monolayers with different composition of Pb and Tl depending on the polarization routine, as shown in Fig. 6.18. The first monolayer may contain either only Pb adatoms (Fig. 6.18a) or both Pb and Tl adatoms (PbjrTly with 0.5 z < 1, x -i- y = 1,... 

Combining these results with those obtained in the system AuQikl)/Ag described above and in Section 4.3, a defined number of Ag monolayers, one PbyTly monolayer and a pure Tl monolayer can be deposited successively in the multicomponent system... 

In all experiments, a constant silver layer thickness of three monolayers was initially deposited. In the multicomponent system, Cd diffuses towards S under thin layer conditions" established by a 2D CdxAgy surface alloy. In the one-component systems, Cd diffuses towards S under semi-infinite conditions. A phase transition... 

Finnie, K. R. Nuzzo, R. G., The phase behavior of multicomponent self-assembled monolayers directs the nanoscale texturing of Si(100) by wet etching, Langmuir 2001, 17, 1250-1254... 

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. 

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