Surface selective adsorption

L. Delle Site, C. F. Abrams, A. Alavi, and K. Kremer (2002) Polymers near metal surfaces Selective adsorption and global conformations. Phys. Rev. Lett. 89, p. 156103 M. Praprotnik, L. DeUe Site, and K. Kremer (2005) J. Chem. Phys. 123, p. 224106... 

We have considered the surface tension behavior of several types of systems, and now it is desirable to discuss in slightly more detail the very important case of aqueous mixtures. If the surface tensions of the separate pure liquids differ appreciably, as in the case of alcohol-water mixtures, then the addition of small amounts of the second component generally results in a marked decrease in surface tension from that of the pure water. The case of ethanol and water is shown in Fig. III-9c. As seen in Section III-5, this effect may be accounted for in terms of selective adsorption of the alcohol at the interface. Dilute aqueous solutions of organic substances can be treated with a semiempirical equation attributed to von Szyszkowski [89,90]... 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Figure Bl.22.2. RAIRS data from molecular ethyl bromide adsorbed on a Pt(l 11) surface at 100 K. The two traces shown, which correspond to coverages of 20% and 100% saturation, illustrate the use of the RAIRS surface selection nde for the detemiination of adsorption geometries. Only one peak, but a different one, is observed in each case while the signal detected at low coverages is due to the asymmetric defomiation of the...

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

Chromatography is based upon the selective adsorption from solution on the active surface of certain finely divided solids. Closely related substances exhibit different powers of adsorption, so that separations, which are extremely difficult by ordinary chemical methods, may be effected by this means. When, for example, a solution of leaf pigments... 

The enhanced concentration at the surface accounts, in part, for the catalytic activity shown by many solid surfaces, and it is also the basis of the application of adsorbents for low pressure storage of permanent gases such as methane. However, most of the important applications of adsorption depend on the selectivity, ie, the difference in the affinity of the surface for different components. As a result of this selectivity, adsorption offers, at least in principle, a relatively straightforward means of purification (removal of an undesirable trace component from a fluid mixture) and a potentially useflil means of bulk separation. 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

Principle The adsorptive-bubble separation methods, or adsub-ble methods for short [Lemlich, Chem. Eng. 73(21), 7 (1966)], are based on the selective adsorption or attachment of material on the surfaces of gas bubbles passing through a solution or suspension. In most of the methods, the bubbles rise to form a foam or froth which carries the material off overhead. Thus the material (desirable or undesirable) is removed from the liquid, and not vice versa as in, say, filtration. Accordingly, the foaming methods appear to be particularly (although not exclusively) suited to the remov of small amounts of material from large volumes of hquid. 

To simplify our discussion, we will consider two specific cases spherical micelles in a selective solvent and selective adsorption on to a solid surface from a selective solvent. 

Adsorption phenomena from solutions onto sohd surfaces have been one of the important subjects in colloid and surface chemistry. Sophisticated application of adsorption has been demonstrated recently in the formation of self-assembhng monolayers and multilayers on various substrates [4,7], However, only a limited number of researchers have been devoted to the study of adsorption in binary hquid systems. The adsorption isotherm and colloidal stabihty measmement have been the main tools for these studies. The molecular level of characterization is needed to elucidate the phenomenon. We have employed the combination of smface forces measmement and Fomier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR) to study the preferential (selective) adsorption of alcohol (methanol, ethanol, and propanol) onto glass surfaces from their binary mixtures with cyclohexane. Om studies have demonstrated the cluster formation of alcohol adsorbed on the surfaces and the long-range attraction associated with such adsorption. We may call these clusters macroclusters, because the thickness of the adsorbed alcohol layer is about 15 mn, which is quite large compared to the size of the alcohol. The following describes the results for the ethanol-cycohexane mixtures [10],... 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

In fact, different techniques revealed cadmium segregation and decrease of the Pb/Se ratio near the InP/PbSe interface, indicating that during the first steps of deposition a CdSe layer is formed on InP prior to the PbSe growth. It was suggested that selective adsorption of Cd(0) on the InP surface gives rise to an epitaxial CdSe monolayer, which facilitates an ordered PbSe growth on account of the small lattice mismatch (0.7%) at the CdSe/(rock salt)PbSe interface. Importantly, it was found... 

Thus the IR active modes will be determined by the matrix elements of the polarlsablllty matrix and not by a combination of the surface selection rule and the normal IR selection rules l.e. all of the Raman active modes could become accessible. This effect has been formalized and Its significance assessed In a discussion (12) which compares Its magnitude for a number of different molecules. In the case of acrylonitrile adsorption discussed In the previous section, the Intensity of the C=N stretch band appears to vary with the square of the electric field strength as expected for the Stark effect mechanism. 

Pabalan RT, Turner DR, Bertetti FP, Prikryl JD (1998) Uranium(VI) sorption onto selected mineral surfaces. In Adsorption of Metals by Geomedia. Jenne EA (ed) Academic Press, San Francisco, p 99-130... 

The surface selection rule operates in addition to the normal IR selection rules in determining which vibrational modes are observed. As a result of the SSR the relative intensities of the fundamental IR adsorption bands of an adsorbed species can be used to give information on the orientation of the species with respect to the surface. Both S- and P-polarised light interact equally with the randomly oriented solution species. 

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