Surface hydroxyl groups description

A number of different surface complexation models have been applied to describe and predict divalent metal ion sorption data over the past 20 to 30 yr. All of Ihe models incorporate surface acidity and the formation of metal ion complexes with surface hydroxyl groups via equilibrium mass law expressions such at those presented in Tabic 7-2. In addition, each model employs a description of the elec-Irical double layer lo correcl for electrostatic effects at the mineral/water interface (as shown in Fig. 7 4 lor (he triple layer model and described in Table 7-3). These... 

Most oxide and hydroxides, as well as the broken edge sites and basal-plane hydroxyl groups of clay minerals exhibit amphoteric behavior. The formation of eleciric charge can be explained by the acid-base behavior of surface hydroxyl groups. A detailed description can be found in several excellent book chapters and articles [1,2,6,8,13,14,35,45], Charge development on amphoteric surface sites (S—OH) could occur by direct proton transfer. The surface hydroxyl groups can be capable of ionization [8,46], Surface ionization (protonation and deprotonation) reactions can take place on these sites, depending on the pH of the solution ... 

Operando DRIFTS measurements suggest that bridged hydroxyl groups are in extensive interaction with hexane molecules during the reaction even at 553 K. However adsorbed alkene or surface alkoxide could not be detected. These findings questions, whether the Haag-Dessau mechanism [4] gives true description of the alkane activation process over zeolite catalysts. 

It is important to establish the origin and magnitude of the acidity (and hence, the charge) of mineral surfaces, because the reactivity of the surface is directly related to its acidity. Several microscopic-mechanistic models have been proposed to describe the acidity of hydroxyl groups on oxide surfaces most describe the surface in terms of amphoteric weak acid groups (14-17), but recently a monoprotic weak acid model for the surface was proposed (U3). The models differ primarily in their description of the EDL and the assumptions used to describe interfacial structure. "Intrinsic" acidity constants that are derived from these models can have substantially different values because of the different assumptions employed in each model for the structure of the EDL (5). Westall (Chapter 4) reviews several different amphoteric models which describe the acidity of oxide surfaces and compares the applicability of these models with the monoprotic weak acid model. The assumptions employed by each of the models to estimate values of thermodynamic constants are critically examined. 

The surface of silica (for detailed description of results see refs. 5, 11 and 12) contains a variable amount of hydroxyl groups and adsorbed wa-... 

Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [118,119]. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption a good fit to the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation a good description of integral conversion data was achieved [119]. 

On the other hand, there is evidence that the pH may affect the reactivity of the hydroxyl groups, in that OH" ions have been found to catalyse the condensation of two hydroxyl groups on different silica particles to water and an oxygen bridge. In analogy, an alternative description of the initial reaction between a hydrolysed metal species and the silica surface would be... 

It is generally known that metal oxide surface is covered with hydroxyl groups when oxide is placed in water. The presence of two free electron pairs of oxygen atom and possibility of hydrogen ion dissociation is the evidence of amphoteric character of these groups. On account of this, the most useful parameter in description of the water/metal oxide interface is pH of the solution being in contact with the surface. Adsorption of H" " or OH ions causes protonization or deprotonization of the surface according to the Eqs. (31a) and (31b). 

A description of silica surfaces should not be made only in terms of hydroxyl groups without reference to the underlying atoms. Various experimental results suggest some local order on the surface, even though the bulk network is amorphous. The presence of typical sites on the atomic scale is all the more likely because constraints due to disorder should relax on a hydroxylated surface. Therefore, network and surface models based... 

Figure 21.32 shows the architecture of an LDL particle. The interior consists of many molecules of cholesteryl esters (the hydroxyl group of the cholesterol is esterified to an unsaturated fatty acid, such as linoleate). On the surface, protein (apoprotein B-lOO), phospholipids, and unesterified cholesterol are in contact with the aqueous medium of the plasma. The protein portions of LDL particles bind to receptor sites on the surface of a typical cell. Refer to the discussion of membrane receptors in Section 8.6 for a description of the process by which LDL particles are taken into the cell as one aspect of receptor action. This process is typical of the mechanism of uptake of lipids by cells, and we shall use the processing of LDL as a case study. LDL is the major player in the development of atherosclerosis. 

The constant capacitance modeP" is a molecular description of surface complexation reactions involving the inorganic hydroxyl group. The chemical basis of the model can be developed from its three principal assumptions concerning the interfacial region ... 

Recently, a new method to synthesize aluminum oxide that contains high surface Lewis acidity relative to surface hydroxyls without dehydroxylation at elevated temperatures was reported [3]. It was shown that this oxide, compared with one prepared by a conventional method, contains a much higher density of surface Lewis acid sites relative to hydroxyl groups. Furthermore, these surface Lewis acid sites are catalytically active. The solid catalyzes ring opening of cyclopentene oxide with piperidine to form 2-piperidylcyclopentanol selectively [4]. In this chapter, a description of the preparation method and characterization of the steps in the synthesis will be presented. 

As a first step, the simulation of a mineral-aqueous interface requires treatment of the issue of surface hydroxylation, which is fundamentally tied to the dissociation of water and the energetics of acid-base reactions on mineral surfaces (Blesa et al. 2000). Even just setting the problem up requires some knowledge of the protonation states of the oxide ions at the surface are they aquo, hydroxo, or oxo functional groups If one cannot describe the processes behind Figure 1, it is not possible to go further. This description is... 

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