Surface concentrations, amphoteric

Use of the above treatment implies surface concentrations of charged species (T) and different kinds of surface potentials (). In solving the equilibria at the interface, one should define the surface charge densities of different postulated planes (cr). For the amphoteric (2-pK) concept the surface charge densities in the 0- and / -planes are defined as ... 

Thus, our results oppose the popular opinion, that it is not ix>ssible to sorb simultaneously in comparable amounts chloroplatinate anion and dope cation on the surface of amphoteric and basic supports from the acid impregnating solutions at the synthesis of bimetal catalysts. Note, however, that a noticeable suppression of copper sorption by Pt limits the catalyst synthesis via a simultaneous deposition from the solutions with the known Pt content and Cu/Pt ratio. Our experiment results show, that only for Pt concentrations less than 0.5 wt.% we can obtain a wide range of Cu/Pt ratios. At the Pt concentration exceeding 1 wt.%, the obtained ratio shall be less than 2. Moreover, the registered acceleration of Pt... 

In addition to providing information regarding the capacity of the solid surface for the liquid phase adsorbate, the adsorption isotherm can also provide valuable conformational and thermodynamic information. The data of O Fig. 10.14 presents the adsorption isotherms for poly(methylmethacrylate) (PMMA) on oxidized aluminum and silicon surfaces (Watts et al. 2000). O Figure 10.14a shows the behavior at low and medium concentrations, and this follows the expected form for chemisorption. The data at higher concentration, O Tig 10.14b, shows a sharp rise in adsorption that is consistent with multilayer rather than monolayer adsorption. The answer, however, lies in the conformation of the molecules at low concentrations they are in an extended form (illustrated by the schematic inset of Fig. 10.14a), while at higher concentrations they are in a more compact form and pack more efficiently on the surface (shown in the schematic of O Fig. 10.14b). Another usefiil feature that can be established qualitatively from inspection of the isotherms of O Fig. 10.14a is the heat of adsorption. The sharpness of the knee at low concentration provides an indication of this value, thus for the data of PMMA on aluminum and silicon the heat of adsorption for PMMA on aluminum is more exothermic than on silicon. This is as one would expect as the silicon surface will be rich in acidic silanol groups very receptive to the basic PMMA, whilst the aluminum oxide surface is amphoteric and will not react so readily with PMMA. 

Beryllium is readily attacked by most acids and, being amphoteric, is slowly attacked by caustic alkalis with the evolution of hydrogen. As might be anticipated, in view of the controlling influence of the surface film of beryllia on corrosion behaviour, concentrated nitric acid has little effect on beryllium , while the dilute acid results in slow attack. Hot acid is much more reactive. Nitric acid is in fact often used to pickle-off residual mild steel from hot-extruded clad beryllium. 

High polarity is one of the reasons why both the ionic and amphoteric surfactants, and especially their metabolites, are difficult to detect. This property, however, is important for the application tasks of surface-active compounds, but is also the reason for their high water solubility. Due to this fact, their extraction and concentration from the water phase, which can be carried out in a number of very different ways, is not always straightforward. Furthermore, they are often not volatile without decomposition, which thus prevents application of gas chromatographic (GC) separation techniques combined with appropriate detection. This very effective separation method in environmental analysis is thus applicable only for short-chain surfactants and their metabolites following derivatisation of the various polar groups in order to improve their volatility. 

The dependences of pH and C-potential on the adsorbed amount of M(H20)2+ at the total metal ion concentrations of 3 x10-3 mol dm-3 are shown in Figures 7 and 8, respectively. The amount adsorbed for each M2+ increases with the pH, and the inflection points are shifted toward the lower pH region in the order of Co2+, Zn2+, Pb2+, Cu2+, which corresponds to the order of the hydrolysis constant of metal ions. To explain the M2+-adsorption/desorption, Hachiya et al. (16,17) modified the treatment of the computer simulation developed by Davis et al. (4). In this model, M2+ binds coordina-tively to amphoteric surface hydroxyl groups. The equilibrium constants are expressed as... 

Since peptides are amphoteric, Zt and Zc are expected to show nonlinear dependencies on pH. Similar behavior has been observed for various synthetic peptides separated on both strong anion and strong cation HP-IEX sorbents. As a consequence, the minima in the In /t iex i versus pH plots at a defined concentration of displacing salt will not usually occur at the predicted p/ value of the peptide, but rather at another pH value. Implementation of an optimized HP-IEX separation of peptides thus requires that the sequence microlocality and extent of ionization of the surface-accessible amino acid side chains, or the N- and C-terminal amino and carboxy groups, respectively, are taken into account. 

These compounds are permanently anionic and are moderately polar (surfactants are organic molecules that are surface active). This means that they concentrate on the surface of a liquid in which they are dissolved. Generally, these types of analytes contain both a hydrophobic and a hydrophilic segment. There are anionic, cationic, neutral, and amphoteric surfactants. They may be readily sorbed from water by reversed-phase SPE. Elution requires methanol or acetonitrile rather than ethyl acetate because of their polar, ionic functional groups, which are typically sulfate esters or sulfonic acids (Fig. 7.18). 

Figure 3.20. Theoretical net surface charge of an hypothetical amphoteric oxide with PZC (point of zero charge) = 8, calculated at three concentrations (0.01, 0.1, and 1.0 M) of indifferent electrolyte.

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