Direct Ionization of Surface Groups

Materials containing surface groups that can be directly ionized, but in which one of the ions is permanently bound to the surface, illustrate a second important mechanism for the development of surface charge. This group of materials includes many metal oxides as well as many polymer latexes (Fig. 5.1b). Some metal oxides are amphoteric in that they can develop either negative or positive surfaces, depending on the pH of the solution. Such surfaces will obviously exhibit a characteristic point of zero charge such as that found for the silver 

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Direct ionization of surface groups. This is a common mechanism in ionizable polymeric surfaces that contain such groups in their molecules as carboxylic or sulfonic acid.s, sulfuric acid esters, or amino and quaternary ammonium groups. 

An interface may acquire an electrical charge by one or more of several mechanisms, the most common of which include (1) preferential (or differential) solution of surface ions, (2) direct ionization of surface groups, (3) substi-... 

FIGURE 5.1. The principle sources of surface charge in solids include (a) differential ion solubility phenomena, (h) direct ionization of surface groups, (c) isomorphous substitution of ions from solution, and (d) speciflc-ion adsorption from the solution phase (e) anisotropic crystal lattice structures. 

Figure 10.11 Sources of surface charge in colloids, (a) Differential ion solubility (b) direct ionization of surface groups (c) isomorphous substitution (d) specific ion adsorption and (e) anisotropic crystals. Adapted from Myers (1999), with permission from John Wiley Sons, Ltd...

Figure 10.9 Charged interfaces. The interface charge causes a potential difference ( io) between the interface and the bulk of the fluid. This is usually a few hundredth of a volt (mV range) unfortunately, it is not directly measurable but it is often approximated by the measurable "zeta " potential. The upper part of the figure could be, for example, Agl and the lower part could be, for example, ionization of surface groups in latex or biosurfaces (proteins). Adapted from Wesselingh et al. (2007), with permission from John Wiley Sons, Ltd...

For Vivak polyester channels, alkaline hydrolysis of surface groups (e.g., ester) to ionizable groups (e.g., carboxylate) has produced a more reproducible EOF. In combination with a dynamic coating (e.g., CTAB), EOF may be either eliminated or reversed in direction, depending on the CTAB concentration [204]. 

Silyl-substituted bicyclobutonium ions are also accessible from direct ionization of 3-silyl-sub-stituted cyclobutyl chlorides. Matrix co-condensation of cw//ra s-3-(trimethylsilyl)cyclobutyl chloride (Scheme 1, D (R = R = Me)) with SbFs onto a surface of SO2CIF/SO2F2 at -196°C yields after homogenization at -130°C a yellow solution of carbocation 12. The C-NMR spectrum obtained for cation 12, except for the alkyl groups at silicon, is very similar to the C-NMR spectrum of carbocation 9. The differences of the signals for the Co-, Cp/Cp -carbons are smaller than 1 ppm. 

For typical aqueous colloidal dispersions, the particles may carry some charges most likely due to the preferential (or differential) dissolution of particle surface ions, direct ionization of particle surface groups, substitution of particle surface ions, specific adsorption of ions, and particle surface charges originating from specific crystal structures. 

The pH is an important factor that can influence the ionization of the surface silica groups. As a result, C is directly dependent on the pH. Therefore, the relationship of /teof as a function of pH is governed by the behavior of the dissociation of the silanol groups. Different capillary materials result in different profiles of the electroosmotic mobility as a function of the pH (due to differences in Q. Typically a sigmoid curve behavior resembling the titration curve of the surface active groups is observed. ... 

In a related approach, arrays with different types of surface chemistries such as hydrophobic, hydrophilic, anionic, and affinity are used to absorb certain protein groups from biological or patient samples. The chip-absorbed proteins are then directly detected by surface-enhanced laser desorption/ionization time-of-flight MS (SELDl-TOF MS) (Issaq et al. 2002). The resulting protein masses can be used in pattern analysis and thereby provide a useful diagnostic tool. 

The intrinsic constants are thermod3mamic constants written for reactions occurring at a hypothetical isolated site on the surface. Actual activities on the surface cannot be directly determined but Q or apparent stability quotients can be calculated based on measurable bulk concentrations. The intrinsic constants and apparent stability quotients are related by considering the electrostatic correction for an ion in solution near the surface compared to an isolated ion on the surface. In an idealized planar model, is the mean potential at the plane of surface charge created by the ionization of the surface functional groups and the formation of surface complexes and is the mean potential at the plane of adsorbed counter ions at a distance 3 from the surface (17). The electrostatic interaction energies at the surface and at a distance 3 are expressed as exponentials. Therefore ... 

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