Surfaces, charging hydroxylated

Charging Hydroxylated Surfaces Definition of IEP(s). A hydroxyl-ated surface should be expected on all oxidic materials which have had a chance to come to equilibrium with an aqueous environment. Charge can develop on a hydroxylated surface through amphoteric dissociation of the surface hydroxide groups. Dissociation reactions can be written as follows, where underscored symbols refer to species forming part of the surface. Symbols not underscored refer to species assumed aqueous unless otherwise specified. 

When a metal oxide surface is exposed to water, adsorption of water molecules takes place as shown in Equation 2.1. Cation sites can be considered as Lewis acids and interact with donor molecules like water through a combination of ion-dipole attraction and orbital overlap. Subsequent protonation and deprotonation of the surface hydroxyls produce charged oxide surfaces as shown in Equation 2.2 and Equation 2.3, respectively ... 

Point of zero charge PZC (-) Surface ionization constants ApK (-) Dielectric constant of the oxide e (-) Hydroxyl site density Ns (1/nm2)... 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

The interfacial excess charge of semiconductor electrodes consists of the space charge osc on the semiconductor side the charge of surface states o the charge Oh of interfacial hydroxyl groups the charge Oms of adsorbed ions in the compact... 

The clay minerals are layer structures (38, 39, 86) incorporating, for example, sheets of interconnected Si04 tetrahedra cross-linked with sheets of A10<5 octahedra. Particles have broken bonds only on the edges of sheets. The broken bonds hydroxylate they and structural OH on the layer surfaces can dissociate to produce a pH-dependent charge in the same way as described for the surfaces of simpler oxides. Charge from this source may be referred to as originating in the hydroxylated surface. 

Zero Point of Charge. We are operating on the assumption that pH-dependent surface charge arises predominantly from H+ adsorption by reactions of the hydroxylated surface similar to Reactions 1 and 3. Figure 4 shows schematic H+ adsorption isotherms illustrating the pH variation of surface charge and H+ adsorption density, Y/, associated with this source. Curve a represents the total amount of adsorption on a given amount of surface area. Curve b represents more surface. In both... 

The IEP(s) characteristic of the hydroxylated surface is the pH at which the hydrogen ion adsorption density on the hydroxylated surface sites alone is zero. To achieve zero total charge when 07 5 0, however, Yh must be increased by adjusting the pH toward the acid side of the IEP(s) until eVH = —07 hence the ZPC is smaller than the IEP(s). 

In developing Equations 13 and 18 it was assumed that a hydrogen ion adsorption model would adequately describe charge development by either mechanism—i.e., amphoteric dissociation of hydroxylated surfaces or adsorption of hydroxo complexes from solution. This assumption was... 

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