Charging Behavior

Equation 3.74 is a parameterization of ctm =/ (cp ) based on the Stern-Grahame double layer model. This relation requires as input the potential of zero charge and the Helmholtz capacitance. As mentioned, finding =f( p is complicated by the formation of various surface and bulk oxide species, occurring simultaneously with double layer charging. 

It is questionable whether a potential of zero charge could be defined uniquely and measured unambiguously. Indeed, in spite of a tremendous interest in the literature, this subject has remained inconclusive. The presence of adsorption processes led to the definition of a potential of zero total charge pP and a potential of zero free 

The approach of Chan and Eikerling (2011) emphasizes the importance of charging phenomena at the metal-solution interface, but it does not account for the intricate and largely unsettled effects that could arise from the progressive oxidation of Pt at high 0. It treats the potential of zero charge as a variable parameter that could attain values in the range from = 0.3 to 1.1 she- 

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The current chapter discusses mechanisms of charge transfer reactions in double-stranded DNA (we will not deal with single-stranded DNA, or with individual bases or base-pair structures). We will also focus on interpretation of excess charge behavior in DNA molecules in terms of accepted theoretical models. 

We begin with a discussion of the most common minerals present in Earth s crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is (1) a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH (2) the nature of the electrical double layer and how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid and (3) dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. We finish with a discussion of some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces. 

Chronocoulometry as just described involves measurement of charge after what is frequently referred to as the forward potential step. A logical extension of the method is to step the potential to its original value or some other value Ef at which the redox process is reversed, and to monitor the resulting charge behavior (see Fig. 3.5) [13]. 

As noted, metal oxides are not the only minerals with variable charge behavior. Kaolinite, an aluminosilicate, may attain as much as 50% of its negative charge by deprotonation of terminal oxygens and hydroxyls at high pH (pH 7). 

Name the various functional groups of (a) clay mineral surfaces and (b) soil organic matter. Explain which of these functional groups exhibits constant charge or variable charge behavior and discuss the practical significance of this behavior. 

Lumbanraja, J. and V. P. Evangelou. 1991. Influence of acidification and liming on surface charge behavior of three Kentucky subsoils. Soil Sci. Soc. Am. J. 54 26-34. 

As already stated in Section V.2., materials such as acceptor doped Zr02, Ce02 and SrTi03 show a space charge behavior that is characterized by a depletion of oxygen vacancies leading to a depression of the ionic conductivity.158... 

A. C. C. Plette, W. H. Van Riemsdijk, M. F. Benedetti, A. van der Wal, pH Dependent Charging Behavior of Isolated Cell Walls of a Gram-Positive Soil Bacterium, Journal of Colloid and Interface Science 173, 354-363 (1995). 

Many synthetic materials, especially monodispersed particles (for details cf. Section III.2) are prepared in the presence of strongly adsorbing ions. It is often impossible to entirely remove these ions from the final material, even after multiple washing cycles, dialysis, etc. For example, a substantial difference in surface charging behavior between indium hydroxide and oxide prepared in the presence of sulfate on the one hand and nitrate on the other has been reported [31]. 

Figures 5.2-5.5 show that the non-electrostatic model completely fails for alumina (one example of relatively good agreement between the calculated and experimental charging curve in Fig, 5.5 is probably a fortuitous coincidence). On the other hand the sigmoidal model curves roughly reflect the charging behavior of silica, especially at low ionic strengths. For silica, the number of adjustable parameters in the model can be reduced to one by fixing the K or N, . Figures 5.11-5.15 show the model curves calculated for log K (reaction 5.25) = 8 (fixed value). The best-fit values are summarized in Table 5.6.

Therefore it would be much desired to derive the concentrations and acidity constants of particular types of surface sites from some first principles, rather than to fit them as adjustable parameters. This would produce generic charging curves for certain material (rather than for specific sample). Unfortunately verification of such a model is rather difficult in view of contradictory surface charging curves reported in the literature for different samples of the same material (cf Figs. 3,43-3.73). The surface charging depends on many variables, e.g, the PZC is temperature dependent, and the absolute value of ag depends on the nature of the counterions (K versus Na, etc.). Consideration of all these variables would be very tedious, and such effects are mostly ignored in the attempts to predict the surface charging behavior of materials. 

The most successful attempt to derive the surface charging behavior from the properties of the material is known as MUSIC [42] (MUltiSIte Complexation) model. Because of many refinements following the original model, MUSIC should be rather considered as a general framework. 

The uptake(pH) curves were generated for two hypothetical materials, namely, alumina and silica whose surface area is 100 m /g. and whose pristine surface charging behavior corresponds to the model curves presented in Figs. 5.72 and 5.85, respectively. The model curves representing specific adsorption of Pb on these materials at low initial concentration ([surface sites] > > [total Pb]) were calculated to produce pHso S for 10 g solid/dm and in the presence of 10 mol dm inert electrolyte solution. For sufficiently low total Pb concentration the adsorption isotherms at constant pH are linear and the course of the calculated uptake(pH) curves is independent of the Pb initial concentration. The above solid to liquid ratio is typical for studies of specific adsorption, and pHjo = 5 is a realistic value for Pb adsorption on silica and alumina at this solid to liquid ratio, in view of the results of actual adsorption experiments compiled in Table 4.1. Lead has higher affinity to solid surfaces than most other divalent metal cations. The choice of the model curves from Fig. 5.72 (alumina) and 5.85 (silica), rather than model curves derived from any other set of experimental data analyzed in Section III, or calculated using any other model than TLM, or any other set of TLM parameters was arbitrary. This choice does not imply that TLM is favored over other models or that the experimental data used to derive these model curves are more reliable than other results used as examples in Section III,... 

Hiemstra, T., and van Riemsdijk, W. H. (1991). Physical chemical interpretation of primary charging behavior of metal (hydr)oxides. Colloids Surf. 59, 7-25. 

Inspection of the resistivity values shows a definite trend, or correlation, with charging behavior. Thus, one may estimate that contact charging drops off for powders below about 10 ohm-cm, while contact charging becomes predominant above about lO . 

Titania occurs as rutile or anatase, and studies of brookite are rare. Reference [1973] suggests that the PZC of rutile is lower than that of anatase by 1 pH unit. A compilation of PZCs/IEPs focused on the difference between rutile and anatase can be found in [1974]. lEPs are compiled in [404]. Electrokinetic and surface charging behavior of titania is reviewed in [734]. 

Two types of correlation have been investigated between a pH-dependent property of a certain material and the surface charging behavior of that material (based on authors own measurements with the same sample of material), and between a pH-independent property in a series of materials and the PZCs in that series of materials. The second type of correlation was studied using authors own experimental data, data from the literature, or a mixture of both. 

The solution chemistry of nonaqueous solvents is very different from that of water-rich mixed solvents. pH measurement in nonaqueous solvents is difficult or impossible. Salts often show a limited degree of dissociation and limited solubility (see [132] for solubility of salts in organic solvents). Ions that adsorb nonspecifically from water may adsorb specifically from nonaqueous solvents, and vice versa. Therefore, the approach used for water and water-rich mixed solvents is not applicable for nonaqueous solvents, with a few exceptions (heavy water and short-chain alcohols). The potential is practically the only experimentally accessible quantity characterizing surface charging behavior. The physical properties of solvents may be very different from those of water, and have to be taken into account in the interpretation of results. For example, the Smoluchowski equation, which is often valid for aqueous systems, is not recommended for estimation of the potential in a pure nonaqueous solvent. Surface charging and related phenomena in nonaqueous solvents are reviewed in [3120-3127], Low-temperature ionic liquids are very different from other nonaqueous solvents, in that they consist of ions. Surface charging in low-temperature ionic liquids was studied in [3128-3132]. 

Plette, A.C.C. et al., pH dependent charging behavior of isolated ceh wahs of a Gram-positive soil bacterium, 7. Colloid Interf. Sci., 173, 354, 1995. 

Whitman, P.K. and Eeke, D.L., Comparison of the surface charge behavior of commercial sihcon nitride and sihcon carbide powders, 7. Am. Ceram. Soc., 71, 1086, 1988. 

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