Surface charge development

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

We do not know the details at the atomic scale how the surface charge development on a carbonate mineral is established, but formally and schematically one could visualize the following type of charging reactions for a hydrous surface... 

Zeltner,W.A. Anderson, M.A. (1988) Surface charge development at the goethite/aqueous solution interface. Effects of CO2 adsorption. Langmuir 4 469—474... 

Of course, surface charge development has been well documented in the carbon black literature [36, where particle coagulation is the key issue. Thus, for example. Kratohvil and Matijevic [56] measured electrophoretic mobilities and concluded (without alluding to the work of Frampton and Gortner) that [c]arbon particles carry a positive charge at lower pH values." They also studied the pH dependence of the adsorption of a nonionic and an anionic surfactant as well as of electrolytes with counterions of varying charges. 

Charge development on solid surfaces usually results from coordinative interactions at the solid surface. The surface coordinative model describes quantitatively how surface charge develops and permits us to incorporate the central features of the electric double-layer theory. 

Bebie, J. et al.. Surface charge development on transition metal sulfides An electrokinetic study, Geochim. Cosmochim. Acta, 62, 633, 1998. 

FIG. 2 Principal features of the activated carbon surface that give rise to surface charge development in aqueous solution. 

The simplified lattice ion hydration theory describes surface charge development by nonreactive ionic solids placed in water. This theory applies to ionic solids that do not react with water to form weak surface acid groups (hydrolysis) and to ionic solids that do not undergo surface oxidation reactions. Either reaction type would modulate the surface charge that the solid develops in water. The only reaction of consequence then is the hydration of lattice ions, and the differential hydration of these lattice ions at the surface of the solid determines the sign of the surface charge. 

Beattie JK, Best SP, Skelton BW, White AH (1981) Structural studies on the cesium alums, CsM(lll)[S04]2l2H20. J Chem Soc - Dalt Tran 10 2105-2111 Bebie J, Schoonen MAA, Strongin DR, Fuhrmann M (1998) Surface charge development on transition-metal sulphides An electrokinetic study. Geochim Cosmochim Acta 62 633-642 Beimett CH (1975) Exact defect calculations in model substances. In Diffusion in Solids Recent Developments. Nowick AS, Buron JJ (eds), p 73... 

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