Intrinsic surface charge

Intrinsic surface charge density, defined by the number of Coulombs per square meter bound by surface functional groups, either because of isomorphic substitutions, or because of dissociation/protonation reactions. 

It is useful to group the two surface charge densities defined thus far into the intrinsic surface charge density [4]... 

The intrinsic surface charge density reflects particle charge developed from either isomorphic substitutions or adsorption involving H+ or OH-. A widely used technique for measuring intrinsic surface charge density is the Schofield method. In this method [3], clay mineral particles are reacted with an electrolyte solution (e.g., NaCl) at a given pH value and ionic strength the specific surface excess of the cation and the anion adsorbed from the electrolyte is determined and the value of is calculated with the equation... 

The Poisson-Boltzman (P-B) equation commonly serves as the basis from which electrostatic interactions between suspended clay particles in solution are described ([23], see Sec.II. A. 2). In aqueous environments, both inner and outer-sphere complexes may form, and these complexes along with the intrinsic surface charge density are included in the net particle surface charge density (crp, 4). When clay mineral particles are suspended in water, a diffuse double layer (DDL) of ion charge is structured with an associated volumetric charge density (p ) if av 0. Given that the entire system must remain electrically neutral, ap then must equal — f p dx. In its simplest form, the DDL may be described, with the help of the P-B equation, by the traditional Gouy-Chapman [23-27] model, which describes the inner potential variation as a function of distance from the particle surface [23]. 

One refers to the intrinsic surface charge density as ffi = Oq + and sometimes to the Stem surface charge density as as = ais -t- aos. 

The chemical interpretation of o-in measured by the Schofield method depends sensitively on the type and concentration of probe electrolyte used. If these properties are chosen so that the cation in the reacting electrolyte neutralizes precisely the exposed functional group charge associated with isomorphic substitutions and dissociated hydroxyls and so that the anion neutralizes only the exposed protonated functional groups, then q+ and q. will have optimal magnitude for the chosen pH value and CTjn will be truly an intrinsic surface charge density. On the other hand, if the cation in the probe electrolyte is not able to displace all of the native adsorbed cations in, e.g., inner-sphere surface complexes, or if the anion cannot displace all of the native anions bound to protonated functional groups, or if either ion does not form only neutral surface complexes in the soil clay, then Ojn will differ from its optimal value. 

A detailed model of the interfacial region requires the specification of the position of the plane where the diffuse ion swarm begins, A popular choice in the literature of soil chemistry has been jc = 0, which means that outer-sphere surface complexes are neglected entirely and inner-sphere surface complexes are ignored if they would protrude beyond the plane to which (Tin, the intrinsic surface charge density, refers. (See Secs. 1.5 and 3.1 for a discussion of trjn ) That this choice is not reasonable physically, however, can be seen from a simple calculation involving Eq. 5.16. Consider a 1 1 electrolyte at the concentration Cq == 100 mol m" and suppose that /r(0) = SRT/F, a value that is not unrealistic for a smectite siloxane surface. Then k = = 1.04 x 10 m" at 298 K, a =... 

Butman, M.F., Smirnov, A. A., Kudin, L.S., and Munir, Z.A. (2000) Determination of the sign of the intrinsic surface charge in alkali halides from ionic sublimation measurements, Surf. Sci. 458, 106. Using an electric field to study ion vaporization. 

The particles have a characteristic double-layer structure that is determined by the intrinsic surface charge, specifically adsorbed ions and ions compensating the surface... 

The surface charge characterization of clay minerals, when permanent charges from isomorphic substitutions of ions in a clay crystal lattice are present besides the variable edge charges, is more complicated than that of metal oxides. In this case, the intrinsic surface charge density, Cin, can be defined as the sum of the net permanent structural charge density, oq, and the net proton surface charge density, ffo.H, i-C-, [2,... 

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