The Compact Layer at Semiconductor Electrodes

The surface acid or base sites in combination with adsorbed hydrated cations or anions, represented by -0 (a) K or -OH2 (b) A , are called the interfacial ion pair [Tamura-Puruichi, 1991]. In some cases the surface acid or base site is covalent-bonded with dehydrated cations or anions to form an interfacial complex [Stumm, 1992]. 

The electrochemical standard free enthalpy, of dissociation of the surface acid or base sites consists of the chemical standard free enthalpy, AG°, an electrostatic energy, eA, and an interaction energy, m0, for the adsorption coverage in the Frumkin adsorption model is the potential across the compact layer, 0 is the adsorption coverage, and m is the Frumkin parameter [Frumkin, 1925]  

The equilibria of dissociation of the surface acid and base sites are, respectively, represented by Eqn. 5-80 and Eqn. 5-81  

In general, the associated acid sites of -OH(a) and the dissociated (ionized) base sites of -OH2 (b) predominate in acidic solutions, while the associated base sites of -OH(b) and the dissociated add sites of -0 (a) predominate in basic solutions. It then follows that the hydroxsdated solid interface of semiconductor electrodes is positively charged in acidic solutions and negatively charged in basic solutions. 

Assuming that the activities of the interfacial addle and basic sites remain nearly constant in the range of the coverage from 6 = 0.1 to 0.9 and that the relation of [K, ] [A, ] holds in aqueous solutions, we obtain Eqn. 5-82 from Eqns. 5-80 and 5-81  

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