Hard Sphere Electrolyte Model for Specific Adsorption

Hard Sphere Electrolyte Model for Specific Adsorption 

Camie and Chan30 treated an ensemble of hard-sphere ions and dipoles in contact with a hard wall. Specifically, they considered solvent molecules of radius rw with a point dipole at their center, and two kinds of ions, positive and negative, both with the same radius r,. One kind of ion can be adsorbed on the surface of the hard wall by a potential proportional to a Dirac delta function. Charge transfer between the metal (i.e. the hard wall) and the adsorbate was not 

This shows that, in the absence of charge transfer, / is determined by the relative sizes (cf. the notion of the thickness ratio introduced by Grahame31) of the solvent and the adsorbed ions, and by the dielectric response at the interface. In essence, this equation shows how much image charge flows onto the metal at constant potential when an ion is adsorbed. When the solvent molecules are smaller than the adsorbed 

Even though this equation is based on an overly simple model, it illustrates well the double-layer properties that govern the electrosorption valency in the absence of pet. In particular, it shows that a fractional value of / need not necessarily indicate pet. We shall return to the hard-sphere electrolyte model when we discuss dipole moments of adsorbates. 

Since l is a thermodynamic quantity, the most reliable procedures for its determination are based on a thermodynamic analysis of adsorption data, possibly at low coverages. Adsorption data to be analyzed by the Gibbs adsorption equation can be obtained by measuring the interfacial tension y, the charge density crM or the differential capacity C. Direct y measurements are equilibrium measurements that can only be carried out on mercury. Direct charge measurements are conveniently carried out by the potential-step chronocoulometric technique, which can be 

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