Physics – Condensed Matter – Soft Condensed Matter
Scientific paper
1999-11-30
Electrochimica Acta, 46, p. 221 (2000)
Physics
Condensed Matter
Soft Condensed Matter
18 pages, 7 postscript figures included. Submitted to Electrochimica Acta
Scientific paper
The behavior of electrolyte solutions close to a charged surface is studied theoretically. A modified Poisson-Boltzmann equation which takes into account the volume excluded by the ions in addition to the electrostatic interactions is presented. In a formal lattice gas formalism the modified Poisson-Boltzmann equation can be obtained from a mean-field approximation of the partition function. In an alternative phenomenological approach, the same equation can be derived by including the entropy of the solvent molecules in the free energy. In order to visualize the effect of the steric repulsion, a simple case of a single, highly charged, flat surface is discussed. This situation resembles recent adsorption experiments of large ions onto a charged monolayer. A simple criterion for the importance of the steric effects is expressed in terms of the surface charge density and the size of the ions. It is shown that when these effects are important a saturated layer is formed near the surface. A modified Grahame equation relating the ion concentration at the surface to the surface charge density is obtained.
Andelman David
Borukhov Itamar
Orland Henri
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