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Effects of Permanent Charge on the Electrical Double‐Layer Properties of Clays and Oxides
Author(s) -
Kleijn W. B.,
Oster J. D.
Publication year - 1983
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1983.03615995004700040040x
Subject(s) - adsorption , helmholtz free energy , ion , aqueous solution , charge density , surface charge , chemistry , oxide , plane (geometry) , effective nuclear charge , inorganic chemistry , chemical physics , materials science , thermodynamics , geometry , physics , organic chemistry , mathematics , quantum mechanics
Two complimentary models which describe the surface chemistry of, respectively, clay minerals and oxides submersed in an aqueous solution are presented. Both models incorporate isomorphous‐substitution charge, specific adsorption of counter ions at the inner Helmholtz plane, and a diffuse double layer outside the outer Helmholtz plane. In addition, one of the models allows for adsorption of potential‐determining ions at the surface plane, which is located inward of the inner Helmholtz plane. General algorithms to solve surface chemistry models are given and methods to determine the associated empirical parameters are discussed. With the model which includes adsorption of potential‐determining ions, it is shown that isomorphous substitution may be of great importance to the adsorption properties of oxides. Based on published data for NaCl/Al 2 O 3 and KCl/SiO 2 , computations are made which show that changes in the charge density of the potential‐determining ions adsorbed at the surface plane tend to be equal in magnitude but opposite in sign to changes in the isomorphous‐substitution charge density (σ o ). However, this cancellation is not perfect, and especially for KCl/SiO 2 systems, the counter‐ion exchange capacity and its pH dependency and the zeta potential are strongly dependent on σ o . The dependence on σ o is particularly strong in the region where σ o is small (−0.04 < σ o < 0.02 C/m 2 ). This implies that small amounts of isomorphous substitution may have a dramatic effect on the physico‐chemical properties of oxides. The ion‐adsorption behavior of montmorillonite cannot be explained with our generalized model for oxide surfaces. Thus, our calculations confirm the long‐standing view that the basal planes of clay minerals do not adsorb H + and OH ‐ as potential‐determining ions.