Evaluation of the Structural Formula and Alkylammonium Methods of Determining Layer Charge
David A. Laird, 1994. "Evaluation of the Structural Formula and Alkylammonium Methods of Determining Layer Charge", Layer Charge Characteristics of 2:1 Silicate Clay Minerals, A. R. Mermut, S. A. Boyd, W. J. Farmer, W. F. Jaynes, G. Lagaly, D. A. Laird, A. R. Mermut
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The structural formula and alkylammonium methods are completely independent analytical techniques for determining the layer charge of 2:1 expanding phyllosilicates (i.e., smectites and vermiculites). Other methods of determining layer charge are empirical and rely on calibrations with values of layer charge determined by either the alkylammonium or structural formula methods. The intent of this chapter is to provide a critical evaluation of both the alkylammonium and structural formula methods. Theoretical bases for these methods are discussed as are the theoretical and practical problems inherent to each method. A direct comparison of values of layer charge determined by the structural formula and alkylammonium methods is presented.
Within the context of this review, "tetrahedral charge" and "octahedral charge" are defined as the net negative charge due to isomorphous substitution in the tetrahedral and octahedral sheets of 2:1 phyllosilicates, respectively. "Permanent charge" is the sum of thetetrahedral and octahedral charges. "Variable charge" is the net negative charge, which is pH dependent, associated with broken bonds on lateral edges of 2:1 phyllosilicates, and "total charge" is defined as the sum of the permanent and variable charges. Although, the term "layer charge" is widely used in discussions of 2:1 phyllosilicates it is rarely defined, In some instances layer charge is used synonymously with permanent charge, whereas in others layer charge is identified with total charge. Most commonly, however, layer charge is operationally defined by a particular analytical method, and that is the sense in which the term is used in this chapter.
Layer charge is
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Layer charge is recognized as the single most important characteristic of 2:1 phyllosilicates. It indicates a mineral's capacity to retain cations and to adsorb water and various polar organic molecules. The affinity of organic-clays for sorption of organic contaminants in aqueous systems is now attracting considerable attention. The layer charge is also considered an important criterion for the classification of 2:1 silicate clay minerals. It is generally agreed that the magnitude of the layer charge can be taken into account to resolve the classification problem. Studies on structural chemistry also confirm the importance of layer charge for characterization of silicate clays.”