The Diffuse-Ion Swarm Near Smectite Particles Suspended in 1:1 Electrolyte Solutions: Modified Gouy-Chapman Theory and Quasicrystal Formation
Garrison Sposito, 1992. "The Diffuse-Ion Swarm Near Smectite Particles Suspended in 1:1 Electrolyte Solutions: Modified Gouy-Chapman Theory and Quasicrystal Formation", Clay-Water Interface and its Rheological Implications, P. F. Low, J. K. Mitchell, G. Sposito, H. van Olphen, N. Güven, R. M. Pollastro
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Despite the long history of continual investigation of the surface and colloid chemistry of smectites (van Olphen, 1977; Sposito, 1984), the structure of the electrical double layer at smectite surfaces and its influence on the rheological properties of smectite suspensions remain topics of lively controversy. One of the most contentious issues is the partitioning of adsorbed monovalent cations among the three possible surface species on the basal planes of smectite particles, such as montmorillonite (see, e.g., Low, 1981, 1987). As illustrated in Figure 1, a monovalent cation can be adsorbed on the basal planes by three different mechanisms: inner-sphere surface complexes, in which the cation desolvates and is captured by a ditrigonal cavity; outer-sphere surface complexes, in which the cation remains solvated but still is captured by a ditrigonal cavity and immobilized; and the diffuse-ion swarm, in which the cation is attracted to the basal plane, but remains fully dissociated from the smectite surface (Sposito, 1989a, Chap. 7). Clearly, the colloidal properties of smectite particles in suspension will depend sensitively on which of the three mechanisms of adsorption predominates for a given type of monovalent cation.
The present chapter is intended to be a heuristic account of the relationship between smectite particle structure and a monovalent diffuse-ion swarm near particle basal planes. Experimental evidence is examined for aggregate structures (quasicrystals) in suspensions of smectites containing monovalent cations. The data then are used to interpret and apply the modified Gouy-Chapman theory of the electrical double layer (Carnie and Torrie, 1984). The
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Clay-Water Interface and its Rheological Implications
Rheology is the science of the flow of fluids and deformation of solids. Of special interest to the clay scientist are the flow behavior and stability of clay suspensions, and the time-dependent deformation of clays in a solid or semi-solid state. The physical state of a clay may change with increasing water content; from a solid, to a semi-rigid plastic, then to a gel, and finally to a suspension. In each state, the main factors determining the rheological behavior of the system are related to: (a) the molecular configuration and dynamics of the clay-water interface, and (b) the nature of the particle interactions at this interface. The hydration of the ions and the clay surfaces plays a special role in clay rheology because flow and deformation directly involve molecular movements along the clay-water interface.