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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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