Aqueous versus surface complexes

Reviews (9, 63, 64) of the reactions between hydroxylated mineral surfaces and aqueous solutions brought out the richness of variety found in surface phenomena involving natural particles. Isolated surface complexes, the principal topic of this chapter, are expected when reaction times are short and the adsorbate content is low [Figure 6, inspired by Schindler and Stumm 63)]. Thus, surface complexes occupy a reasonably well-defined domain in the tableau of reaction time scale versus sorbate concentration. Localized clusters of adsorbate (47, 48, 65, 66) that contain two or more adsorbate ions bonded together can form if the amount sorbed is increased by accretion or bv the direct adsorption of polymeric species (multinuclear surface complexes). Surface clusters can erase the hyperfine structure in the ESR spectrum of an immobilized adsorbate (33, 67) or produce new second-neighbor peaks from ions like the absorber in its EXAFS spectrum (47, 66). 

IEP and SIMS characterizations can be used to compare the particle surface compositions dry and in aqueous solution. In Figure 11, the initial Si surface coverages and the IEP data are plotted together versus the amount of silica added. Schwarz et al. (9) showed a linear relationship between zero point of charge and silica content in a set of aluminosilicates. A more complex relationship is suggested by the curves in Figure 11 for these coated alumina particles, but the data suggest similar surface compositions in both dry and aqueous environments. 

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