Polyvalent cation bridging

Polyvalent cation bridging may be direct [scheme (1.1)] or effected by hydrogen bonding to water molecules in the primary hydration shell of a polyvalent cation [scheme (1.2)] ... 

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

In combination with clay minerals, such as through bridging by polyvalent cations (clay-metal-humus), hydrogen bonding, van der Waal s forces, and in other ways as discussed by Greenland (1971) and Theng (1979). 

In the presence of polyvalent cations, the electric potential can be affected by the formation of bridges between cations and the adsorbents or by the formation of organic phosphate salts that precipitate on the reacting surfaces. For instance, with calcium ions the adsorption of myo-inositol hexakisphosphate increases above pH 5,... 

FIGURE 9.13. In systems of negatively charged surfaces and anionic snrfactants, the presence of polyvalent cations snch as Ca+ and AF, can prodnce bridging phenomena that result in strong snrface-snrfactant interactions that would not occur in the presence of monovalent ions such as Na+ or K+. 

Figure 10.12. The role of polyvalent ions as potential bridging ions for the adsorption of anionic surfactants on negatively charged surfaces (a) normal adsorption of a surfactant on a surface of the same electrical charge (b) cation bridging leading to inverted adsorption.

As far as the significance of the ion valency is concerned one can among other things, consider also that polyvalent ions, can form bridges between ionised groups of two or more neighbouring colloid ions. Nevertheless polyvalency of the micro ion is not a conditio sine qua non since cases are also known in which monovalent ions (e.g. dye cations, picrate anions) cause complex coacervation or flocculation (see further p. 404, 3h). 

Numerous studies have been made of the interaction of polyvalent metal ions with the silica surface. The development of Zr-O-Si bridges has been examined by Nikolskii et al. (432), who concluded that as the basic Zr ions polymerized they were less reactive. Healy, cooper, and James (433) found that Fe and Cr are adsorbed on silica particles, giving them a positive charge. The chemistry of hydrolysis of cations Al, Zr, Th, Cr has been summarized by Baes and Mesmer (434). Matijevic, Janauer, and Kerker (435) have shown that it is the polymeric hydrolyzed species in aluminum nitrate solution and not the single AF ion that can cause charge reversal in silver halide sols. The same is true in charge reversal of silica sols. 

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