Coagulation by multivalent counterions and the Schulze-Hardy rule.

Starting from a number of empirical papers by Matijević et al., the issue of coagulation of hydrophobic colloids by multivalent inorganic counterions is revisited. More recent information is also considered, and some important trends are confirmed and made more quantitative. It can be concluded that the high coagulating power of multivalent ions is mostly not a direct result of their high valence but caused by their propensity of building complexes with water that adsorb specifically. This mechanism requires a revision of the Schulze-Hardy rule interpretations. In addition, this finding helps to define conditions where overcharging by multivalent counterions can be attributed to ion correlations.