Coagulation by simple multivalent counterions

The origin of the strong screening power of multivalent counterions in electric double layers is reconsidered. Even for simple multivalent ions like La3+ and Th4+ more than one mechanism may be held responsible. The most critical alternatives are adsorption by specific non-electrostatic forces and enrichment by ion correlations. Many multivalent ions tend to hydrolyse in water, thereby producing highly absorbable hydrolysis products. This leads to a pH dependence of overcharging by specific adsorption, but as ion correlations are virtually independent of pH, this influence helps to discriminate between the two mechanisms. Some critical literature illustrations show that the number of publications that prove the effect of ion correlations unambiguously is very limited.