Contact Activation By Ellagic Acid - The Concept Of Soluble Activator Disputed

Ellagic acid (EA) has often been described as a “soluble” activator of the contact mechanism in contrast to kaolin, silica or glass which are insoluble. We have found that EA is not effective as a contact activator when in true solution but is active only when present as a particulate solid or dispersion.

A pH titration of EA was carried out and it was found that EA dissolved readily in alkaline solutions. When adjusted to neutral pH these formed supersaturated solutions which slowly precipitated forming fine microcrystalline dispersions. At high EA concentrations there was concurrent development of turbidity and contact activity. At lower EA levels no turbidity was visible, yet contact activity increased on standing, presumably as EA particle size increased. Contact activity was reduced in all ellagic acid preparations by centrifugation after standing.

Substancesof low molecular weight in true solution do not sediment under these conditions and do not show turbidity. These are properties of particles or aggregates. These observations support the old theory that only particulate materials providing a liquid/negatively charged solid interface of substantial size function as true activators.

It is probably important to distinguish between activation of the contact mechanism by substances which may be in true solution such as various organic sulphates and those which are dispersions of solids, functioning possibly by a different mechanism.