Surface Complexation of Calcite by Carboxylates in Water.

Small molecules that have two carboxylic functions can adsorb from water onto calcite. The adsorption site is a -Ca+ site. The mechanism of adsorption is a complexation of the -Ca+ site by the two carboxylates, similar to the solution complexation of Ca++ ions. The complex has a ring structure where the two carboxylates are joined on one side by the -Ca+ ion and on the other by the n CH2 groups of the small molecule. Five-bond rings (n = 0) are the most stable, followed by six-bond rings (n = 1) and seven-bond rings (n = 2). Five-bond rings can also be formed with one carboxylate and one hydroxyl group (this is the case for alpha-hydroxycarboxylates) or with one enolate and one hydroxyl group (catechol). The sequence of binding strengths is enolate > carboxylate > hydroxyl; it matches the sequence of complexation efficiencies of these groups in solution and their characters as electron donors toward the metal cation. Copyright 1999 Academic Press.