Small molecules that have two carboxylic functions can adsorb from water on
to 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 oth
er by the n CH2 groups of the small molecule. Five-bond rings (n = 0) are t
he most stable, followed by six-bond rings (n = I) and seven-bond rings (n
= 2). Five-bond rings can also be formed with one carboxylate and one hydro
xyl group (this is the case for alpha-hydroxycarboxylates) or with one enol
ate 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 d
onors toward the metal cation. (C) 1999 Academic Press.