Modeling the Acid–Base Properties of Montmorillonite Edge Surfaces

The
surface reactivity of clay minerals remains challenging to
characterize because of a duality of adsorption surfaces and mechanisms
that does not exist in the case of simple oxide surfaces: edge surfaces
of clay minerals have a variable proton surface charge arising from
hydroxyl functional groups, whereas basal surfaces have a permanent
negative charge arising from isomorphic substitutions. Hence, the
relationship between surface charge and surface potential on edge
surfaces cannot be described using the Gouy–Chapman relation,
because of a spillover of negative electrostatic potential from the
basal surface onto the edge surface. While surface complexation models
can be modified to account for these features, a predictive fit of
experimental data was not possible until recently, because of uncertainty
regarding the densities and intrinsic pKa values of edge functional groups. Here, we reexamine this problem
in light of new knowledge on intrinsic pKa values obtained over the past decade using ab initio molecular dynamics
simulations, and we propose a new formalism to describe edge functional
groups. Our simulation results yield reasonable predictions of the
best available experimental acid–base titration data.