An interlayer expansion model for counterion-intercalated montmorillonite from first-principles calculations

From density functional theory calculations on dry and hydrated MMT, Na-MMT, NH4+-MMT, and formamide-MMT structures at various interlayer spacings, we proposed an interlayer expansion model for the counterion-intercalated MMT systems. Complex interactions exist among the counterions, lower and upper MMT surfaces, including ionic bonding, hydrogen bonding, electrostatic attraction/repulsion, and van der Waals forces. These interactions vary with different counterions and different interlayer spacings, playing a crucial role in the energy landscape of interlayer expansion. The equilibrium spacings are related to counterion volume, and the competition between interlayer electrostatic attraction and repulsion. The expansion energy potential near the equilibrium spacings can be approximately described with a quadratic function of spacing increase. At large spacings, however, the counterions have a tendency to form relatively strong interaction with either surface, making their energy potential in MMT approach to constant values. (C) 2014 Elsevier B. V. All rights reserved.