Structure and cleavage energy of surfactant-modified clay minerals: Influence of CEC, head group and chain length

Abstract

The interlayer structure and cleavage energy of surfactant-modified clay minerals at the molecular level were determined for 50 different alkylammonium-modified montmorillonites of different cation exchange capacity (CECs of 91 meq/100 g and 143 meq/100 g), head group structure, and chain length using molecular dynamics simulation with specifically developed sampling techniques. The interlayer density and the cleavage energy are lower for partially packed, flat-on alkyl multilayers in the interlayer space and higher for densely packed, flat-on alkyl multilayers in the interlayer space, in agreement with trends in gallery spacing and molecular conformation. Particularly high cleavage energies, in excess of 60 mJ m⁻², are found when charged head groups of surfactants are not vertically separated between the clay mineral layers at equilibrium distance due to strong contributions from Coulomb energy. A widely tunable range of cleavage energies between 25 and 210 mJ m⁻² was identified by simulation, whereas experimental data on the separation of modified layered clay minerals remain difficult to obtain. Alkylammonium-modified clay minerals with the lowest cleavage energy are of particular interest as additives for nanocomposites with increased potential for exfoliation without further chemical functionalization.

Keywords:

• surface reconstruction
• layered material
• nanocomposite
• nanostructured thin film
• interfacial thermodynamics
• molecular dynamics simulation
• mineral surface

Acknowledgements

We acknowledge support from the Air Force Research Laboratory, Wright-Patterson AFB, Ohio, the University of Akron, the Ohio Department of Development, and the allocation of computer resources at the Ohio Supercomputing Center.