Molecular Dynamics Modeling of CO2 and Poly(ethylene glycol) in Montmorillonite: The Structure of Clay-Polymer Composites and the Incorporation of CO2

Composite materials composed of aluminosilicate clays with organic molecules or biomolecules in the interlayer galleries are readily synthesized and have many applications in agriculture, medicine, environmental science, engineering, and geochemistry. Detailed characterization of the molecular-scale structure and dynamics of the interlayer galleries is difficult experimentally because of static and dynamic disorder but can be obtained by molecular dynamics (MD) simulation using classical force fields. This Article presents an MD study of smectite clay montmorillonite (MMT) intercalated with poly(ethylene glycol) (PEG) with and without CO2 also present in the interlayer. Bulk PEG can incorporate large amounts of CO2 under supercritical conditions, and MMT PEG composites have potential as gas-separation materials. The MD simulations of anhydrous MMT containing interlayer CO2 (MMT-CO2), PEG (MMT PEG), and PEG + CO2 (MMT-PEG-CO2) provide new atomic-level insight into the molecular ordering of CO2 near the basal surface and CO2-induced changes in the structure, conformation, and energetics of PEG in the interlayer. The results show that the structural arrangement among the CO2 molecules in the MMT CO2 system is similar to that in supercritical CO2 and is analogous to that of crystalline CO2. All Na ions in this system remain coordinated by the basal oxygens (O-b) but are also coordinated by the oxygens (O-CO2) of the CO2 molecules, and a few are displaced similar to 2 angstrom above their surface sites. The cooperative motion of the Na ions and CO2 molecules increases the Na diffusion and the CO2 reorientation in the interlayers. The critical interactions among cations, CO2, PEG, and the basal surface in the MMT-PEG-CO2 system result in a layer of CO2 trapped between the basal surface and cation-permeable PEG film formed on the basal surface. In the MMT-PEG and MMT-PEG-CO2 systems, PEG is highly disordered and exhibits conformational and orientational heterogeneity in the interlayer confinement with many of the molecules lying in a layer parallel and close to the basal surface of the clay. Some of the PEG molecules span from one basal surface to the other across the interlayer. Many of the Na ions are displaced from the basal surface and are coordinated by both basal oxygens and oxygens of the PEG. Some are completely displaced from the surface and are dissolved in the PEG. In the MMT-PEG-CO2 system, the CO2 molecules occur in well-defined layers parallel to the basal surface and are most commonly dissolved in the PEG. In the full MMT-PEG-CO2 system, Na ions have few O-CO2 nearest neighbors, and the presence of CO2 causes the basal spacing to expand but does not change the conformation of the PEG. The MD results provide detailed and otherwise unobtainable information about the coordination environments and distributions of interatomic distances and angles in the interlayer.