Molecular level investigations of organoclays
This project aims to investigate the molecular organization and mobility of small organics in smectite interlayers (i.e. in lamellar pores between 1nm thick clay layers) of organo-clay nanocomposites. Smectite clays have a layered structure and an extremely high surface area, enabling intercalation of large quantities of substances leading to nanomaterials with novel and/or improved properties compared to a simple isotropic mixtures. It has been reported that clay-polymer nanocomposites demonstrate improved mechanical properties and increased heat resistance, while significantly the reducing gas permeability, flammability and biodegradability of polymers. Due to the tunable porosity and high surface area, organoclays and so-called PILC’s (pillared clays) are also currently being investigated for various catalytic reactions and H2-storage. Furthermore, clays also comprise one of the most dominant absorbents for organics in the atmosphere and in natural waters. Our ultimate goal is to improve our understanding of the main interactions controlling intercalation and surface adsorption of organics to clay mineral surfaces. Using mostly classical molecular dynamics simulations we will study adsorption and interaction behavior of clay minerals with a range of organic molecules, covering a selected range of functional groups (initially alcohols and amines), using methods developed during our previous (and ongoing) work of similarly hydrated systems, such as free energy and umbrella sampling methods, see: Holmboe and Bourg, JPC C 2014,118,1001-1013, Tinnacher and Holmboe, 2016 Geo. et Cosm. Acta 177 (2016) 130–149, Tournassat, 2016, Clays and Clay Min. pre-publication doi: 10.1346/CCMN.2016.0640403, Hellrup, Holmboe et al, submitted to Langmuir in July 2016, Holmboe, Larsson, Jamshed and Bergström, submitted to Langmuir Aug 2016. In our group at Umeå University, we have currently implemented two clay-specific forcefields, Clayff (Cygan, 2004) and the charmm-like INTERFACE ff (Heinz, 2005, 2013) (currently also evaluating TraPPE) for Gromacs, and have prior experience of OPLS/aa, gaff and charmms general ff for organic molecules. As an initial benchmarking study, we will model i) smectite solvation (of clay surfaces + counter-cations) of pure alcohol and mixed water/alcohol mixtures, to find the dependence on clay basal spacings versus loading, to aid interpretation of experimental XRD diffraction data on the corresponding systems (by subsequent calculation of structure factors and 1D XRD modeling). If successful, these simulations will also be performed for other layered materials such as graphene and graphite oxide, for which a vast amount of experimental XRD data already exists. In another project we will address ion-exchange reactions of the natural occurring cations in clays (Na+, Ca2+, K+) with small tertiary amines and lipids like CTAB, since they are known to easily undergo intercalation, replacing the existing cations. The motivation for this subproject is the potential applications of hydrophobic clay interlayers with tunable pore size. Reported batch sorption experiments have however shown that the ability for intercalation varies substantially with charge, head-group ‘bulkiness’ and change characteristics with the clay lattice - parameters that easily can be controlled in molecular dynamics simulations.