H. Chris Greenwell

On the application of computer simulation techniques to anionic and cationic clays: A materials chemistry perspective

The use of computational methods for the study of clay minerals has become an essential adjunct to experimental techniques for the analysis of these poorly ordered materials. Although information may be obtained through conventional methods of analysis regarding macroscopic properties of clay minerals, information about the spatial arrangement of molecules within the interlayers is hard to obtain without the aid of computer simulation. The interpretation of experimental data from techniques such as solid-state nuclear magnetic resonance or neutron diffraction studies is considerably assisted by the application of computer simulations. Using a series of case studies, we review the techniques, applications and insight gained from the use of molecular simulation applied to the study of clay systems (particularly for materials applications). The amount of information that can be gleaned from such simulations continues to grow, and is leading to ever larger-scale and hence more realistic classical and quantum mechanical studies which promise to reveal new and unexpected phenomena.

Preparation of zinc oxide free, transparent rubber nanocomposites using a layered double hydroxide filler

A layered double hydroxide (LDH) mineral filler particle has been designed and employed in rubber vulcanization to prepare a more environmentally friendly rubber composite. The LDH delivers zinc ions in the vulcanization process as accelerators, stearate anions as activators and simultaneously the mineral sheets act as a nanofiller to reinforce the rubber matrix whilst totally replacing the separate zinc oxide (ZnO) and stearic acid conventionally used in the formulation of rubber. This method leads to a significant reduction (nearly 10 times) of the zinc level and yields excellent transparent properties in the final rubber product. The morphological characterization, rheometric curing behaviour, mechanical properties and uniaxial multi-hysteresis behaviours of the resultant rubber/LDH nanocomposite are studied in this paper.

Intercalation and in situ polymerization of poly(alkylene oxide) derivatives within M+-montmorillonite (M = Li, Na, K)

We have synthesized a range of montmorillonite-based clay–polymer nanocomposites by intercalation of a variety of functionalized molecules having poly(ethylene oxide) and poly(propylene oxide) backbones from aqueous solution using a facile batch process. We focus on montmorillonite clays charge-balanced by cation exchange with Li+ and K+, but otherwise unmodified. Analysis by X-ray diffraction and thermal methods showed that intercalation occurred in all cases and that the composites displayed a range of interlayer spacings and organic content, from monolayer arrangements to pseudo-trilayer arrangements. Intercalated K+-montmorillonites had a propensity to exfoliate, in marked contrast to their resistance to swelling by water. Large-scale molecular dynamics simulations of selected composites were used to elucidate possible interlayer arrangements of the composites. Materials property studies showed that these clay–polymer composites had significantly increased Youngs moduli compared to the unfilled polymer.

A one-pot synthesis of hybrid organo-layered double hydroxide catalyst precursors

Organo-layered double hydroxides (LDHs) have attracted much attention recently for their utility as solid base catalysts, providing an environmentally friendly heterogeneous phase alternative to conventional stoichiometric liquid bases. Here we describe a synthetic approach for the production of layered double hydroxides containing organic (acetate) anions by a method that requires no excess of base, filtering or washing of the product. A mixture of a slurry of aluminium trihydroxide (or its thermally treated form), magnesium oxide, and magnesium acetate are reacted under atmospheric or autogenous conditions to prepare MgAl-acetate LDHs of general formula Mg1−xAlx(OH)2(CH3CO2)x·yH2O. Using this synthetic procedure it has been found that MgAl-acetate LDHs with either a full or partial acetate loading may be prepared for a range of Mg/Al stoichiometries. The LDHs formed have been characterised by X-ray diffraction, infra-red spectroscopy, solid state nmr and thermal methods. Depending on the synthesis method, the acetate LDHs had an interlayer spacing of between ca. 8.4 and 14.8 A indicating the formation of monolayers and bilayers of acetate anions, respectively, in the LDH host.

Ion adsorption at clay mineral surfaces : the Hofmeister series for hydrated smectite minerals.

Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form: K+ > Na+ > Ca2+ > Cs+ > Ba2+The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide-soil interactions, and enhanced oil recovery.

Mineral surface chemistry control for origin of prebiotic peptides

Some seventy years ago, John Desmond Bernal proposed a role for clays in the origin of life. While much research has since been dedicated to the study of silicate clays, layered double hydroxides, believed to be common on the early Earth, have received only limited attention. Here we examine the role that layered hydroxides could have played in prebiotic peptide formation. We demonstrate how these minerals can concentrate, align and act as adsorption templates for amino acids, and during wetting—drying cycles, promote peptide bond formation. This enables us to propose a testable mechanism for the growth of peptides at layered double hydroxide interfaces in an early Earth environment. Our results provide insights into the potential role of mineral surfaces in mimicking aspects of biochemical reaction pathways.Clay is thought to have played a part in the origin of life. Here, the authors show that layered double hydroxides, a type of clay little studied despite its presumed prevalence on the early Earth, can facilitate the formation of small proteins.

Understanding surface interactions in aqueous miscible organic solvent treated layered double hydroxides

Layered materials are of interest for use in a wealth of technological applications, many of which require a high surface area for optimal properties and performance. Recently, an industrially scalable method to create high surface area layered double hydroxide (LDH) materials, which may be readily dispersed in non-polar solvents, has been developed. This method involves treatment of LDHs with aqueous miscible organic (AMO) solvents. Here, molecular modeling is exploited to elucidate the AMO solvent–LDH interactions, in order to understand how the dispersion process is facilitated by the AMO treatment. The simulations show how hydrogen-bond networks within the LDH interlayer are disrupted by AMO solvents, leading to delamination.

The Water-Alkane Interface at Various NaCl Salt Concentrations: A Molecular Dynamics Study of the Readily Available Force Fields

In this study, classical molecular dynamic simulations have been used to examine the molecular properties of the water-alkane interface at various NaCl salt concentrations (up to 3.0 mol/kg). A variety of different force field combinations have been compared against experimental surface/interfacial tension values for the water-vapour, decane-vapour and water-decane interfaces. Six different force fields for water (SPC, SPC/E, TIP3P, TIP3Pcharmm, TIP4P & TIP4P2005), and three further force fields for alkane (TraPPE-UA, CGenFF & OPLS) have been compared to experimental data. CGenFF, OPLS-AA and TraPPE-UA all accurately reproduce the interfacial properties of decane. The TIP4P2005 (four-point) water model is shown to be the most accurate water model for predicting the interfacial properties of water. The SPC/E water model is the best three-point parameterisation of water for this purpose. The CGenFF and TraPPE parameterisations of oil accurately reproduce the interfacial tension with water using either the TIP4P2005 or SPC/E water model. The salinity dependence on surface/interfacial tension is accurately captured using the Smith & Dang parameterisation of NaCl. We observe that the Smith & Dang model slightly overestimates the surface/interfacial tensions at higher salinities (>1.5 mol/kg). This is ascribed to an overestimation of the ion exclusion at the interface.