The mechanical response of the interlayer of hydrated montmorillonite was evaluated using steered molecular dynamics. An atomic model of the sodium montmorillonite was previously constructed. In the current study, the interlayer of the model was hydrated with multiple layers of water. Using steered molecular dynamics, external forces were applied to individual atoms of the clay surface, and the response of the model was studied. The displacement versus applied stress and stress versus strain relationships of various parts of the interlayer were studied. The paper describes the construction of the model, the simulation procedure, and results of the simulations. Some results of the previous work are further interpreted in the light of the current research. The simulations provide quantitative stress deformation relationships as well as an insight into the molecular interactions taking place between the clay surface and interlayer water and cations.
Pyrophyllite is the precursor to other smectite-group minerals which exhibit swelling. The mineral structure of pyrophyllite can lead to other minerals in the smectite group, including montmorillonite, through appropriate isomorphous substitutions. In this work, an atomic model of the pyrophyllite interlayer was constructed. The response of the interlayer was evaluated using steered molecular dynamics simulations. In steered molecular dynamics, external forces were applied to individual atoms to study the response of the model to applied forces. In this work, forces are applied to the surface clay atoms to evaluate the displacement vs. applied stress in the interlayer between clay layers. This paper describes the construction of the model, the simulation procedure, and the results of the simulations which show that under the applied loading, deformation occurs mainly in the interlayer. The clay layers show relatively little deformation. The results show that the relationship between applied stress and displacement of the interlayer is linear. The stress-deformation relationship for the interlayer is presented.
Nanosized montmorillonite clay dispersed in small amounts in polymer results in polymer nanocomposites having superior engineering properties compared to those of the native polymer. These nanoinclusions are created by treating clay with an organic modifier which makes clay organophilic and results in intercalation or exfoliation of the montmorillonite. The modifiers used are usually long carbon chains with alkylammonium or alkylphosphonium cations. In this work, we have investigated the use of some alternative molecules which can act as modifiers for clay composites using clay for reinforcing a matrix of biopeptides or proteins. Such composites have potential applications in the fields of biomedical engineering and pharmaceutical science. In this work, the amino acids arginine and lysine are used as modifiers. The intercalation and mechanical behavior of the interlayer spacing with these amino acids as inclusions under compression and tension are studied using molecular dynamics simulations. Significant differences in the responses are observed. This work also provides an insight into the orientation and interaction of amino acids in the interlayer under different stress paths.
The organic phase of nacre, which is composed primarily of proteins, has an extremely high elastic modulus as compared to that of bulk proteins, and also undergoes large deformation before failure. One reason for this unusually high modulus could be the mineral-organic interactions. In this work, we elucidate the specific role of mineral proximity on the structural response of proteins in biological structural composites such as nacre through molecular modeling. The "glycine-serine" domain of a nacre protein Lustrin A has been used as a model system. It is found that the amount of work needed to unfold is significantly higher when the GS domain is pulled in the proximity of aragonite. These results indicate that the proximity of aragonite has a significant effect on the unfolding mechanisms of proteins when pulled. These results will provide very useful information in designing synthetic biocomposites, as well as further our understanding of mechanical response in structural composites in nature.
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