Molecular insights into the temperature and pressure dependence of mechanical behavior and dynamics of Na-montmorillonite clay.

Sodium montmorillonite (Na-MMT) clay mineral is a common type of swelling clay that has potential applications for nuclear waste storage at high temperatures and pressures. However, there is a limited understanding of the mechanical properties, local molecular stiffness, and dynamic heterogeneity of this material at elevated temperatures and pressures. To address this, we employ all-atomistic (AA) molecular dynamics (MD) simulation to investigate the tensile behavior of Na-MMT clay over a wide temperature range (500 K to 1700 K) and pressures (200 atm to 100 000 atm). The results show that increasing the temperature significantly reduces the tensile modulus, strength, and failure strain, while pressure has a minor effect compared to temperature, as seen in the normalized pressure-temperature plot. Mean-square displacement (MSD) analysis reveals increased molecular stiffness with increasing pressure and decreasing temperature, indicating suppressed atomic mobility. Our simulations indicate temperature-dependent dynamical heterogeneity in the Na-MMT model, supported by experimental studies and quantified local molecular stiffness distribution. These findings enhance our understanding of the tensile response and dynamical heterogeneity of Na-MMT clay under extreme conditions, aiding the development of clay minerals for engineering applications such as nuclear waste storage and shale gas extraction.

First-Principles Study on Optoelectronic Properties of Fe-Doped Montmorillonite Clay.

A theoretical investigation is conducted to describe optoelectronic properties of Fe-doped montmorillonite nanoclay under spin states of low spin (LS), intermediate spin (IS), and high spin (HS). Ground state electronic properties are studied using spin-polarized density functional theory calculations. The nonradiative and radiative relaxation channels of charge carriers are studied by computing nonadiabatic couplings (NACs) using an "on-the-fly" approach from adiabatic molecular dynamics trajectories. The NACs are further processed using a reduced density matrix approach with the Redfield formalism. The computational results are presented for electronic density of states, absorption spectra, charge carrier dynamics, and photoluminescence (PL) by comparing various spin multiplicities. Results on spin α and spin ß components are independent and quite different because of the partial occupation of Fe 3d states. Overall, HS is the most stable with the largest Fe-O distances. One finds different nonradiative relaxation pathways in space and on the time scale for electrons and holes. The Redfield PL reveals obvious Fe 3d-3d transitions for LS and IS.

A Coarse-Grained Model for the Mechanical Behavior of Na-Montmorillonite Clay.

Sodium montmorillonite (Na-MMT) is one of the most commonly found swelling clay minerals with diverse engineering and technological applications. The nanomechanical properties of this mineral have been extensively investigated computationally utilizing molecular dynamics (MD) simulations to portray the molecular-level changes at different environmental conditions. As the environmentally found Na-MMT clays are generally sized within hundreds of nanometers, all-atomistic (AA) MD simulations of clays within such size range are particularly challenging due to computational inefficiency. Informed from atomistic modeling, a coarse-grained (CG) modeling technique can be employed to overcome the spatiotemporal limitation. The current study presents a modeling strategy to develop a computationally efficient model of Na-MMT clay with a typical size over ≃100 nm by shrinking the atomistic platelet thickness and reducing the number of center-layer atoms. Using the "strain-energy conservation" approach, the force field parameters for the CG model are obtained and the developed CG model can well preserve in-plane tension, shear, and bending behaviors of atomistic counterparts. Remarkably, the CG tactoid model of Na-MMT, a hierarchical multilayer structure, can reproduce the interlayer shear and adhesion as well as d-spacing among the clay sheets as of atomistic one to a good approximation while gaining significantly improved computational speed. Our study demonstrates the efficacy of the CG modeling framework, paving the way for the bottom-up multiscale prediction of mechanical behaviors of clay and related minerals.

Understanding the Role of Self-Adhesion in Crumpling Behaviors of Sheet Macromolecules.

Understanding the crumpling behavior of two-dimensional (2D) macromolecular sheet materials is of fundamental importance in engineering and technological applications. Among the various properties of these sheets, interfacial adhesion critically contributes to the formation of crumpled structures. Here, we present a coarse-grained molecular dynamics (CG-MD) simulation study to explore the fundamental role of self-adhesion in the crumpling behaviors of macromolecular sheets having varying masses or sizes. By evaluating the potential energy evolution, our results show that the self-adhesion plays a dominant role in the crumpling behavior of the sheets compared to in-plane and out-of-plane stiffnesses. The macromolecular sheets with higher adhesion tend to form a self-folding planar structure at the quasi-equilibrium state of the crumpling and exhibit a lower packing efficiency as evaluated by the fractal dimension of the system. Notably, during the crumpling process, both the radius of gyration Rg and the hydrodynamic radius Rh of the macromolecular sheet can be quantitatively described by the power-law scaling relationships associated with adhesion. The evaluation of the shape descriptors indicates that the overall crumpling behavior of macromolecular sheets can be characterized by three regimes, i.e., the less bent, intermediate, and highly crumpled regimes, dominated by edge-bending, self-adhesion, and further compression, respectively. The internal structural analysis further reveals that the sheet transforms from the initially ordered state to the disordered glassy state upon crumpling, which can be facilitated by greater self-adhesion. Our study provides fundamental insights into the adhesion-dependent structural behavior of macromolecular sheets under crumpling, which is essential for establishing the structure-processing-property relationships for crumpled macromolecular sheets.