Molecular Structural Transformation of 2:1 Clay Minerals by a Constant-Pressure Molecular Dynamics Simulation Method

Jianfeng Wang,Marte S Gutierrez

doi:10.1155/2010/795174

Abstract

This paper presents results of a molecular dynamics simulation study of dehydrated 2:1 clay minerals using the Parrinello-Rahman constant-pressure molecular dynamics method. The method is capable of simulating a system under the most general applied stress conditions by considering the changes of MD cell size and shape. Given the advantage of the method, it is the major goal of the paper to investigate the influence of imposed cell boundary conditions on the molecular structural transformation of 2:1 clay minerals under different normal pressures. Simulation results show that the degrees of freedom of the simulation cell (i.e., whether the cell size or shape change is allowed) determines the final equilibrated crystal structure of clay minerals. Both the MD method and the static method have successfully revealed unforeseen structural transformations of clay minerals upon relaxation under different normal pressures. It is found that large shear distortions of clay minerals occur when full allowance is given to the cell size and shape change. A complete elimination of the interlayer spacing is observed in a static simulation. However, when only the cell size change is allowed, interlayer spacing is retained, but large internal shear stresses also exist.

Highlights

In the last two decades, computer simulation methods, including the Monte Carlo (MC) method and the Molecular Dynamics (MD) method, have been successfully used in the studies of dehydrates/hydrates of 2:1 clay minerals [1,2,3,4]
A first MD simulation of the dehydrated mica relaxed under zero external stress is carried out, where only the cell size change is allowed
In the second case where full freedom is given to the cell size and shape change, it is interesting to observe that the entire tensor π decays to zero after about 16 000 iterations (Figure 7(c)), and in order to allow that to happen, remarkable changes have occurred to the matrix h (Figure 7(d)): (2) h11 and h33 undergo a significant increase and decrease to reach a value of 3.36 nm and 2.24 nm, respectively, with the latter essentially eliminating any existing interlayer spacing (see Figure 8(a)); (2) h12 and h21 undergo significant increases to reach an approximate value of 9.8 A, implying a much larger shear distortion in the sheet plane (see Figure 8(b))

Summary

Introduction

In the last two decades, computer simulation methods, including the Monte Carlo (MC) method and the Molecular Dynamics (MD) method, have been successfully used in the studies of dehydrates/hydrates of 2:1 clay minerals [1,2,3,4]. For the conventional MD method, the energy, volume, and number of particles are fixed for a particular system, and it is assumed that time averages of properties of the system are equal to the microcanonical (NVE) ensemble averages of the same properties When such a system moves along its trajectory, the pressure and temperature will change. Such a limitation hinders the application of the MD method to the study of clay minerals to a large extent. The coupled changes of cell configuration and atomic positions driven by the external and internal stress imbalance, which cannot be predicted by the conventional MD method, provide new insights on the atomic-scale behavior of clay minerals. The static simulation provides results that represent the behavior of clay minerals at zero Kelvin

MD Simulation Method

Simulation Results

Concluding Remarks