Large-scale molecular dynamics study of montmorillonite clay: Emergence of undulatory fluctuations and determination of material properties

Large-scale atomistic simulations, which we define as containing more than 100 000 atoms, are becoming more commonplace as computational resources increase and efficient classical molecular dynamics algorithms are developed. With the advent of grid-computing methods, it is now possible to simulate even larger systems efficiently. Using this new technology, we have simulated montmorillonite clay systems containing up to approximately ten million atoms whose dimensions approach those of a realistic clay platelet. This considerably extends the spatial dimensions of microscopic simulation into a domain normally encountered in mesoscopic simulation. The simulations exhibit emergent behavior with increasing size, manifesting collective thermal motion of clay sheet atoms over lengths greater than 150 A. This motion produces low-amplitude, long-wavelength undulations of the clay sheets, implicitly inhibited by the small system sizes normally encountered in atomistic simulation. The thermal bending fluctuations allow us to calculate material properties, which are hard to obtain experimentally due to the small size of clay platelets. Montmorillonite is commonly used as a filler in clay-polymer nanocomposites, and estimation of the elastic properties of the composite requires accurate knowledge of the elastic moduli of the components. We estimate the bending modulus to be 1.6 x 10(-17) J, corresponding to an in-plane Young's modulus of 230 GPa. We encounter a clay sheet persistence length of approximately 1400 A, which dampens the undulations at long wavelengths for the largest system in our study.