Molecular Simulation for Flexibility of a Single Clay Layer

Molecular dynamics (MD) simulations have been performed to study the flexibility of smectite clay minerals. We aim at the quantitative understanding of the mechanical behavior of a single clay layer in a completely exfoliated state. The repeating unit of a clay layer is taken to be a o = 0.52 nm and b o = 0.902 nm with formula of 2Na 1/3 Al 2 [Si 11/3 Al 1/3 ]O 10 (OH) 2 which corresponds to that of beidellite. When the size of the basic cell ( A = 9.3 nm, B = 2.6 nm, and C = 5 nm) (denoted by A-type cell) is reduced by 3−40% in the A -direction, the stationary structure of a clay layer is obtained as a curved sheet with a 2:1 smectite-type layer structure. In such a curved state, the layer experiences a stress of 0.5−0.7 GPa. The layer structure of a clay fractures when the size of the same basic cell is reduced by more than 40%. The bending constant is estimated for a curved layer by plotting the inverse of the average radius against stress. The similar calculations are performed by reducing the size of the basic cell ( A = 3.1 nm, B = 10.7 nm, and C = 5 nm) (denoted by B-type cell) in the B -direction. The clay layer is found to be more flexible along the A -axis direction than along the B -axis direction. When the microscopic structure of a curved clay layer is examined, it is concluded that the main origin of flexibility lies in the change of Si−O−Si angles in the silicate tetrahedral sheets rather than in the change of bond lengths.