Computer simulation of simple fluid properties in a mica slit pore

Abstract

When liquids are confined between solid surfaces whose separation is comparable to the molecular dimensions, the properties of liquid are significantly different from bulk properties. It has been shown that the structure and properties of a confined fluid can be profoundly affected by the molecular structure of confining surfaces. Molecular-scale computer simulations play an important role to elucidate thin film properties. Molecular dynamics and grand canonical Monte Carlo simulations are used to study diffusion of monolayer octamethylcyclotetrasiloxane (OMCTS) and cyclohexane films confined between atomically structured uncharged mica surfaces. Diffusion parallel to the walls is found to be anisotropic due to the influence of the atomically structured surfaces. If the surfaces are aligned perfectly, the fluid occupies isolated regions of the pore space and diffusion is the same in all lateral directions and is a minimum. If one of the surfaces is shifted laterally in the x-direction by one half unit cell diffusion is enhanced in the x-direction along conduits formed by the overlapping potential energy fields of the surfaces. Nonequilibrium molecular dynamics are used to study the flow of octamethylcyclotetrasiloxane (OMCTS) films confined between atomically structured micalike surfaces in the presence of a chemical potential gradient. The relationship between the fluid flux and the atomic structure of confining mica surfaces, the distance separating the surfaces (∼1 to 10 nm), and the difference in chemical potential at the pore ends was studied. The results show that when the pore width is small enough and under large chemical potential differences, the liquid molecules form a nearly pure monolayer and the alignment of the mica surfaces affects the flux value. The flux is also affected by the pore width, which affects the structure of the thin film. The liquid molecules are packed loosely or tightly at different pore widths so the fluid flow varies with pore width. So, for certain pore widths, the flux is higher and for others it is lower. The chemical potential difference drives the fluid movement and when this difference is less than a critical level there is no fluid flow at least on the time scale of the computer simulation. We also studied the transport of the simple liquid, octamethylcyclotetrasiloxane (OMCTS), in a slit pore of finite length, where the pore-ends are explicitly included in the model. The results show that when the pore width is small enough, the liquid molecules form a nearly pure monolayer and the flux value is close to zero. When the pore width increases, the alignment of the mica surfaces affects the flux value. The fluid tracks forced by the alignment enhance the molecular movement in the same direction. The pore width also can affect the structure of the thin films. This information will enhance our understanding of phase transitions and rheology at the molecular level, and additionally will improve our ability to manipulate fluids flowing through or trapped in natural and synthetic nanoporous systems.