My seminar will focus on two recent developments in my research group both of which involve
molecular simulations albeit for two contrasting purposes; realistic simulations of membrane
separations and the conceptual design of molecular machines.
The first is a new non-equilibrium molecular dynamics simulation method we developed recently
in order to simulate the permeation of pure fluids and mixtures through membranes, namely
Concentration Gradient Driven Molecular Dynamics (CGD-MD). This new method works by
employing bias forces to fix the concentration of fluids (pure or mixture) at the inlet and outlet of a
membrane in order to maintain a concentration gradient and drive the diffusion of the molecules
through the membrane (Figure 1). This is aimed at mimicking membrane separation experiments;
for instance, high pressure at the feed side and vacuum at the permeate side. CGD-MD addresses
two main shortcomings of previous non-equilibrium MD methods used for simulating membrane
separation processes at the molecular scale. First, it avoids the feed depletion issue and allows
running steady state and continuous simulations for unrestricted simulation times. Second, it
maintains the feed composition at a target value without the need of any complex Monte Carlo-MD
coupling. We demonstrate the new method for methane, ethane, ethylene permeation and
ethane/ethylene separation through a flexible metal-organic framework (MOF) membrane.

In the second study we propose and computationally demonstrate the concept of electric field
controlled molecular gates mounted on the open-metal coordination sites in MOF materials. The
MOF-molecular gate complex works by opening and closing under the effect of an electric field. By
carrying out density functional theory (DFT) calculations and molecular dynamics (MD) simulations
we show that the MOF-molecular gate complex can be switched between two stable
configurations, open and closed, by turning on and off an external electric field (Figure 1). We
further show that the molecular gate can be controlled to block or allow the diffusion of methane
molecules through the channels of the MOF like a nanoscale butterfly valve.

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By: Sean Paul Ogilvie
Last updated: Tuesday, 12 September 2017