Computational simulation of time-dependent porosity in micro-porous molecular materials
M. A. Lewis, J. Jordanovic, B. Tumilty, A. I. Cooper, A. Trewin
University of Liverpool, Liverpool L69 3BX United Kingdom
Materials with molecular-scale porosity are important in a wide range of applications such as gas storage, molecular separation, and heterogeneous catalysis. Discrete molecules tend to pack efficiently in the solid state resulting in minimal void volume, hence very few molecular materials exhibit permanent porosity. [1] Molecular porous materials are able to respond locally to the presence of a guest leading to the potential for diffusion of molecules that would not be expected by analysis of the static crystalline structure alone.
For example, although experimentally porous to N2, molecular crystal Cage 3 [1,2] displays disconnected voids in the calculated solvent accessible surface area of the structure obtained from single crystal data suggesting a formally non-porous system. It is only when the time averaged calculated surface area for a molecular dynamic simulation is considered that the structure appears to exhibit connected porosity. Understanding the time dependence of pore connectivity can give essential information about the structural properties of a material and its gas uptake mechanisms.
We have developed a novel methodology for investigating the time-dependent relationship between structure and pore connectivity, and hence we are able to investigate co-operative diffusion mechanisms. By considering Dianin’s molecule as a test case we investigate the fluctuations of solvent accessible surface area through an entire molecular dynamics run. The technique yields maximum and minimum surfaces, surface change over time, and, the potential to identify periods in time or specific structural volumes in which dynamic porosity mechanisms are evident.
[1] J. R. Holst, A. Trewin, A. Cooper, Nature Chem., 2010, 2, 915-920.
[2] Tomokazu Tozawa, James T. A. Jones, Shashikala I. Swamy, Shan Jiang, Dave J. Adams, Stephen Shakespeare, Rob Clowes, Darren Bradshaw, Tom Hasell, Samantha Y. Chong, Chiu Tang, Stephen Thompson, Julia Parker, Abbie Trewin, John Bacsa, Alexandra M. Z. Slawin, Alexander Steiner & Andrew I. Cooper, Nature Materials, 2009, 8, 973 – 978,