Paper title: Molecular dynamics simulation of the transport of small molecules across a polymer membrane

Journal: J. Chem. Phys. 96, 4699 (1992)

Author/s: R. M. Sok [1], H. J. C. Berendsen [1] and W. F. van Gunsteren [2]
[1] Laboratory of Biophysical Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
[2] Laboratory of Physical Chemistry, ETll-Zentrum, CH-8092 Zurich, Switzerland


Summary: 
Molecular dynamics (MD) simulations of helium and methane in polydimethylsiloxane (PDMS) were used to model the transport of small molecules across a polymeric membrane. The authors calculated the permeability coefficient (P) from two properties that are directly derivable from the MD simulations: diffusion coefficient (D) and solubility factor (S).

MD simulations were carried out using the leap-frog algorithm for the equations of motion and a time step of 5 fs, under NPT (T=300k, P=1 atm) conditions.

Diffusion constants were reproduced by calculating the time average of the root mean square displacement of the particles. Chemical potential is calculated by using the Widom particle insertion method and the ensemble average of the relationship of interaction energy and temperature. The fraction of accessible volume (volume not occupied by the polymer chain) was also calculated using the relationship between position of polymer and penetrant. Excess free energies were determined using particle insertion method.

Results from MD simulations were compared to experimental data. Discrepancies were attributed to the limited accuracy of interaction potentials used and also to the estimation of the polymer flexibility.

For a polymer membrane, “jumps” happen because of the fluctuations in size and shape of holes (sets of grid points without diagonal connections). It was observed that both He and CH4 seem to be experiencing jump diffusion. However, they differ in the residence time per hole. For He, it was very short that it looks like it is freely diffusing. But for Ch4, the residence time considerably exceeds the transition time for holes. 


Contribution and application:
Despite being focused on gas transport, this work can contribute to our work in ESEL because it shows how to calculate diffusion constants, solubility factor and excess free energy, which are needed to describe the transport phenomena in a system. In addition, this proves that MD is an appropriate tool for studying diffusion processes. It has also shown that MD results may be compared to experimental results.



By: Hannah Ebro
hannah@gist.ac.kr