High-Resolution Coarse-Grained Model of Hydrated Anion-Exchange Membranes that Accounts for Hydrophobic and Ionic Interactions through Short-Ranged Potentials

Molecular
simulations provide a versatile tool to study the structure,
anion conductivity, and stability of anion-exchange membrane (AEM)
materials and can provide a fundamental understanding of the relation
between structure and property of membranes that is key for their
use in fuel cells and other applications. The quest for large spatial
and temporal scales required to model the multiscale structure and
transport processes in the polymer electrolyte membranes, however,
cannot be met with fully atomistic models, and the available coarse-grained
(CG) models suffer from several challenges associated with their low-resolution.
Here, we develop a high-resolution CG force field for hydrated polyphenylene
oxide/trimethylamine chloride (PPO/TMACl) membranes compatible with
the mW water model using a hierarchical parametrization approach based
on Uncertainty Quantification and reference atomistic simulations
modeled with the Generalized Amber Force Field (GAFF) and TIP4P/2005
water. The parametrization weighs multiple properties, including coordination
numbers, radial distribution functions (RDFs), self-diffusion coefficients
of water and ions, relative vapor pressure of water in the solution,
hydration enthalpy of the tetramethylammonium chloride (TMACl) salt,
and cohesive energy of its aqueous solutions. We analyze the interdependence
between properties and address how to compromise between the accuracies
of the properties to achieve an overall best representability. Our
optimized CG model FFcomp quantitatively reproduces the
diffusivities and RDFs of the reference atomistic model and qualitatively
reproduces the experimental relative vapor pressure of water in solutions
of tetramethylammonium chloride. These properties are of utmost relevance
for the design and operation of fuel cell membranes. To our knowledge,
this is the first CG model that includes explicitly each water and
ion and accounts for hydrophobic, ionic, and intramolecular interactions
explicitly parametrized to reproduce multiple properties of interest
for hydrated polyelectrolyte membranes. The CG model of hydrated PPO/TMACl
water is about 100 times faster than the reference atomistic GAFF-TIP4P/2005
model. The strategy implemented here can be used in the parametrization
of CG models for other substances, such as biomolecular systems and
membranes for desalination, water purification, and redox flow batteries.
We anticipate that the large spatial and temporal simulations made
possible by the CG model will advance the quest for anion-exchange
membranes with improved transport and mechanical properties.