Reactive molecular dynamics simulations of an excess proton in polyethylene glycol-water solutions^*

* Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doepublic-access-plan).

ABSTRACT

We investigate the effects of both triethylene glycol (TEG) and polyethylene glycol (PEG) on the structure and transport of an excess proton (H⁺) in aqueous solution. A new reactive force field (ReaxFF) parameterisation is carried out to better describe proton transfer of H⁺ with PEG polymers in aqueous solution via training against density functional theory (DFT) calculations. Using this ReaxFF parameterisation, reactive molecular dynamics simulations are carried out for three different systems containing H⁺: bulk water, aqueous TEG, and aqueous PEG with a single, oriented polymer chain. The H⁺ diffusivity is suppressed due to the presence of TEG and enhanced in the oriented chain PEG system, particularly along the axis parallel to the PEG chain, relative to bulk water. The enhancement in proton mobility along the chain axis is mainly due to the structural, rather than vehicular, component. From free energy calculations for H+ transfer, the increase in proton mobility is not due to a reduction in activation energy. From pair distribution function analysis of the local structure for water and hydronium we hypothesise the increased mobility is due to an entropic effect, specifically a favourable hydrogen bond network for long range proton transport due to the oriented PEG chain.

KEYWORDS:

• Proton transport
• polyethylene glycol
• water
• ReaxFF

Acknowledgements

During this work, M.M. was partially supported by a grant from the National Science Foundation (DGE-0801470). This research used resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231 and also the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562. The XSEDE resources included the Comet and Gordon supercomputers at the San Diego Supercomputer Center through allocation TG-DMR 160076. The authors gratefully thank Dr. Adri van Duin for correspondence regarding an initial starting ReaxFF parameter set for the simulated systems and also Dr. Andrés Jaramillo-Botero for help and providing the GARFfield software necessary for this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Marshall T. McDonnell http://orcid.org/0000-0002-3713-2117

David J. Keffer http://orcid.org/0000-0002-6246-0286

Notes

* Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doepublic-access-plan).

Additional information

Funding

This work was supported by National Science Foundation [grant number DGE-0801470].