Ab Initio Molecular Dynamics Simulations of an Excess Proton in a Triethylene Glycol-Water Solution: Solvation Structure, Mechanism, and Kinetics

MT McDonnell and HX Xu and DJ Keffer, JOURNAL OF PHYSICAL CHEMISTRY B, 120, 5223-5242 (2016).

DOI: 10.1021/acs.jpcb.6b02445

We investigate the solvation shell structures, the distribution of protonic defects, mechanistic details; kinetics, and dynamies of proton transfer for an excess protbn in bulk water and for an excess proton in an aqueous solution of triethylene glycol (TEG) via Car-Parrinello molecular dynamics simulations, The PW91, PBE, and PBE with the Tkatchenko-Schefflet (TS) density-dependent dispersion functionals were used and compared for bulk water and the TEG-water mixtures. The excess proton is found to reside predominantly on water molecules but also resides on hydroxyl groups of TEG. The lifetimes associated with structural diffusion time scales of the protonated water were found to be on the Order of similar to 1 ps. All three functionals studied support the presolvation requirement for structural diffusion: The highest level of theory shows a reduction in the free energy barrier for water-water proton transfer in TEG water mixtures compared to bulk water. The effect of TEG shows no strong change in the kinetics for TEG- water mixtures compared to bulk water for this same-level of theory. The excess proton displays burst-rest behavior in the presence of TEG, similar to that found in bulk water. We find that the TEG chain disrupts the hydrogen-bond network, causing the solvation shell around water to be populated by TEG chain groups instead of other waters, reducing the rigidity of the hydrogen-bond network: Methylene is a dominant hydrogen bond donor for the protonated water in hydrogen-bond networks associated with proton transfer and structural diffusion. This is consistent with previous studies that have found the hydronium ion to be amphiphilic in nature and to have higher proton mobility at oil-water interfaces.

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