Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

Abstract

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse nonionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms.W ei dentify that only at acritical micelle size of W 0 = 9dosolvated proton complexes form in the water pool, accompanied by ac hangei n mechanism from Grotthuss forwards huttling to one that favors local oscillatory hopping.This is due to apreference for H + and Cl À ions to adsorb to the micelle interface,together with an acid concentration effect that causes a"traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle sizei ncreases.T hese findings have implications for atmospheric chemistry,b iochemical and biophysical environments,and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Monitoring the photochemistry of a formazan over 15 orders of magnitude in time

“…In principle, also an intermediate [as is of relevance in formazan synthesis (Hegarty and Scott, 1966;Hegarty and Scott, 1967;King and Murrin, 2004)] with two azo groups and the H atom at the interjacent C atom is conceivable, but much less stable than the formazan tautomers (Buemi et al, 1998) (see also Supplementary Figure S7). However, the transfer might occur via a proton wire mechanism, i.e., in a Grotthusstype fashion (Agmon, 1995;Miyake and Rolandi, 2015;Adams et al, 2021) as was found in water but also identified in other protic solvents (Stoyanov et al, 2008;Fujii et al, 2018;Long et al, 2020). Hence, the isomerization involving proton transfer should only be possible in protic solvents or in aprotic solvents containing at least traces of protic cosolvents.…”

Grotthuss Molecular Dynamics Simulations for Modeling Proton Hopping in Electrosprayed Water Droplets

“…18 The latter undergo vehicular diffusion (Brownian motion). 19,20 In contrast, H 3 O + is subject to both Brownian motion and Grotthuss diffusion. The Grotthuss mechanism allows protons to rapidly migrate through water as a charge defect, by swapping covalent bonds for H-bonds.…”

“…The transport properties of H 3 O + are unique, with a diffusion coefficient that is roughly 1 order of magnitude larger than that of other small cations such as Li + or Na + . The latter undergo vehicular diffusion (Brownian motion). , In contrast, H 3 O + is subject to both Brownian motion and Grotthuss diffusion. The Grotthuss mechanism allows protons to rapidly migrate through water as a charge defect, by swapping covalent bonds for H-bonds. − Scheme indicates how this mechanism can result in H 3 O + translation along a H-bonded water wire. − Liquid water contains an extensive branched network of such water wires because each H 2 O is ligated by ∼four other H 2 O molecules, albeit in a highly dynamic fashion. − …”

“…Techniques that have been applied in this context include ab initio molecular dynamics (AIMD), where potentials are calculated on the fly using density functional theory, ,,,− quantum mechanical/molecular mechanical (QM/MM) methods, ,, multistate empirical valence bond (MS-EVB) simulations, λ-dynamics, and other approaches. , While these techniques have uncovered many atomistic details, high computational cost tends to restrict their application to short times (picoseconds) and small systems, often less than 100 waters with a single H 3 O + . ,,,,,, Some studies were able to extend this range to significantly more water molecules and longer times, particularly, when using MS-EVB and related approaches (see refs ,– and references therein). Efforts to use simplified reactive force fields have yielded interesting data as well. ,,− Nonetheless, simulations of large systems that involve Grotthuss diffusion remain challenging.…”

“…ESI nanodroplets contain roughly 10 4 –10 5 solvent molecules, and the time range of interest stretches over nano- to microseconds . This size and temporal regime precludes the application of existing Grotthuss simulation methods. ,,,,,,,− ,− There have been numerous recent efforts to study ESI nanodroplets using MD simulations. − Most of those simulations focused on droplets charged with metal ions such as Na + that can be modeled with standard MD force fields. − However, [M + z Na] z + ions generated in such simulations are of limited relevance for practical ESI conditions, where the [M + z H] z + species dominates (implying that interactions with H 3 O + are an essential component of analyte charging) . A handful of MD studies examined droplets containing H 3 O + , but this was done by treating these ions as Brownian particles without Grotthuss diffusion and without allowing for protonation/deprotonation events. ,,, …”