How confined protons migrate

Date:
September 8, 2021
Source:
Ruhr-University Bochum
Summary:
Protons in aqueous solution can usually migrate very quickly --
much faster compared to other ions. However, this only applies
when they are in a space greater than two nanometers, as a new
study shows. In confined spaces the so-called Grotthuss mechanism
no longer works, in which protons diffuse faster than ions.



FULL STORY ==========================================================================
The results described by the team of the Bochum Excellence Cluster
Ruhr Explores Solvation, RESOLV for short, together with colleagues of
the sister research network CALSOLV in Berkeley were published in the
journal Angewandte Chemie on September 3rd, 2021. The reviewers rated
the results as a Highlight Paper (Top 10%).


========================================================================== Protons (H+) and hydronium ions (H3O+) in free aqueous
solutions seem to migrate faster than other ions due to the the Grotthuss-mechanism. Individual protons do not really migrate at
all. Instead, bonds of the hydronium ions are broken and new bonds to
other water molecules are formed, so that the individual proton does not migrate. Rather charges are transported directly from one water molecule
to the next. This process is quicker than the diffusion of an ion through
the solution.

Behavior in Confined Spaces Unexplored So far, many studies have
investigated the transport of protons in free aqueous solution. "In
real life such conditions are relatively rare," says Professor Martina Havenith, speaker of RESOLV and an author of the study. "Most protons
transport processes actually occur in confined spaces or in nanopores." Hydronium ions are involved in defining the pH value. Up to now, the
effect the of confinement has not yet been completely understood.

To change that, researchers from Bochum and Berkeley combined theoretical
and experimental methods. They created tiny water pools, whose size
could be precisely controlled. As soon as the diameter of the droplets
became smaller than two nanometers, the proton transport mechanism in
the experiment and simulations changed abruptly. "Under two nanometers
the proton migration is restricted by confinement effects. This
effect is reduced when the water pool is enlarged," explains Martina
Havenith. "Suprisingly we found that above two nanometres, where the
formation of hydronium ions is possible, there is a proton traffic
jam." The proton is stuck in an oscillatory state, where it bounces back
and forth along the surface of the water pool, but makes no progress
forward, resulting in the conductivity not increasing further -- as
originally expected.

Short-Circuit in the Hydrogen-Bonding Network In addition to the size of
the pools, the acid concentration also influences the proton migration behavior. When the research team increased the acid content, they created
a type of short-circuit in the hydrogen bonding network of the droplet,
so that the proton no longer migrated from its position, but rather
paused in oscillatory bouncing state. "That has consequences for every
system that relies on proton transport, because the size of the system
or the proton concentration can lead to a traffic jam and for example
disrupt the signaling process," concludes Havenith.

========================================================================== Story Source: Materials provided by Ruhr-University_Bochum. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Martina Havenith-Newen, Ellen M Adams, Teresa Head-Gordon,
Hongxia Hao,
Maximilian Ru"ttermann, Itai Leven, Hanna Wirtz. Proton Traffic
Jam: Effect of Nanoconfinement and Acid Concentration on Proton
Hopping Mechanism. Angewandte Chemie International Edition, 2021;
DOI: 10.1002/ anie.202108766 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210908180435.htm

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