Study Explains the Migration of Confined Protons

A new study from Ruhr University-Bochum and the University of California Berkeley reveals that protons present in aqueous solution tend to migrate very rapidly — much quicker than other ions. Yet, this is only applicable when they are in a space greater than 2 nm.

Study Explains the Migration of Confined Protons
When water is present in tiny quantities—much less than in this droplet—it develops special properties. Image Credit: © RUB, Marquard.

The so-called Grotthuss mechanism loses its ability to work in confined spaces, in which protons tend to diffuse faster compared to ions. The outcomes of the study were explained by the team of the Bochum Excellence Cluster Ruhr Explores Solvation, called (RESOLV in short), together with collaborators of the sister research network CALSOLV in Berkeley.

The study was published in the journal Angewandte Chemie on September 3rd, 2021. The reviewers rated the findings as a Highlight Paper (Top 10%).

The Grotthuss-mechanism makes the protons (H+) and hydronium ions (H3O+) present in free aqueous solutions migrate faster compared to other ions. In fact, separate protons do not migrate at all. Rather, bonds of the hydronium ions are broken and new bonds to other water molecules are developed so that the individual proton does not migrate. Instead, charges are carried directly from one water molecule to the next. This process takes place comparatively faster than the diffusion of an ion via the solution.

Behavior in Confined Spaces Unexplored

Until now, numerous studies have examined the transport of protons in a free aqueous solution.

In real life such conditions are relatively rare. Most protons transport processes actually occur in confined spaces or in nanopores.

Dr. Martina Havenith, Study Author and Professor, Speaker of Bochum Excellence Cluster Ruhr Explores Solvation

Hydronium ions play a part in specifying the pH value. So far, the effect of confinement is yet to be understood fully.

To alter that, scientists from Bochum and Berkeley integrated theoretical and experimental methods. They made tiny water pools whose size could be accurately controlled. Once the droplets’ diameter turned smaller than 2 nm, the proton transport mechanism in the experiment and simulations varied suddenly.

Under two nanometers the proton migration is restricted by confinement effects. This effect is reduced when the water pool is enlarged. Surprisingly we found that above two nanometers, where the formation of hydronium ions is possible, there is a proton traffic jam.

Dr. Martina Havenith, Study Author and Professor, Speaker of Bochum Excellence Cluster Ruhr Explores Solvation

In an oscillatory state, the proton is stuck, where it bounces back and forth across the surface of the water pool. However, it shows no progression, thereby leading to no further increase in conductivity — as originally predicted.

Short-Circuit in the Hydrogen-Bonding Network

Besides the size of the pools, the acid concentration also impacts the proton migration behavior. When the acid content was increased by the researchers, a kind of short-circuit was developed in the hydrogen bonding network of the droplet, so that the proton did not migrate from its position, but instead paused in an 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.

Dr. Martina Havenith, Study Author and Professor, Speaker of Bochum Excellence Cluster Ruhr Explores Solvation

This study was financially supported by the Excellence Cluster RESOLV EXC 2033 - Project number 390677874 through the Deutschen Forschungsgemeinschaft (DFG, German Research Foundation) and the Graduate School “Confinement-controlled Chemistry” GRK2376-331085229.

HH and THG are assisted by the CPIMS program of the Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The simulations were performed in the National Energy Research Scientific Computing Center, financed via the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Journal Reference:

Havenith-Newen, M., et al. (2021) Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism. Angewandte Chemie International Edition. doi.org/10.1002/anie.202108766.

Source: https://www.ruhr-uni-bochum.de/en

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