Donnan's potential, finally revealed

Donnan’s potential, finally revealed

Donnan's potential, finally revealed

Overview of direct measurement of Donnan potential using the APXPS Tender. a A schematic of the experimental setup when acquiring data from the CR-61 membrane equilibrated in aqueous saline solution. b Ion sharing at equilibrium between a membrane in contact with a monovalent electrolyte solution. At equilibrium, electroneutrality requires an equivalent number of counterions (which, for a monovalent system, is equal to the fixed charge concentration) to balance the fixed charge groups. The co-ions sorbing in the membrane are accompanied by an equivalent number of counterions, leading to significantly higher counterion concentrations than the co-ions in the membrane. vs Illustration of the potential drop at the membrane/solution interface. D The corresponding binding energy (BE) moves into the membrane-bound core levels. e Relationship between measured BE of membrane-bound core levels and Donnan potential as a function of external solution concentration. Credit: Nature Communication (2022). DOI: 10.1038/s41467-022-33592-3

Donnan’s electric potential arises from an imbalance of charges at the interface of a charged membrane and a liquid, and for over a century it has stubbornly eluded direct measurement. Many researchers have even called such a measurement impossible.

But those days are finally over. Using a tool conventionally used to probe the chemical composition of materials, scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) recently conducted the first direct measurement of Donnan’s potential.

“We were naive enough to believe we could do the impossible,” said Ethan Crumlin, a scientist with Berkeley Lab’s Advanced Light Source (ALS), which generated the bright X-rays used in the experiment. Crumlin and co-workers recently reported the measurement in Communication Nature.

Such a measurement could bring new insights into many areas focused on membranes. The Donnan potential plays a critical role in transporting ions across a cell membrane, for example, which links it to biological functions ranging from muscle contractions to neural signaling. Ion exchange membranes are also important in energy storage strategies and water purification technologies.

“Knowing about Donnan’s potential is relevant to many applications, from energy to biology to water treatment,” said Pinar Aydogan-Gokturk, early career scientist and postdoctoral researcher at the Berkeley Lab. who performed the measurements.

Aydogan-Gokturk said the new measurement will also improve previous thermodynamic models of Donnan’s equilibrium. These models have long been based on uncertain assumptions and indirect measurements. “Using our method, we hope to be able to answer questions about fluid dynamics under non-ideal conditions at membrane interfaces,” she said.

Frederick Donnan, an Anglo-Irish chemist, first probed the phenomenon in the early 20th century using a solution of Congo red, a dye now known to be toxic and carcinogenic to many organisms. In a paper published in 1911, Donnan describes experiments in which a membrane separates two charged solutions and allows only a few ions to pass. When the two solutions reach equilibrium, he found, they can also scatter charges unevenly across the membrane – and therefore produce an electric potential.

Donnan's potential, finally revealed

Credit: Marilyn Sargent/Berkeley Lab

The Donnan potential plays a role in any system that combines a material with fixed ions, such as a charged polymer or the membrane of a cell, and an electrolyte solution. The charges of the solution are free to move and some can pass through the membrane.

To make the measurement, Aydogan-Gokturk, Crumlin and their collaborators at the University of Texas at the Center for Materials for Water and Energy Systems in Austin used a technique called “tender” ambient pressure X-ray photoelectron spectroscopy, or tender -APXPS.

It’s a sophisticated application of X-ray Photoelectron Spectroscopy, or XPS, that can reveal the lesser-known (but equally important) chemical makeup and local potentials of a material’s surface. When X-rays are focused on the surface of materials, they trigger the release of electrons, and the energy levels of these electrons release the constituent atoms. In 1981, Swedish physicist Kai Siegbahn won the Nobel Prize in Physics for his work using XPS.

Surface spectroscopy tools such as XPS typically require vacuum environments to operate, but pioneering work at Berkeley Lab led to the use of ambient pressure XPS. About 10 years ago, ALS scientists took the technology a step further, combining room-pressure XPS with higher-energy X-rays. This advance allowed them to probe solid-liquid interfaces.

“Until recently, Berkeley Lab’s ALS was the only place in the world where you could do this with a solid-liquid interface,” Crumlin said.

During the pandemic, Crumlin, Aydogan-Gokturk and their team collected time-consuming spectroscopic datasets to probe Donnan’s potential. They immersed a charged membrane in a saline solution, shot X-rays at the interface and studied the electrons emerging from it. To help validate the experiments, Berkeley lab scientist Jin Qian compared measured values ​​of the Donnan potential to simulated thermodynamic models.

A tool usually used to probe chemical composition may not seem like an obvious instrument for studying membranes, but Crumlin predicted that the use of soft APXPS in membrane science will continue to reveal new insights into interfacial phenomena. .

“The membrane community is a totally new world in this science space,” he said. “This work really combines two worlds together.”

More information:
Pinar Aydogan Gokturk et al, Donnan’s potential revealed, Nature Communication (2022). DOI: 10.1038/s41467-022-33592-3

1911 paper:… hist-68-Donnan-1.pdf

Provided by Lawrence Berkeley National Laboratory

Quote: Donnan’s Potential, Finally Revealed (December 19, 2022) Retrieved December 19, 2022 from

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