Germany: Chemistry The Role of Hydrophobic Molecules in Catalytic Reactions

Author / Editor: Dr. Julia Weiler* / Ahlam Rais

Optimizing electrochemical processes is one of the challenges in developing technologies for renewable energies. New research findings could provide assistance here.

Water molecules may appear insignificant at first glance, but they have a major influence on chemical reactions. The Franco-German team is investigating what happens at the interface between water and gold.
Water molecules may appear insignificant at first glance, but they have a major influence on chemical reactions. The Franco-German team is investigating what happens at the interface between water and gold.
(Source: © Elmar Weiler)

Bochum/Germany – Electrochemical processes could convert CO2 into useful raw materials for the industry. In order to optimize the processes, chemists try to calculate in detail what energy costs arise from the various reaction partners and steps. Researchers at the Ruhr University Bochum (RUB) and Sorbonne Université in Paris have found out how small water-repellent molecules such as CO2 contribute to the energy costs of such reactions by analyzing the interaction of the molecules in water at the interface.

The team describes the results in the journal ‘Proceedings of the National Academy of Sciences’, PNAS for short, published online on April 13, 2021. To conduct the work, Dr. Alessandra Serva and Prof. Dr. Mathieu Salanne from the Laboratoire Phenix of the Université Sorbonne with Prof. Dr. Martina Havenith and Dr. Simone Pezzotti from the Bochum Chair for Physical Chemistry II came together.

Crucial role for small hydrophobic molecules

In many electrochemical processes, small water-avoiding molecules react on catalyst surfaces, which are often made of precious metals. Such reactions often take place in aqueous solution, with the water molecules forming so-called hydration shells around the other molecules: They are deposited around the other molecules. In the vicinity of polar, i.e. water-attracting molecules, the water behaves differently than in the vicinity of non-polar molecules, which are also referred to as hydrophobic, i.e. water-repellent. The Franco-German research team was interested in this hydrophobic hydration.

Using molecular dynamics simulations, the scientists analyzed the hydrophobic hydration of small molecules such as carbon dioxide (CO2) or nitrogen (N2) at the interface between gold and water. They showed that the interaction of water molecules in the vicinity of the hydrophobic small molecules makes a decisive contribution to the energy costs of electrochemical reactions.

Model for calculating energy costs expanded

The researchers implemented these findings in the Lum-Chandler-Weeks theory. This can be used to calculate the energy that is required for the formation of water networks. “The energy costs for hydrophobic hydration were calculated in the previous model for a large volume, here this model has now been extended to hydrophobic molecules near interfaces. This case was not previously included,” explains Prof. Dr. Martina Havenith, spokesperson for the Ruhr Explores Solvation Cluster of Excellence, Resolv for short, at RUB. With the adapted model, the energy costs for the hydrophobic hydration at the interface of gold and water can now be calculated, depending on the size of the hydrophobic molecules. "Due to the influence of the water, the size of the molecules plays a decisive role in the chemical reaction at the interface," says Dr. Simone Pezzotti from the Bochum Chair for Physical Chemistry II.

For example, the model predicts that small hydrophobic molecules would tend to collect at the interface due to interactions with the water, while larger molecules would remain further away in the solution.

* Dr. Julia Weiler, Department of Higher Education Communication at the Ruhr University Bochum

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