Combining neutron reflectometry and electrochemistry to understand and control liquid-liquid interfaces

Post-doc highlight [4/5]

This article was writen in anticipation of the Electrochimica Acta publication, where Andrea Tummino’s work developed as part of his FILL2030 post-doc mission is highlighted.



First structural investigation of
electrified liquid-liquid interfaces with neutrons

Many scientific phenomena occur at the interface between two different substances. Liquid-liquid (LL) interfaces represent the softest possible distinction between two different phases. Moreover, they allow us to investigate how solutes distribute on both sides of the interface and how molecules can cross through the interface. Similar studies are not possible at solid-liquid interfaces and air-liquid interfaces since solutes are constrained to the liquid half-system. Furthermore, LL interfaces are ubiquitous; they are found in biology, oil-recovery technologies, and even in our kitchen, when mixing olive oil and vinegar for instance. It is therefore of great interest to researchers – investigating how the interface functions and behaves in varying conditions. This understanding can help us to create models for a range of studies, such as how drug molecules enter cells.

When an oil and water are mixed together, the vastly different structures and molecular interactions of the two liquids results in a clear phase separation with a visible interface. This may appear horizontal between two layers in a glass, or spherical as droplets of oil in a salad dressing rise to the surface. Understanding how these interfaces function can have important industrial and environmental applications however. For example, in environmental sciences, oil recovery demonstrates a long-standing issue of how to control liquids of different structures and phases. When a major oil spill occurs, understanding how to stabilise or destabilise the interface between sea water and crude oil can aid clean-up operations. It can also help to prevent devastating impact on the surrounding environment.

The application of an electric field to the interface can stimulate chemical reactions. Over the last few decades, electrochemical techniques have proven to give accurate kinetic information of processes taking place at liquid-liquid interfaces, such as ion and charge transfer and chemical reactions. Thanks to these studies, scientists have gained insights into how materials can be produced at a liquid interface, or how ion transfer occurs from one phase to another. Nevertheless, the direct determination of the interfacial structure was missing to date because there are very few techniques that can probe LL interfaces in the sub-nanometer length scale.

Characterising the structure of liquid interfaces also provides a huge advantage when it comes to understanding the steps within a chemical process. An advanced investigation of the interface prior to- and following a chemical process can provide valuable data. To fully understand the evolution of a process taking place at any interface, several analysis techniques must be carried out simultaneously, describing the interface in detail.

For the first time, a group of scientists from Institut Laue-Langevin (ILL) developed a method for performing both electrochemistry and neutron reflectometry on a liquid-liquid interface. A technique that brings a huge opportunity to researchers from across the scientific community, by allowing the characterisation of the interfacial structures while monitoring in situ the electrochemical behaviour.

Liquid-liquid interfaces also allow the reaction to be continually modified from both sides if desired, with the novel technique enabling uninterrupted analysis. For example, in a polymerisation, the interface between two liquids can act as a ‘template’ for the precise point where two (or more) reactants meet, thus enabling a fine control over the process itself.

Neutrons are among the most powerful tools for revealing structural details of materials. When a neutron beam is directed at a sample, the subsequent scattering can reveal information about the size, shape, structure, and distribution of atoms in any given material, such as liquids. Neutrons can penetrate deep into materials, reaching sites of interest in a sample beyond the reach of other analytical techniques. The instruments and sample environments at ILL are highly adaptable to a broad range of experiments and scientific problems. The new research uses neutron reflectometry (NR), which can characterise interfaces between materials at the nanometre scale. ILL’s FIGARO is a time of flight reflectometer optimised for measurements at free liquid surfaces, ideal for developing the novel sample environment.

The combination of electrochemical characterisation techniques and NR has the potential to provide answers for many scientific fields, in particular the soft matter and environmental sciences. Biological models of cell membranes, critical to early-stage drug development and toxicity studies, can be mimicked with a ‘floating’ membrane interface between two liquids. This could allow the study of whether a drug reacts differently in an oil or aqueous medium, or how it passes through the membrane, as an electric field modifies the conditions while structural analysis is performed. Usually, studies on model membranes are performed on a solid substrate, with no opportunity to modify the interface from both sides, or allowing permeation studies to be carried out. Thus, the new technique could enhance fundamental research into how drugs or ions may act in a cell.

From food sciences to cosmetics, liquid-liquid interfaces are ubiquitous in chemistry, biology, and throughout our surroundings, and are critical to our ability to control interactions of substances.

Advances in analytical techniques to observe these interactions in immense detail are essential for providing answers to long-standing scientific questions. Scientists at ILL are constantly innovating new instrument adaptations and environments to provide a home for structural analysis in all areas of research.

Contact : Andrea Tummino (ILL)

Email: tumminoa@ill.fr

Re: Neutron reflectometry study of the interface between two immiscible electrolyte solutions: effects of electrolyte concentration, applied electric field, and lipid adsorption. Andrea Tummino  et al. (2021)
doi.org/10.1016/j.electacta.2021.138336

You can find out more about the FIGARO instrument here: https://www.ill.eu/FIGARO

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