New method to measure chemical kinetics

Led by the École polytechnique fédérale de Lausanne (EPFL), scientists have developed a new technique to quantify the chemical kinetics by imaging the development of a reaction at a liquid-liquid interface that has been fixed in a microjet laminar flow liquid.

A flat jet showing blue luminescence due to the oxidation of Luminol. The photograph shows the sheets formed by the crossing of two liquid microjets, flowing from left to right, and shows that the first sheet is characterized by a laminar flow. As a result, a liquid-liquid interface is formed which can be used to measure chemical kinetics. Image credit: A. Osterwalder (Federal Polytechnic School of Lausanne).

This technique appears to be perfect for sub-millisecond time scale dynamics studies, which is very difficult to do with current applications.

This is a new application of so-called flat water jets. We prepare a controlled interface between two aqueous solutions and use it to measure chemical kinetics.

Andreas Osterwalder, Faculty of Basic Sciences, Swiss Federal Institute of Technology Lausanne

It is possible for chemists to design a controllable smooth (and in some cases flat) surface of a liquid that could be used for surface scattering or spectroscopy studies using free-flowing liquid microjets. The free flow of liquid present in air or vacuum allows unhindered optical access to gas-liquid and liquid-vacuum interfaces.

Some major applications of microjets include evaporation dynamics, attosecond pulse generation, X-ray photoelectron spectroscopy, and gas-liquid chemistry. A famous implementation is a single cylindrical jet, created by forcing a liquid out through a nozzle measuring 10–50 μm in diameter and under a pressure of a few bars, resulting in a laminar jet with a flow velocity of several tens of meters per second.

Lately, these microjets have gained enormous interest in vacuum applications, where the jets tend to move freely and remain liquid for a few millimeters before breaking up into droplets and being subjected to freezing.

Many experiments require a flat surface that prevents unwanted averaging over the angle-dependent surface effects.

Andreas Osterwalder, Faculty of Basic Sciences, Swiss Federal Institute of Technology Lausanne

Therefore, due to this need, researchers have proposed various arrangements of laminar flow planar surfaces, thereby generating so-called liquid flat jets.

Liquid leaves

A general form of such an arrangement is to protrude two cylindrical jets of a liquid. The resulting flat jet is known as a chain of sheet-like structures of the flowing liquid. “Sheets” are sheets that are only a few microns thick, and each has been bound by a relatively thick fluid rim and stabilized by fluid inertia and surface tension.

At the point where the two cylindrical jets tend to intersect, the solutions are forced outward, while moving in a full forward direction. However, the surface tension of flowing solutions counteracts this, so that eventually the outer boundaries tend to coalesce to form the so-called “sheet” shape.

These free-flowing impinging jets produce a sheet structure, where we hypothesized that due to the absence of turbulence, the fluids flow next to each other in the first sheet, forming an interface between two liquids.

Andreas Osterwalder, Faculty of Basic Sciences, Swiss Federal Institute of Technology Lausanne

Osterwalder added:We thought this would make it a great tool for accessing the liquid-liquid interface of even miscible fluids – fluids that mix homogeneously, and even two identical solvent samples..”

The flat jet arrangement was tested by the researchers using it to study the kinetics of the chemiluminescent reaction of luminol oxidation. This is known as the glow-in-the-dark reaction which discharges blue light when the organic compound luminol has been oxidized. The reaction is famous among criminal investigators who trace blood, but it is also widely used in biological research tests.

Using the luminol reaction, the scientists verified that the flat jet is indeed made up of a liquid-liquid interface, instead of solutions mixed by turbulent processes. In addition, they illustrate a method of chemical kinetic studies under controlled conditions.

The advantage of the flat jet technique is that it removes the requirement for rapid mixing of solutions and takes advantage of fluid jets which are undisturbed by friction on the vessel walls.

We believe this is a promising approach for measuring chemical kinetics at the sub-millisecond scale, a range very difficult to achieve with existing technologies, and for studying fundamental dynamics at liquid-liquid interfaces.“said Osterwalder.

The study was financially supported by the Swiss National Science Foundation, the German Research Foundation (DFG), EPFL-Max Planck Center for Molecular Nanosciences and Technology

Journal reference:

Schewe, HC, et al. (2022) Imaging chemical kinetics at the water-water interface in a free-flowing liquid flat jet. Journal of the American Chemical Society.


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