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New Method to Measure Chemical Kinetics by Imaging Progress of a Reaction at a Liquid–Liquid Interface

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

New Method to Measure Chemical Kinetics by Imaging Progress of a Reaction at a Liquid–Liquid Interface.
A flat-jet exhibiting blue luminescence due to the oxidation of Luminol. The photograph depicts the leaves formed through the crossing of two liquid microjets, flowing from left to right, and shows that the first leaf is characterized by laminar flow. As a consequence, a liquid-liquid interface is formed which can be used to measure chemical kinetics. Image Credit: A. Osterwalder (École polytechnique fédérale de Lausanne).

This technique seems to be perfect for studies of dynamics on the sub-millisecond timescale, which is very hard to do with present applications.

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

Andreas Osterwalder, School of Basic Sciences, École polytechnique fédérale de Lausanne

It is possible for chemists to design a controllable smooth (and in certain cases flat) surface of a liquid that could be utilized for surface scattering or spectroscopy studies with the help of free-flowing liquid microjets. The free flow of the liquid present in the air or a vacuum makes unimpeded optical access to gas–liquid and liquid–vacuum interfaces.

Some primary 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 to exit via a nozzle measuring a diameter of 10–50 μm and under a pressure of a few bars, leading to a laminar jet with a flow velocity of tens of meters per second.

In recent times, such microjets have acquired huge interest for in-vacuum applications, where the jets tend to travel freely, and remain liquid, for a few millimeters before decaying into droplets and being subjected to freezing.

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

Andreas Osterwalder, School of Basic Sciences, École Polytechnique Fédérale de Lausanne

Consequently, because of this need, researchers have been coming up with various arrangements of laminar-flow planar surfaces, thereby generating alleged liquid flat-jets.

Liquid Leaves

A general form of such an arrangement is to surpass two cylindrical jets of a liquid. The consequent flat-jet is known as a chain of leaf-shaped structures of the flowing liquid. The “leaves” are sheets that measure a thickness of only a few microns, and each one has been bound by a comparatively thick fluid rim and stabilized by fluid inertia and surface tension.

At the point where the two cylindrical jets tend to cross, the solutions are driven outwards, while moving in a complete forward direction. However, the surface tension of the flowing solutions neutralizes this, so ultimately the outer boundaries tend to merge to make the so-called “leaf” shape.

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

Andreas Osterwalder, School of Basic Sciences, École polytechnique fédérale de Lausanne

Osterwalder added, “We believed that this would make them an excellent tool for gaining access to the liquid–liquid interface even of miscible fluids—fluids that mix homogeneously, and even two samples of identical solvents.”

The flat-jet arrangement was tested by the researchers by utilizing it to study the kinetics of the luminol oxidation chemiluminescence reaction. This is known as a glow-in-the-dark reaction that discharges a blue light when the organic compound luminol has been oxidized. The reaction is famous among criminal investigators who trace blood but is also utilized extensively in biological research assays.

With the help of the luminol reaction, the scientists verified that the flat-jet indeed does consist of a liquid–liquid interface, instead of solutions that are mixed by turbulent processes. Also, they illustrate a method for chemical kinetics studies under controlled conditions.

The benefit of the flat-jet technique is that it does away with the requirement for quick mixing of solutions and profits from the free-flowing jets that are not disturbed by friction on the container walls.

We believe this is a promising approach towards measuring chemical kinetics on the sub-millisecond timescale, a range that is very difficult to reach with currently existing technologies, and to study fundamental dynamics at liquid-liquid interfaces,” stated Osterwalder.

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

Journal Reference:

Schewe, H. C., et al. (2022) Imaging of Chemical Kinetics at the Water–Water Interface in a Free-Flowing Liquid Flat-Jet. Journal of American Chemical Society.


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