Imaging chemical kinetics at liquid-liquid interfaces

“It is a new application of so-called water flat-jets,” says Andreas Osterwalder at EPFL’s School of Basic Sciences. “We prepare a controlled interface between two aqueous solutions and use it to measure chemical kinetics.”

Free-flowing liquid microjets allow chemists to create a controllable smooth (and in some cases flat) surface of a liquid that can be used for surface scattering or spectroscopy studies. The free flow of the liquid in air or in a vacuum creates unobstructed optical access to gas-liquid and liquid-vacuum interfaces.

Some of the main applications of microjets include X-ray photoelectron spectroscopy, evaporation dynamics, attosecond-pulse generation, and gas-liquid chemistry. A popular implementation is a single cylindrical jet, made by forcing a liquid to exit through a nozzle 10–50 micrometers in diameter and under a pressure of a few bars, resulting in a laminar jet with a flow velocity of tens of meters per second.

These microjets have recently garnered a lot of interest for in-vacuum applications, where the jets travel freely, and remain liquid, for some millimeters before decaying into droplets and freezing. “Many experiments require a planar surface that prevents unwanted averaging over effects from the angle-dependent surface,” says Osterwalder. As a result of this need, scientists have been developing different arrangements of laminar-flow planar surfaces, producing so-called liquid flat-jets.

Liquid leaves

A common form of such an arrangement is to cross two cylindrical jets of a liquid. The resulting flat-jet is a chain of leaf-shaped structures of the flowing liquid. The “leaves” are sheets only a few microns thick, and each one is bound by a relatively thick fluid rim and stabilized by surface tension and fluid inertia.

At the point where the two cylindrical jets cross, the solutions are pushed outwards, while continuing to move in an overall forward direction. But the surface tension of the flowing solutions counteracts this, so eventually the outer boundaries merge to create the “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,” says Osterwalder. “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 scientists tested the flat-jet arrangement by using it to study the kinetics of the luminol oxidation chemiluminescence reaction, a glow-in-the-dark reaction that emits a blue light when the organic compound luminol is oxidized. The reaction is popular with criminal investigators looking for trace amounts of blood, but is also widely used in biological research assays.

Using the luminol reaction, the researchers confirmed that the flat-jet indeed does contain a liquid-liquid interface, rather than solutions that are mixed by turbulent processes, and they demonstrate a technique for chemical kinetics studies under controlled conditions. The advantage of the flat-jet method is that does away with the need for rapid mixing of solutions, and benefits from the free-flowing jets that are not perturbed by friction on 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,” says Osterwalder.

Other contributors

• Fritz-Haber-Institute of the Max-Planck-Society
• Czech Academy of Sciences
• University of Kassel