But directly observing how water solvents participate in critical pH-dependent processes, such as protein assembly, is no easy matter. Scientists need a technique that can resolve the mutual interactions occurring between acids and millions of chemically similar surrounding water molecules.
Now, researchers led by Shik Shin from the RIKEN SPring-8 Center in Hyogo prefecture, Japan, have combined state-of-the-art technical engineering with x-rays to detect quantum electronic state changes during an aqueous acid–base reaction. The resulting data paint a surprising picture of water-induced stability that differs sharply from typical views of chemical dynamic
A delicate balance
The researchers used a molecule called acetic acid—best known as the main component of vinegar—as a model system to explore pH-dependent reactions. When acetic acid is dissolved in water, it can either remain intact or it can break its oxygen–hydrogen (O–H) bond and become a charged anion. At acidic pH, the neutral species dominates whereas at basic pH, the anion form prevails. Equilibrium always exists between the two states, however, regardless of the pH level.
During this chemical reaction, acetic acid moves through and interacts with a multitude of water molecules. Shin and colleagues found that the best way to understand how water influences the acid–base equilibrium was to probe the quantum electronic states that hold acetic acid together.
The imaging abilities of x-rays are familiar to anyone who has visited a dentist’s clinic. However, x-ray generated by electrotons accelerated to near the speed of light in the RIKEN SPring-8 synchrotron also can be used to peer deep inside molecules to reveal their underlying chemical framework.
In x-ray emission spectroscopy (XES), a photon first excites an electron, which is tightly bound to the nucleus, to a higher, excited-energy state. This creates a ‘hole’—an absent electron—in the core level. This hole is quickly filled by a valence electron in the molecule, and, to conserve energy, a new x-ray photon is emitted.
The emitted x-rays are uniquely dependent on the chemical and physical environment of the excited atom—a characteristic that let the researchers distinguish the electronic signal of acetic acid from the surrounding water molecules. Following this signal at varying pH levels provided nearly instantaneous information on chemical interactions during the acid–base reaction.
Because the intensity of x-rays emitted by light elements such as carbon, nitrogen and oxygen is quite low, it was essential to use high-energy synchrotron light to produce detectable electronic signals. “Observing such kinds of weak x-ray emissions requires an intense excitation light source and precise energy tuning of the incident light,” says co-author Takashi Tokushima, a research scientist at RIKEN.
Engineering at the interface
To perform the experiment, the researchers developed a new liquid cell2 that could be bolted onto the vacuum chamber and connected to the beamline of the synchrotron light source. According to Tokushima, two challenges had to be overcome during the engineering process.
“First, we had to make a window as thin as possible to separate the vacuum chamber from the liquid cell,” says Tokushima. Once the team had determined the optimum window—a 150 nm thin silicon nitride film—they faced their second challenge: how to flow a liquid sample past it.
“The 150 nm window is very fragile,” states Tokushima. “Our liquid cell is designed to make liquid flow parallel to the window, with no pushing on its surface.” Otherwise, the window cracks—spilling water into the vacuum chamber.
Using their liquid cell, the scientists recorded x-ray emissions from a double-bonded oxygen atom located in acetic acid. The XES spectra showed significant differences between low and high pH levels—reflecting the change in chemical structures between the neutral and ionized species.
“Each peak structure mostly corresponds to a unique valence electronic state,” says Tokushima. “Thus, we can use XES to fingerprint the forms of acetic acid.”
At intermediate pH levels, chemists typically picture a dynamic acid–base equilibrium, in which the neutral and ionized forms of acetic acid rapidly change states. According to this model, the XES spectra of aqueous acetic acid at intermediate pH should be quite different than the acidic or basic spectra.
However, the researchers saw only gradual, systematic changes to the original XES spectra. In fact, the spectra from intermediate pH levels could be reproduced as ratios of either the pure neutral or ionic form. These results indicated that the two states of acetic acid did not interact with each other; instead, each state was static and stable, with populations determined by the solution pH.
Tokushima says that the unexpected perfect match between the ratio analysis of the XES spectra and a static acid–base equilibrium equation provides strong evidence that water molecules stabilize the different acetic acid states, possibly by forming a ‘shell’ of water around the acid molecules.
The next plan for the researchers is to install a microfluidic mixer into their liquid cell to study chemical reactions involving two different reagents. Because the XES technique is extremely fast, the researchers hope to observe electronic structure changes with real-time precision. “In XES, the emission process occurs within the core hole lifetime—a few femtoseconds (1015 seconds),” says Tokushima. “That means it is possible to observe dynamics down to this timescale.”
About the Researcher
Shik Shin obtained his doctorate of science in 1983 from the University of Tokyo in Japan. He then joined the Research Institute of Measurement at Tohoku University as a research associate, and was later promoted to associate professor. In 1991, he returned to the University of Tokyo as an associate professor in the Institute for Solid State Physics, where he later obtained the position of professor. Shin joined RIKEN in 2008 as team leader of the Excitation Order Research Team in the Quantum Order Research Group.
Journal information1. Horikawa, Y., Tokushima, T., Harada, Y., Takahashi, O., Chainani, A., Senba, Y., Ohashi, H., Hiraya, A., & Shin, S. Identification of valence electronic states of aqueous acetic acid in acid–base equilibrium using site-selective X-ray emission spectroscopy. Physical Chemistry Chemical Physics 11, 8676–8679 (2009).
2. Tokushima, T., Horikawa, Y., Harada, Y., Takahashi, O., Hiraya, A., & Shin, S. Selective observation of the two oxygen atoms at different sites in the carboxyl group (–COOH) of liquid acetic acid. Physical Chemistry Chemical Physics 11, 1679–1682 (2009).
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