Contrary to expectations, structurally different molecules can display different
solvent properties at an interface between air and water, researchers in Japan
have discovered1. Tahei Tahara and colleagues from the RIKEN
Advanced Science Institute in Wako showed that polarity at this interface
cannot be defined simply, because it depends on the nature of the solute molecule
at the interface. The finding could have significant consequences for chemistry
at interfaces, since the polarity of a molecule's environment affects how it
reacts with other molecules. Fields such as atmospheric science, where air/water
interfaces abound, will be particularly affected.
The researchers made their discovery using an interface-selective spectroscopic
technique that they developed earlier2. The spectra that the technique produces
are of comparable quality to those of bulk solutions, enabling previously impossible
comparisons between systems.
The researchers looked at the electronic spectra of five coumarin dyes at the
interface between air and water; electronic spectra are essentially a graphical
representation of a molecule's color. Coumarin dyes all share the same
basic chemical structure and are used to probe the polarity of solvents because
their spectra differ depending on the molecules' environment.
Tahara and colleagues found that the spectra of all five coumarin dyes at the
air/water interface resembled a cross between the bulk spectra of coumarin in
polar water and non-polar hexane. This is because the dye molecules were positioned
partly in the polar water and partly in the non-polar air at the interface.
However, the closeness of the spectra to either the spectrum in water or in
hexane changed depending on the precise structure of each coumarin dye.
Previously it was thought that, in ordinary cases, molecules experience the
same polarity—the average of that of polar water and non-polar air. The
spectra Tahara and colleagues measured, however, showed that even molecules
having similar structures experience substantially different polarity at the
air/water interface.
The researchers found that the different molecules were positioned at slightly
different angles at the interface of air and water so have different sections
of their structures submerged and are, consequently, in quantitatively different
surroundings.
“This work showed that, even at the same air/water interface, the interaction
between the solute and solvent is significantly varied,” says Tahara.
This means the molecules experience different environments at the interface,
similar to being in different solvents from a view point of the stabilization
energy. “This fundamental understanding of molecular behavior will be
very important when people consider chemical reactivity at liquid interfaces.”
Posted October 15th, 2009
