In chemistry, there are many interfaces between liquids and gases, such as in biological processes and the atmosphere. Understanding the reactivity of molecules at these interfaces is important for predicting and controlling chemistry in these environments.

Tahei Tahara and colleagues at RIKEN in Japan1 have discovered that, contrary to expectations, the interface ‘polarity’ — the ability of the interface to stabilize polar molecules — of an air/water interface is not uniform but differs according to the molecule that comes into contact with it. They found that the structure of a molecule affects the effective polarity it experiences, which in turn affects its chemical reactivity, necessitating a rethink of our understanding of chemistry at interfaces.

The researchers used coumarin dye molecules to evaluate effective interface polarity because the electronic spectrum of these molecules varies depending on the nature of the molecular environment. The experiments were conducted using the researchers' specially developed spectroscopic technique, which allowed spectra for molecules at interfaces to be obtained at comparable quality to those in bulk solution despite the much smaller numbers of molecules present at the interface.

By comparing their spectra with those of coumarin dye in water (a polar solvent) or hexane (non-polar), the researchers were able draw conclusions about the polarity of the environment of the molecules at the interface. The spectra for molecules in hexane are representative of results that would be expected if the spectra could be taken in air (non-polar).

Fig. 1: Different molecules adopt different positions at the air/water interface.

Tahara and his colleagues found that the spectra of the different coumarin dyes resembled that of the dye in either hexane or water depending on the structure of the molecule. Furthermore, they discovered that the degree to which the spectrum was similar to that in either hexane or water depended on how the molecule was oriented at the interface. As the angle of the molecule to the interface changed, different parts became submerged, changing the environment of the molecule and consequently the polarity it encountered (Fig. 1).

“We provided clear data that change the present understanding of one of the most fundamental concepts in the chemistry at liquid interfaces, that is, the polarity of liquid interfaces,” says Tahara. “This work also showed that the spectroscopy technique we developed is a very powerful new tool to study soft interfaces.”