Much work by many clever folks over the last 25 years has shown that, contra expectations from classical theories of solutions, some — mostly larger, polarizable — anions adsorb on the air/water interface. While this phenomena has been extensively studied both theoretically and experimentally, understanding the contribution of anion polarizability to the free energy of adsorption has proven surprisingly challenging: anion polarization cannot rationalize known trends in anion adsorption some simulation approaches and interface active anions appear not to be polarized in others. One possible explanation for this state of the affairs is that, in the absence of experimental constraints on interfacial anion polarizability to parameterize classical descriptions or validate interfacial anion polarization calculated from ab-initio simulations, theoretical description of interfacial polarizability is beset by systematic error.

Yujin has just submitted a paper showing that the symmetry of the polarizability tensor of perchlorate, a known interface-active anion, changes by more than 2x in moving from bulk liquid to the air/water interface and that this change is concentration dependent: as the interfacial population of perchlorate increases the polarizability tensor grows still more anisotropic. This work is the first experimental observation of an anion’s interfacial polarizability and the effect that it describes not explicitly parameterized or shown to occur in any theoretical approach. It thus seems likely that reproducing it in simulation is a useful step in trying to resolve the role of polarizability in anion adsorption at the air/water interface (for the preprint see the arXiv).