Experimentally quantifying anion polarizability at the air/water interface

Understanding anion specific interactions with hydrophobic interfaces is challenging due to an absence of local structural probes. Here, the authors experimentally quantify the anisotropy of perchlorate’s polarisability at the air/water interface, a window onto anion and solvation shell structure.
Published in Chemistry
Experimentally quantifying anion polarizability at the air/water interface

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The paper in Nature Communications is here: http://rdcu.be/KvcW

It is known that different anions have different interfacial properties that relate to various biological, environmental and physical phenomena.  For example, in biology, the Hofmeister series classifies ions in order of their ability to salt out or salt in proteins.  A similar trend has been found for the propensity of anions at the air/water interface. While the phenomena had been found in various fields a long time ago, the underlying mechanism is still debatable. So far, the main problem is the lack of experimental input for the properties of the anions at the interface. Owing to its surface selectivity and high sensitivity, vibrational sum frequency spectroscopy (VSFS) that is commonly used in our group (http://www.fhi-berlin.mpg.de/pc/NSECh/), might be able to provide novel information regarding the anion structure at the air/water interface. By choosing an anion with high symmetry, namely perchlorate and studying its vibrational response at the air/water interface with VSFS, we expect to gain insight into the properties of the anions that are related to, for example, the Hofmeister series.

It was very exciting to observe generally silent vibrational mode of perchlorate to be VSF active at the interface. This suggested that the symmetry of the anion is broken at the interface due to an unknown reason. To this end, we have performed control experiments such as replacing the cations from hydronium to lithium, sodium and potassium; exchanging the anion to sulfate which has the same symmetry but a different surface propensity; examining solutions at various concentrations, etc. Finally, we concluded that the symmetry break was due to the anisotropic solvation environment at the interface. Furthermore, the ability to obtain signals with different polarization combinations allowed us to quantify the anion’s depolarization ratio at the interface.  In comparison with its bulk counterpart, a significant increase was observed, which suggests strong structural changes as the anion is adsorbed to the interface. To relate this to the exact structural changes, our colleague Igor Ying Zhang from the theory department of FHI has performed electronic structure calculations, which reveal the corresponding structures of the anions at the interface. 

During the submission process, we got constructive comments from the reviewers. According additional surface tension measurements were done with the help of Irina Shekova from the Max Planck Institute of Colloids and Interfaces which support the conclusions drawn from spectroscopy.

In this work we demonstrated that the presence of the interface can induce deformation of the anion and its solvation environment which should relate to the propensity of the anion at the air/water interface. The close connection we describe in the current study between the dipole moment, structure and polarizability of interfacial anions with increasing interfacial field has not, to our knowledge, been previously considered but should be a quite general feature of anions, particularly polyvalent anions at the hydrophobic interfaces. An extension of the current study will provide insight into the mechanisms for the Hofmeister series.

The paper in Nature Communications is here: https://go.nature.com/2H7Bb0r

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