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Published May 14, 2013 | Published
Journal Article Open

Long-range specific ion-ion interactions in hydrogen-bonded liquid films


Anions populate fluid interfaces specifically. Here, we report experiments showing that on hydrogen-bonded interfaces anions interact specifically over unexpectedly long distances. The composition of binary electrolyte (Na+, X−/Y−) films was investigated as a function of solvent, film thickness, and third ion additions in free-standing films produced by blowing up drops with a high-speed gas. These films soon fragment into charged sub-micrometer droplets carrying excess anions detectable in situ by online electrospray ionization mass spectrometry. We found that (1) the larger anions are enriched in the thinner (nanoscopic air-liquid-air) films produced at higher gas velocities in all (water, methanol, 2-propanol, and acetonitrile) tested solvents, (2) third ions (beginning at sub-μM levels) specifically perturb X−/Y− ratios in water and methanol but have no effect in acetonitrile or 2-propanol. Thus, among these polar organic liquids (of similar viscosities but much smaller surface tensions and dielectric permittivities than water) only on methanol do anions interact specifically over long, viz.: ⟨ri − rj⟩/nm = 150 (c/μM)^(−1/3), distances. Our findings point to the extended hydrogen-bond networks of water and methanol as likely conduits for such interactions.

Additional Information

© 2013 AIP Publishing LLC. Received 21 January 2013; accepted 18 April 2013; published online 9 May 2013. This work was financially supported by the Japan Science and Technology Agency (JST) PRESTO program, and by the U.S. National Science Foundation (NSF) Grant No. AGS-1238977. We thank H. Mishra and M. R. Hoffmann (Caltech) for helpful discussion. We are grateful to Professor Theofanous (UC Santa Barbara) for valuable advice.

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