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Published May 23, 2013 | public
Journal Article

Long-Range Hofmeister Effects of Anionic and Cationic Amphiphiles


Specific ion effects at aqueous interfaces play key roles in many important phenomena. We recently reported that ions interact specifically over unexpectedly long distances on the surface of sub-micromolar electrolyte solutions (Enami et al. J. Chem. Phys. 2012, 136, 154707). Whether the anionic and cationic headgroups of the organic amphiphiles present at most water/hydrophobe interfaces act similarly or display new behaviors, however, is not known. Here we report the results of experiments in which we apply online electrospray ionization mass spectrometry (ESI-MS) to investigate how carboxylate, RCOO– (R = CH3, C5H11, C7H15), and alkylammonium, R′(CH_3)_(3)N+ (R′ = CH_3, C_(14)H_29), ions affect the ratio χ = I–/Br– at the aerial interface of 1 μM (NaI + NaBr) aqueous solutions. We found that χ is systematically but selectively depressed by these ionic amphiphiles and minimally affected by the neutral surfactant 1-octanol. The depressing effects induced by cationic headgroups are stronger than those caused by anionic surfactants and always increase with the length of the alkyl chains.

Additional Information

© 2013 American Chemical Society. Received: February 4, 2013; Revised: April 22, 2013; Published: April 26, 2013. This work was financially supported by the Japan Science and Technology Agency (JST) PRESTO program. We are grateful to Michael Hoffmann and Himanshu Mishra of Caltech for helpful discussion.

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