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

Shinichi Enami, Agustín J. Colussi
2013 Journal of Chemical Physics  
Anions populate fluid interfaces specifically. Here, we report experiments showing that on hydrogenbonded 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
more » ... 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.: r i − r j /nm = 150 (c/μM) −1/3 , distances. Our findings point to the extended hydrogenbond networks of water and methanol as likely conduits for such interactions. © 2013 AIP Publishing LLC. [http://dx.
doi:10.1063/1.4803652 pmid:23676063 fatcat:ssdgfgbdjjdenamrm75gpuh2xq