Spectroscopic evidence for the specific Na+ and K+ interactions with the hydrogen-bonded water molecules at the electrolyte aqueous solution surfaces

Abstract

Sum frequency generation vibrational spectra of the water molecules at the NaF and KF aqueous solution surfaces showed significantly different spectral features and different concentration dependence. This result is the first direct observation of the cation effects of the simple alkali cations, which have been believed to be depleted from the aqueous surface, on the hydrogen bonding structure of the water molecules at the electrolyte solution surfaces. These observations may provide important clue to understand the fundamental phenomenon of ions at the air/water interface. † Also Graduate University of the Chinese Academy of Sciences.* To whom correspondence should be addressed. E-mail: hongfei@iccas.ac.cn; Tel: 86-10-62555347; Fax:86-10-62563167. 1In this letter we would like to report the significantly different sum-frequency generation vibrational spectra of the NaF and KF aqueous solution surfaces. These results suggested that the ion effects on the interfacial water hydrogen bonding structure are quite different for the NaF and KF salts. These results may provide direct counterexamples to the physical picture based on the currently prevalent molecular dynamics (MD) simulations with the polarizable force field, which have predicted that the adsorption of the more polarizable anions, such as the I − and Br − anions, are significantly enhanced at the electrolyte aqueous solution surfaces, while the less polarizable anions, such as the F − and Cl − anions, and the non polarizable cations, such as the Na + and K + cations, are repelled from the electrolyte aqueous solution surfaces. [1,2,3,4,5] Recently, there have been a great deal of theoretical and experimental studies on the ion solvation and ion effects at the electrolyte aqueous solution surfaces. [1,2,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] It has been well established that the ssp SFG spectra for the neat air/water interface has two prominent features, one is the narrow peak centered at 3700cm −1 and the other is the fairly broad band in the 3000-3600cm −1 . The former is the signature of the non hydrogenbonded O-H group which sticks out of the liquid phase, i.e. the so called 'free O-H'; while the latter is the signature of the O-H stretching vibrations of the differently hydrogen-bonded water molecules on the liquid side in the vicinity of the air/water interface. [38,39,40,41,42] These signatures of the interfacial water molecules can be observed in the SFG-VS because the water molecules at the air/water interface are orientationally ordered. [38,39,40,41,42,46] It has been known that addition of the electrolytes into the bulk aqueous phase can introduce changes in the ssp SFG spectra in the hydrogen-bonded water region, i.e. 3000- [3,6,17,18,24,39,47] In order to systematically investigate the anion effects on the hydrogen-bonded water molecules at the electrolyte aqueous solution surfaces, the ssp 3 SFG-VS spectra of the NaF, NaCl, NaBr and NaI slat solution surfaces were measured by two independent groups. [17...

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

“…The decrease in the broad-band intensity of water SFG spectrum at low salt concentrations has also been reported by others. 25,26 Allen et al recorded the SFG spectra for 0.8 M NaF solution and 0.8 and 2 M solutions of NaCl, NaBr, and NaI. 19 Contrary to the other works, the SFG spectra did not vary much for 0.8 M solutions of NaF and NaCl.…”

Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray

“…Aqueous interfaces play a fundamental role in many chemical, biological, and environmental processes including geo-mineralization/dissolution and atmospheric aerosol chemistry. , To advance our understanding of these processes, a molecular level picture of the aqueous interface itself is required. So far, water structure and ion distributions for mostly simple mono- and divalent salts at air/aqueous interfaces have been investigated by a variety of techniques including vibrational sum frequency generation (SFG) spectroscopy, − second harmonic generation, − photoelectron spectroscopy, − X-ray reflectivity measurements, , as well as theoretical calculation and simulation. − …”

“…Understanding the spatial extent to which the ionic effects are important is under debate. − Recent studies demonstrate ion-induced long-range orientational order of bulk water molecules, and the long-range effects show specificity associated with the size and charge. , Smaller ions with larger charges tend to perturb water structures more effectively. While specific ionic effects have been observed at the air/aqueous interface, ,,,, those studies have focused mainly on alkali and alkaline earth metal ions.…”

“…62,63 Smaller ions with larger charges tend to perturb water structures more effectively. While specific ionic effects have been observed at the air/aqueous interface, 13,20,26,43,65 those studies have focused mainly on alkali and alkaline earth metal ions.…”

Methylguanidinium at the Air/Water Interface: A Simulation Study with the Drude Polarizable Force Field

“…9 In the 1990s Perera and Berkowitz found that heavy halide ions such as Br − and I − would accumulate at the air/water interface employing molecule dynamics (MD) simulations with a polarizable force field (FF). 10,11 Since then, the propensity of simple ions at the interface has been studied by a variety of surfacesensitive experimental [12][13][14][15] and theoretical approaches. [16][17][18][19][20][21] The specific effects that ions can play on protein stabilities 22 and chemical reactions at the interface 23 have also been realized.…”