Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces.

Takakazu Seki,Shuji Ye,Yuki Nagata,Junjun Tan,Chun-Chieh Yu,Shumei Sun,Mischa Bonn,Kuo-Yang Chiang

doi:10.1021/acs.jpcb.1c03258

Abstract

The origin of the sum-frequency generation (SFG) signal of the water bending mode has been controversially debated in the past decade. Unveiling the origin of the signal is essential, because different assignments lead to different views on the molecular structure of interfacial water. Here, we combine collinear heterodyne-detected SFG spectroscopy at the water-charged lipid interfaces with systematic variation of the salt concentration. The results show that the bending mode response is of a dipolar, rather than a quadrupolar, nature and allows us to disentangle the response of water in the Stern and the diffuse layers. While the diffuse layer response is identical for the oppositely charged surfaces, the Stern layer responses reflect interfacial hydrogen bonding. Our findings thus corroborate that the water bending mode signal is a suitable probe for the structure of interfacial water.

Highlights

The bending mode around 1550−1700 cmof−1H. 2TOhishamsodaechhaasrabcteeernistpicrobfreedquuesnincgy vibrational spectroscopies, because it reports on the local structure of the hydrogen-bond network in water; when water is strongly hydrogen-bonded, the frequency of the bending mode is blue-shifted.[1,2] Probing the bending mode of water has several advantages over probing theO−H stretch mode
Whereas the O−H stretch mode of water cannot be spectrally distinguished from other molecules containing OH-groups, the H−O−H water bending mode is specific to water.[3−8] the vibrational coupling between bending modes has a limited impact on its spectral response,[3,9] in sharp contrast to the O−H stretch mode.[10,11]
The H−O−H bending mode of interfacial water molecules has been probed with sum-frequency generation (SFG) spectroscopy.[3,22−28] SFG spectroscopy is surface-specific, the precise origin of the SFG signal has been highly debated

Summary

INTRODUCTION

The bending mode around 1550−1700 cmof−1H. 2TOhishamsodaechhaasrabcteeernistpicrobfreedquuesnincgy vibrational spectroscopies, because it reports on the local structure of the hydrogen-bond network in water; when water is strongly (weakly) hydrogen-bonded, the frequency of the bending mode is blue-shifted (red-shifted).[1,2] Probing the bending mode of water has several advantages over probing the. Because the frequency shift cannot be accounted for via the bulk quadrupole mechanism, Tahara and co-workers proposed that the bending mode SFG signal is generated by the higher-order term arising from the interface (interfacial quadrupole mechanism).[28]. Clarifying this apparent contradiction by unveiling the origin of the SFG signal is important, because the different assignments of the origin of the signal lead to different interpretations of the bending mode of water and thereby of the structure of interfacial water. We highlight the importance of the homogeneous sampling of the water−lipid interface, which could be achieved by rotating the sample in the collinear HDSFG setup

METHODS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES