Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two‐Dimensional Heterodyne‐Detected VSFG Spectroscopy

Abstract

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular‐level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate‐ion (TPB−)/water and tetraphenylarsonium‐ion (TPA+)/water interfaces, which are clarified by two‐dimensional heterodyne‐detected vibrational sum‐frequency generation (2D HD‐VSFG) spectroscopy. The obtained 2D HD‐VSFG spectra of the anionic TPB− interface reveal the existence of distinct π‐hydrogen bonded OH groups in addition to the usual hydrogen‐bonded OH groups, which are hidden in the steady‐state spectrum. In contrast, 2D HD‐VSFG spectra of the cationic TPA+ interface only show the presence of usual hydrogen‐bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

“…In order to extract quantitative information about the vibrational relaxation dynamics of the free OH by separating the thermalization signal, we carried out a singular value decomposition (SVD) analysis of the time-resolved ΔIm χ (2) spectra obtained at the delay times from −0.2 to 6.0 ps. SVD is a mathematical technique to decompose a set of the time-resolved ΔIm χ (2) spectra into independent spectral components and their temporal profiles (see Supplementary Note 2 for details) 38 – 40 . We stress that the SVD analysis is not applicable to time-resolved Δ| χ (2) | 2 spectra measured with conventional homodyne detection and that it can be used for the time-resolved ΔIm χ (2) spectra obtained with heterodyne detection.…”

Unified picture of vibrational relaxation of OH stretch at the air/water interface

“…On the other hand, time-resolved heterodyne-detected VSFG (TR-HD-VSFG) provides Δχ (2) 28 , 33 – 35 , and ΔImχ (2) spectra can be directly compared to the time-resolved infrared or Raman spectra that correspond to ΔImχ (1) and ΔImχ (3) spectra, respectively. Furthermore, TR-HD-VSFG measurements carried out with various pump frequencies can provide 2D spectra, i.e., two-dimensional (2D) HD-VSFG spectra 34 , 36 , which are interface counterparts of 2D-IR spectra 33 , 34 , 36 – 40 .…”