Phase-Sensitive Sum Frequency Revealing Accommodation of Bicarbonate Ions, and Charge Separation of Sodium and Carbonate Ions within the Air/Water Interface

Abstract

Interfacial water structure plays a key role in many chemical, biological, and environmental processes. Here, in addition to conventional VSFG, we employ phase-sensitive sum frequency generation (PS-SFG) to investigate the average direction of the transition dipole of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ((2)). The orientation of water at air/aqueous inorganic salt interfaces of Na(2)CO(3) and NaHCO(3) was inferred from the direct measurement of the transition dipole moment of the interfacial water molecules. It is found that bicarbonate and its counterion sodium do not significantly perturb the interfacial water structure, whereas carbonate strongly orients water so that the water hydrogens point down toward the bulk solution. This is consistent with the picture of carbonate anions residing many layers below the water surface with a preference for the sodium cations to be above the anions and thereby closer to the topmost layer of the water surface.

Mapping Molecular Orientation with Phase Sensitive Vibrationally Resonant Sum-Frequency Generation Microscopy

“…Recent tomographic work has demonstrated that interferometric detection of SHG signals gives access to phase information that cannot be probed with conventional SHG imaging. 27,29,43 Indeed, our observation is in concert with recent phase sensitive SHG measurements on collagen (through private communication with François Légaré, Institut National de la Recherche Scientifique, Centre Énergie Matériaux et Télécommunications, Canada). Nonetheless, phase-sensitive SFG enables a direct identification of the molecular modes whose differences in orientation are responsible for the sign reversal, providing a more complete molecular picture than what can be obtained with SHG.…”

“…It was previously shown in macroscopic SFG spectroscopic experiments that phase sensitive (PS) detection allows a direct measurement of the amplitude and phase of vibrationally resonant χ (2) components. 24–27 Here, we extend this principle to microscopic examination of collagen tissue, providing information on the effective polarity of the collagen fibrils within the probing volume. Using PSVR-SFG, we show that the methylene mode exhibits a highly conserved orientation within individual bundles of fibrils.…”

Time-resolved phase-sensitive second harmonic generation spectroscopy

“…More recently, this has been overcome by the use of broadband lasers and the collection of the entire spectrum using heterodyne-detected (HD) SHG 19,20 and SFG spectroscopy. [21][22][23][24][25][26][27][28][29][30] In this, the signal and local oscillator are separated in time at a fixed delay and the interference is essentially measured in the frequency domain. By back-Fourier transformation, filtering and forward-Fourier transformation, the real and imaginary parts of the non-linear susceptibility can be measured across a spectral range.…”

Study of Water Interfaces With Phase-Sensitive Sum Frequency Vibrational Spectroscopy

“…[83][84][85][86] The differential Imχ (2) S spectra for different salts with respect to that of neat water are presented in Fig. 12.…”