Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration

Abstract

Spectroscopic studies of aqueous surfaces can provide a fundamental understanding of interfacial processes. These studies have largely focused on the OH stretch vibrations of water, which is unfortunate as the bending mode is an attractive feature to probe as it is relatively free of inter-and intramolecular couplings. However, the origin of the response of the bending mode is highly debated and has been assigned to either surface-specific or to bulk effects. Here, we study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG) to quantify the two effects. Our results show a low -(1626 cm −1 ) and a high -(1656 cm −1 ) frequency component which can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.

Direct Observation of the Orientation of Urea Molecules at Charged Interfaces

“…For SDS and DTAB solutions, the response of the water bending mode is strongly affected by adding NaCl. 25 We observe that, after adding NaCl to the solution, the amplitudes of the vibrational modes of urea substantially decrease. This decrease indicates that the net orientation of the urea molecules strongly decrease upon the addition of Na + and Cl – ions.…”

“…The observed decrease in the signal of urea is similar to the one observed for the dipolar response of the bending mode of water. 25 This observation indicates that the orientation of urea and water molecules in a local electric field are highly correlated, which indicates that urea and water form a collective hydrogen bond network. Application of an electric field induces a net orientation of this collective hydrogen bond network, thus leading to a similar orientational effect on the responses of the urea and water molecules forming the network.…”

“…A more detailed explanation of this procedure can be found elsewhere. 25 For all measurements, we used water from a Millipore Nanopure system (18.2 MΩ cm, “Simplicity”). Urea, DTAB (dodecyltrimethylammonium bromide), and SDS (sodium dodecyl sulfate) were purchased from Sigma-Aldrich and are used without further purification.…”

Bulk Response of Carboxylic Acid Solutions Observed with Surface Sum-Frequency Generation Spectroscopy

“…The remaining modulation, which largely represents the structural noise on the CCD camera induced by the etaloning effect, can now be used as a scaling factor to remove the structural noise from the data. 12 For all measurements we used water from a Millipore Nanopure system (18.2 MΩ·cm). Acetic acid (99.8%) and formic acid (98%) were purchased from Sigma-Aldrich and are used without further purification.…”

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent

“…As noted in Figure 1 , this region contains the vibrational modes of urea (⟩C=O stretch and −NH 2 bend) and that of the water bending mode. 7 , 30 , 71 − 73 The experimental details of the setup and the measurement procedure are presented in the Supporting Information . The HD-VSFG spectrum in the 1500–1800 cm –1 region shows two vibrational bands centered at ∼1590 and ∼1660 cm –1 , similar to the FTIR spectrum.…”