Molecular Orientation of Carboxylate Anions at the Water–Air Interface Studied with Heterodyne-Detected Vibrational Sum-Frequency Generation

Abstract

The carboxylate anion group plays an important role in many (bio)chemical systems and polymeric materials. In this work, we study the orientation of carboxylate anions with various aliphatic and aromatic substituents at the water–air interface by probing the carboxylate stretch vibrations with heterodyne-detected vibrational sum-frequency generation spectroscopy in different polarization configurations. We find that carboxylate groups with small aliphatic substituents show a large tilt angle with respect to the surface normal and that this angle decreases with increasing size of the substituent. We further use the information about the orientation of the carboxylate group to determine the hyperpolarizability components of this group.

Determining the Surface pKa of Perfluorooctanoic Acid

“…The assignment of the peak at 1370 cm –1 to the coupling between the terminal C–C and C–F 3 bonds of the PFOA alkyl chain is based on studies using heterodyne SFG spectroscopy. , The 1412 and 1760 cm –1 peaks are inversely related as the pH is adjusted. At high pH, the dominant signal is at 1412 cm –1 and is attributed to the symmetric stretch of the carboxylate headgroup (ν SS,COO– ) . As the pH decreases, two peaks emerge at 1705 and 1760 cm –1 .…”

Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces

“…The reported uncertainties in the theoretical R ratios are calculated based on the published uncertainties in the β aca /β ccc ratios. 46 . features in bulk solutions either (Figure S11), we rule out a significant formation of specific ion-associated species.…”

“…In the case of glycine zwitterions, this formalism accounts for the HD-VSFG contributions of the COO – group. The resulting equations involve ratios of the main elements of the molecular hyper-polarizability tensor (β) that were recently experimentally determined for aliphatic COO – groups connecting to alkyl chains (CH 3 (CH 2 ) n COO – ), with n = 0, 1, 4, 6. Using the approximation that the ratios of the β elements are identical for zwitterionic glycine and propionate ( n = 1), the following relations hold: Im ( χ S S P ( 2 ) ) { ν s C O O − } = italica normals = cos ( θ ) · C and Im ( χ S S P ( 2 ) ) { ν a s C O O − } = italica normala normals = prefix− 0.65 · [ cos false( θ false) − cos 3 false( θ false) …”

“…(a) Definition of the angle θ, as the angle between the surface normal and the symmetry axis of the zwitterion’s COO – group. (b) Relative Im(χ (2) ) contribution ( a ) of the two main carboxylate modes of zwitterionic glycine, derived using the theoretical and experimental results of earlier works. − These works use the assumption that the COO – group can freely rotate around the C–C bond. We show italica normala normals / italica normals ratios in Table .…”

“…Using the approximation that the ratios of the β elements are identical for zwitterionic glycine and propionate ( n = 1), the following relations hold: Im and Im , where C is a physical constant, and θ is the angle between the surface normal and the symmetry axis of the COO – group, see Figure (a). To obtain this result, we assumed the angular distribution of the COO – groups is narrow: Δθ = 0, and that the COO – group can freely rotate around the connecting C–C bond . In general, the narrow distribution assumption yields sufficiently accurate results for glycine species oriented by direct surfactant interactions or by strong electric fields.…”