Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal–Liquid Interface

Abstract

Understanding interfacial phenomena is important in processes like corrosion, catalysis, and electrochemical reactions. Specifically, in corrosion inhibition, the assembly of adsorbed surfactants at metal–water interfaces in well-packed, ordered layers is desired. We provide direct evidence of the role of alkyl tails of surfactants in the formation of ordered adsorbed layers at metal–water interfaces. We have employed surface-specific sum frequency generation (SFG) spectroscopy to probe the in situ adsorption and self-assembly of cationic surfactants, alkyldimethylbenzyl ammonium bromides of tail lengths n = 4 (C4) and 12 (C12), without any applied potential or stimulus, at the gold–water interface. Our SFG measurements show that C12 Quat adsorbs as an ordered monolayer, whereas C4 Quat adsorbs in a disordered monolayer. All-atom molecular dynamics (MD) simulations of these surfactants corroborate with SFG results. These findings provide new insights on how hydrophobic interactions between alkyl tails of surfactants affect their self-assembly at metal–water interfaces.

Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

“…In our case, to partially overcome this background, a well-established method of detiming the visible pulse from the IR pulse was employed. 65 This method allows for the selective detection of longer lasting vibrational modes over shorter-lived modes. Required in this setup is an etalon to shape the visible pulse to have a time asymmetric profile with a steep dropoff.…”

“…They confirmed this by use of a windowless VSFSS setup that exhibits no background response. In our case, to partially overcome this background, a well-established method of detiming the visible pulse from the IR pulse was employed . This method allows for the selective detection of longer lasting vibrational modes over shorter-lived modes.…”

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

“…Excellent results have been obtained from research on protein structures in bulk environments (e.g., in solution or in crystal form) using X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, cryo-electron microscopy, etc. − However, it is difficult to examine molecular structures of proteins at interfaces in situ because of the lack of appropriate techniques. In the past 30 years, sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful tool to probe surfaces and interfaces, − including buried interfaces such as solid/liquid and liquid/liquid interfaces. SFG vibrational spectroscopy (SFG in short) is a second-order nonlinear optical spectroscopic method, which can selectively probe surfaces and interfaces with a submonolayer specificity by providing their vibrational spectra.…”

“…SFG vibrational spectroscopy (SFG in short) is a second-order nonlinear optical spectroscopic method, which can selectively probe surfaces and interfaces with a submonolayer specificity by providing their vibrational spectra. The superb surface/interface selectivity of SFG is due to the selection rule of a second-order nonlinear optical process: only a medium without inversion symmetry can generate an SFG signal under the electric dipole approximation. − Most bulk materials have inversion symmetry, and therefore, they cannot produce an SFG signal. Inversion symmetry is broken at a surface/interface, leading to the detection of SFG signals from the surface/interface.…”

“…Inversion symmetry is broken at a surface/interface, leading to the detection of SFG signals from the surface/interface. SFG has been successfully applied to investigate molecular interactions of peptides and proteins at solid/liquid interfaces in the last 20 years. − In a typical SFG experiment, two input pulsed laser beamsa frequency tunable infrared beam and a frequency fixed visible beamspatially and temporally overlap at an interface, generating a signal which has the sum frequency of the two input beams. − The SFG signal intensity is plotted against the infrared input wavenumber to produce a vibrational spectrum of the interface. Figure shows the sample geometry used to collect SFG spectra from interfacial protein molecules and the energy level diagram of the SFG process. In the past 30 years, SFG vibrational spectroscopy has been developed into a powerful tool to probe surfaces and interfaces, including buried interfaces such as solid/liquid and liquid/liquid interfaces.…”