Infrared Hide-and-Seek: Vibrational Excitons Conceal Surfactants at the Air/Water Interface

01 March 2021, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational excitons in infrared reflection-absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling yields a signicant decrease in peak intensities corresponding to C-F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C-H stretching modes. The aqueous phase ionic composition impacts surfactant intermolecular distances, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities in analyses that are ubiquitous in interface science.

Keywords

excitons
air/water interface
vibrational spectroscopic analysis
vibrational excitons
Perfluorooctanoic acid
PFOA adsorption
quantum chemistry approaches
Infrared Reflection Absorption Spectroscopy
IRRAS
surface-sensitive
surface-selective
monolayer adsorption behaviors
vibrational couplings
Arachidic acid
Soluble Surfactants
IR Spectroscopy Analysis
transition dipole moment
exciton Hamiltonians
fragment based quantum chemistry methods
wavefunction delocalization
AMOEBA Force Field
density functional theory

Supplementary materials

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Description
Actions
Title
SI InfraredHideAndSeek II
Description
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