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

preprint
submitted on 23.09.2020 and posted on 24.09.2020 by Kimberly A. Carter-Fenk, Kevin Carter-Fenk, Michelle E Fiamingo, Heather Allen, John M. Herbert

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.

History

Email Address of Submitting Author

carter-fenk.2@osu.edu

Institution

The Ohio State University

Country

United States

ORCID For Submitting Author

0000-0001-8302-4750

Declaration of Conflict of Interest

J. M. H. serves on the board of directors of Q-Chem Inc.

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