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    Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

    Year: 2021

    Journal: Chem. Sci., Volume 12, JUN 28, page 8320–8332

    Authors: Carter-Fenk, Kimberly A.; Carter-Fenk, Kevin; Fiamingo, Michelle E.; Allen, Heather C.; Herbert, John M.

    Organizations: National Science Foundation Center for Aerosol Impacts on Chemistry of the Environment (NSF-CAICE) [CHE-1801971]; National Science Foundation [CHE-1955282]; Ohio Supercomputer Center [PAA-0003]; Ohio State University

    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 exciton delocalization in infrared reflection-absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant 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. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.
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