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    La3+ and Y3+ interactions with the carboxylic acid moiety at the liquid/vapor interface: Identification of binding complexes, charge reversal, and detection limits

    Year: 2022

    Journal: J. Colloid Interface Sci., Volume 608, FEB 15, page 2169–2180

    Authors: Sthoer, Adrien; Adams, Ellen M.; Sengupta, Sanghamitra; Corkery, Robert W.; Allen, Heather C.; Tyrode, Eric C.

    Organizations: Swedish Foundation for Strategic Research [SSF-FFL-5]; Swedish Research Council (VR); Ohio State University Allen Research Fund; DOE-BES [DE-SC0022099]

    Keywords: Ion specific effects; Langmuir monolayer; Trivalent ions; Carboxylic acid moiety; Vibrational sum frequency spectroscopy; Molecular characterization; Carboxylate binding complexes; Charge reversal; Poisson Boltzmann theory; Vibrational spectroscopy; Arachidic acid; Eicosanoic acid monolayer

    Specific interactions of yttrium and lanthanum ions with a fatty acid Langmuir monolayer were investigated using vibrational sum frequency spectroscopy. The trivalent ions were shown to interact with the charged form of the carboxylic acid group from nanomolar concentrations (<300 nM). Analysis of the spectral features from both the symmetric and the asymmetric carboxylate modes reveals the presence of at least three distinct coordination structures linked to specific binding configurations. Although the same species were identified for both La3+ and Y3+, they display a different concentration dependence, highlighting the ion-specificity of the interaction. From the analysis of the response of interfacial water molecules, the reversal of the surface charge, as well as the formation of yttrium hydroxide complexes, were detected upon increasing the amount of salt in solution. The binding interaction and kinetics of absorption are sensitive to the solution pH, showing a distinct ion speciation in the interfacial region when compared to the bulk. Changing the subphase pH or adding a monovalent background electrolyte that promotes deprotonation of the carboxylic acid headgroup could further improve the detection limit of La3+ and Y-3(+) to concentrations < 100 nM. These findings demonstrate that nM concentrations of trace metals contaminants, typically found on monovalent salts, can significantly influence the binding structure and kinetics in Langmuir monolayers. (C) 2021 The Authors. Published by Elsevier Inc.
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