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On the Charging of Carboxylic Acid Monolayers with Monovalent Ions at Low Ionic Strengths: Molecular Insight Revealed by Vibrational Sum Frequency Spectroscopy

preprint
submitted on 17.09.2018 and posted on 19.09.2018 by Eric Tyrode, Robert W. Corkery
The charging of arachidic acid Langmuir monolayers as a function of subphase pH and monovalent ion concentration below 100 mM was investigated using vibrational sum frequency spectroscopy. Molecular information was obtained by targeting the vibrational modes of the carboxylic acid headgroups, alkyl chains, and water molecules in the immediate surface and diffuse double layers. The surface charge in the monolayer was experimentally determined by monitoring the hydrated carboxylate stretching modes. The charging behaviour was found to be in excellent agreement with that predicted by Gouy-Chapman theory using a thermodynamic pKa of 5.1 ± 0.2. This resulted in an apparent pKa of ~10.8 when the only ions present in solution were those associated with adjusting the pH. Water molecules with a preferred orientation in the immediate surface region were found to primarily interact with the uncharged carboxylic acid moiety, decreasing in number as the monolayer further deprotonated. Contributions from water molecules in the diffuse double layer, partly aligned by the exponentially decaying surface electric field, closely followed the predictions of a recently proposed theoretical framework that accounts for interference and screening effects. Finally, the charging of the monolayer was experimentally found to be independent of the identity of either the monovalent cation (i.e., Li+, Na+, Rb+) or anion (i.e., F-, Cl-, I-) at low salt concentrations.

Funding

SSF Swedish Foundation for Strategic Research; and VR Swedish Research Council

History

Email Address of Submitting Author

tyrode@kth.se

Institution

KTH Royal Institute of Technology

Country

Sweden

ORCID For Submitting Author

0000-0003-1221-0227

Declaration of Conflict of Interest

The authors declare no conflicts of interest

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