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

Abstract

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 behavior was found to be in excellent agreement with that predicted by Gouy–Chapman theory using a thermodynamic pK a of 5.1 ± 0.2. This resulted in an apparent pK a 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.

Identifying Eigen-like hydrated protons at negatively charged interfaces

“…The negative charge and accompanying surface potential result in surface proton concentrations that are several orders of magnitude higher than in the bulk. Moreover, the charging behavior upon addition of monovalent ion salts to solution has been shown to be consistent with the Gouy-Chapman theory for concentrations ≤50 mM 26,27 .…”

“…The absence of features from CD 2 groups, indicate that the monolayer is conformationally well-ordered in an all-trans configuration 28 , consistent with the high packing densities of the tilted condensed phase. At lower frequencies, the peak at~1720 cm −1 observed in both polarization combinations, is assigned to the C=O stretch of the uncharged form of the carboxylic acid headgroup 26,29 . The spectra show no features linked to the charged carboxylate moieties, which are expected at~1408 cm −1 and~1535 cm −1 26 .…”

“…1). Although the intrinsic pK a of these fatty acids is similar to that of shorter chain carboxylic acids (i.e., pKa~5), at the surface, when only the pH determining ions are present in solution, the apparent pK a is~10.8 26 . The negative charge and accompanying surface potential result in surface proton concentrations that are several orders of magnitude higher than in the bulk.…”

“…The spectra show no features linked to the charged carboxylate moieties, which are expected at~1408 cm −1 and~1535 cm −1 26 . At pH~6 and a NaCl concentration of 1 μM, the carboxylate modes are below the detection limit, yet the monolayer is expected to be~0.5% deprotonated, as estimated from the Gouy-Chapman model 26 . Nevertheless, despite the relatively small surface charge (i.e.,~−4 mC m −2 ), the low ionic strength of the subphase ensure a substantial surface potential that is in the order of~−200 mV 26 .…”

“…1. The sharp peaks found between 2050 cm −1 and 2250 cm −1 are all linked to the alkyl chain of the fatty acid, and are primarily stretching modes of the terminal methyl group 26 . The absence of features from CD 2 groups, indicate that the monolayer is conformationally well-ordered in an all-trans configuration 28 , consistent with the high packing densities of the tilted condensed phase.…”