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



Specific interactions of yttrium and lanthanum ions with a fatty acid Langmuir monolayer were investigated using vibrational sum frequency spectroscopy (VSFS). 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 response of interfacial water molecules, charge reversion, as well as the formation of yttrium hydroxide complexes, were detected upon increasing the amount of salt in the 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 Y3+ 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.


Supplementary material

Supporting Information
Langmuir isotherms of AA and dAA on LaCl3 and YCl3 solutions at various concentrations and pH, details of the fitting procedure of the VSF spectra, predictions from the Gouy-Chapman model, estimation of proportions of metal-carboxylates contributions, VSF spectra showing improved detection limits in the presence of 100 mM NaCl background electrolyte, correlation between symmetric and asymmetric carboxylate stretches, detailed speciation diagrams for YCl3 and LaCl3, and coarse modelling of the trivalent ion adsorption on the fatty acid monolayer for the charge compensated case.