Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition

Vibrational sum frequency spectroscopy has been used to study the molecular properties upon compression of a highly charged arachidic acid Langmuir monolayer, which displays a first-order phase transition plateau in the surface pressure - molecular area (π-A) isotherm. By targeting vibrational modes from the carboxylic acid headgroup, alkyl chain, and interfacial water molecules, information regarding the surface charge, surface potential, type of ion pair formed, and conformational order of the monolayer could be extracted. The monolayer was found to be fully charged before reaching the 2D-phase transition plateau, where partial reprotonation, as well as the formation of COO⎺ Na+ contact-ion pairs, started to take place. After the transition, three headgroup species, mainly hydrated COO⎺, COOH, and COO⎺ Na+ contact-ion pairs could be identified and their proportions quantified. Comparison with theoretical models shows that predictions from the Gouy Chapman model are only adequate for the lowest surface charge densities (<-0.1 C/m2). In contrast, a modified Poisson-Boltzmann (MPB) model that accounts for finite-size of the cation, captures many of the experimental observables, including the partial reprotonation, and surface potential changes upon compression. The experimental results provide a quantitative molecular insight that could be used to test potential extensions to the theory.QC 20190827. QC 20191015. QC 20200217