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

Abstract

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 two-dimensional 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 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.

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

“…[52] On the contrary, monolayer expansion and appearance of the plateau of phase transition with a LE phase, result from the introduction of new repulsive forces, which in the case of fatty acid monolayers on monovalent chloride salt solutions originate from the charging of the monolayer. [53,54] The Langmuir isotherms recorded for YCl3 and LaCl3 indicate that the interfacial equilibrium is affected in both ways, revealing a complex interaction between the trivalent ion salts and the carboxylic acid headgroup. A molecular insight into these interactions is revealed by vibrational sum frequency spectroscopy.…”

Identifying Eigen-Like Hydrated Protons at Negatively Charged Interfaces

“…For the fatty acid monolayers, the proton has a significantly higher affinity than other monovalent cations such as Na + to interact with the carboxylate headgroup. 42 This is not the case for the sulphate headgroup, 43 where the presence of sodium counterions in the subphase could have a detrimental effect on the measured H3O + core signal.…”

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

“…To prevent trace metal binding to the AA monolayer, 4 µM ethylenediaminetetraacetic acid was added to both subphase solutions. 55 Mean molecular area values were chosen based on the surface pressure-area isotherms in Fig. 5b.…”

“…Monolayer compression yields a plateau region (37 Å 2 /molecule) corresponding to a firstorder phase transition between the LE and tilted condensed (TC) phases, known as the LE-TC coexistence region. 55,56 AA surfactants aggregate into 2D domains at the air/water interface and increase in size with compression. 57 (Brewster angle microscopy images of the AA domain morphologies are shown in Fig.…”