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 (p-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 in the liquid expanded phase is found to be fully charged until
reaching the 2D-phase transition plateau, where partial reprotonation, as well
as the formation of COO⎺ Na<sup>+ </sup>contact-ion
pairs, start to take place. In the condensed phase after the transition, three
headgroup species, mainly hydrated COO⎺, COOH, and COO⎺ Na<sup>+ </sup>contact-ion pairs could be
identified and their proportions quantified. Comparison with theoretical models
shows that despite the low ionic strengths used (i.e. 10 mM), the predictions
from the Gouy Chapman model are only adequate for the lowest surface densities,
when the surface charge does not exceed -0.1 C/m<sup>2</sup>. In contrast, a
modified Poisson-Boltzmann (MPB) model that accounts for the steric effects
associated with the finite ion-size, captures many of the experimental
observables, including the partial reprotonation, and surface potential changes
upon compression. The agreement highlights the importance of hydronium ion –
carboxylate interactions, as well as the layer of sodium ions packed at the
steric limit, for explaining the phase transition behavior. The MPB model,
however, does not explicitly consider the formation of contact ion pairs with
the sodium counterion. The experimental results provide a quantitative molecular
insight that could be used to test potential extensions to the theory.
Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition