The interactions
between ions and lipid monolayers have captivated
the attention of biologists and chemists alike for almost a century.
In the absence of experimentally accessible concentration profiles,
the electrolyte adsorption remains the most informative quantitative
characteristic of the ion-lipid interactions. However, there is no
established procedure to obtain the electrolyte adsorption on spread
lipid monolayers. As a result, in the literature, the ion-lipid monolayer
interactions are discussed qualitatively, based on the electrolyte
effect on more easily accessible variables, e.g., surface tension.
In this letter, we demonstrate how the electrolyte adsorption on lipid
monolayers can be obtained experimentally. The procedure requires
combining surface pressure versus molecular area compression isotherms
with spreading pressure data. For the first time, we report an adsorption
isotherm of NaCl on a lipid monolayer as a function of the density
of the monolayer. The leading interactions seem to be the osmotic
effect from the lipid head groups in the surface layer and ion-lipid
association.