Thermodynamics of ionized monolayers: Surface manometry on very low density spread monolayers of sodium octadecyl sulfate at the air/water interface and analysis of ionic double layer contributions to the isotherms

Double layer theories of ionized monolayers have mostly been developed for the adsorption of soluble ionic surfactants at the air/water and oil/water interfaces, with much disagreement, due in part to the adoption of nonthermodynamic assumptions and the lack of solution activity data for estimating surface excess densities from surface tension experiments. Both surface pressure and density can be measured directly with insoluble spread monolayers of ionized surfactants, allowing simpler analysis of the several contributions to the surface properties. Data are presented on the surface pressure-surface area isotherms for very dilute spread monolayers of sodium octadecyl sulfate (NaODS) at the air/water interface at two temperatures and three NaCl concentrations. The results indicate that the basic Gouy model fails to account for the contribution of the ionic double layer to the surface pressure in the low density region at low NaCl concentrations, both conditions favorable to the model. Of the possible corrections, the discreteness of charge effect, interionic van der Waals forces, headgroup immersion, and ion association with the charged monolayer head groups all appear to be relevant factors. The results indicate that the electrostatic and van der Waals contributions to the surface pressure are not independent additive terms. Further complex electrostatic modifications to traditional ionic double layer models are considered unpromising. Recently developed methods in molecular dynamics and lattice summation methods which avoid the nonphysical Poisson-Boltzmann equation and include ion-ion van der Waals forces have shown more constructive insights for advancing an understanding of ionic double layers.