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

Abstract

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 pressureÀsurface 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 PoissonÀBoltzmann equation and include ionÀion van der Waals forces have shown more constructive insights for advancing an understanding of ionic double layers.

Membrane Hydration

“…For instance, the ratio …A/kT of spread monolayer of sodium octadecyl sulfate on aqueous NaCl solution (0.001-0.01 M) in the surface concentration range up to 3 10 2 molecules/nm 2 and in the surface pressure range of 0-300 N m 1 was greater than unity ( Fig. 10.1) (Middleton et al 2011). This deviation is generally attributed to contribution of … e to the experimental surface pressure.…”

“…For most surfactants, the ratio …A/kT is usually not equal to unity even in very dilute monolayers (Davies 1956;Gershfeld 1970;Middleton et al 2011;Mingins, et al 1992), where the system is expected to obey the twodimensional ideal gas law. For instance, the ratio …A/kT of spread monolayer of sodium octadecyl sulfate on aqueous NaCl solution (0.001-0.01 M) in the surface concentration range up to 3 10 2 molecules/nm 2 and in the surface pressure range of 0-300 N m 1 was greater than unity ( Fig.…”

“…… versus relationship ( ) of sodium octadecyl sulfate spread monolayer at the 0.001 M NaCl-air interface at 20 ı C. The data points were extracted fromMiddleton et al (2011). The dotted line represents theoretical curve predicted by Eq.…”

Water at Biological Phase Boundaries: Its Role in Interfacial Activation of Enzymes and Metabolic Pathways

“…However, it was shown that the Gouy model (Eq. 10.5) completely failed to account for the electrostatic contribution arising from head group repulsion (Middleton et al 2011). On the other hand, the …A/kT ratio was found to be lesser than unity for a very dilute monolayer of n-pentadecanoic acid on a 0.01 M HCl substrate (where … e D 0) ( Fig.…”

“…For most surfactants, the ratio …A/kT is usually not equal to unity even in very dilute monolayers (Davies 1956;Gershfeld 1970;Pallas and Pethica 2009;Middleton et al 2011;Mingins, et al 1992), where the system is expected to obey the twodimensional ideal gas law. For instance, the ratio …A/kT of spread monolayer of sodium octadecyl sulfate on aqueous NaCl solution (0.001-0.01 M) in the surface concentration range up to 3 10 2 molecules/nm 2 and in the surface pressure range of 0-300 N m 1 was greater than unity ( Fig.…”