Surface potential vs concentration isotherms of Na n-dodecyl sulfate (SDDS) adsorbed at the air−solution
interface, measured using the vibrating plate method at various concentrations of added salt, exhibit a
pronounced minimum. The results of surface tension measurements indicate that the minimum occurs within
the concentration range that corresponds to the transition from the Henry regime of adsorption for low surface
coverages to the one typical for adsorption of amphiphiles at high surface coverages. We proposed a simple
model of adsorption of ionic surfactants at air−fluid interfaces based on the assumption that surfactant
headgroups and counterions can adsorb in the Stern layer at the same Helmholtz plane. The electric potential
in the electric double layer was calculated according to the Gouy−Chapman model for the diffuse part of the
double layer and a modified Stern model for the inner layer with corrections for the discrete charge effects.
The total potential drop across the interface was assumed to consist of two contributions: (i) the potential
drop in the diffuse and compact double layers, negative for n-alkyl sulfate ions adsorbed at the air−solution
interface, and (ii) a positive contribution due to the effective dipole moment of adsorbed surfactant molecules
attributed mainly to the terminal CH3 groups. Our model correctly describes the dependence of the surface
tension and surface potential of SDDS solution on its concentration and the amount of added salt.
Generally, surfactants contain trace impurities that will falsify their adsorption properties enormously. To carry out reliable experiments with surfactant solutions necessitates removing those trace impurities beforehand. A programmed apparatus for removing surface-active trace impurities from surfactant solutions has been constructed and tested. It takes advantage of the contaminants’ stronger surface activity and considerably lower content in comparison with the main surfactant component. Surface-active material of the solution to be purified is allowed to adsorb at its surface. After considerable reduction of the solution surface area, the adsorbed material is sucked off from the surface in a definite manner by using a fine capillary. The single operating steps are repeated periodically and automatically until the solution reaches the state of ‘‘surface chemical purity.’’ The apparatus can be utilized for a wide range of different conditions given by the individual surfactant properties simply by changing the operating parameters. The instrument favorably and effectively provides that peculiar grade of surfactant purity necessary for all kinds of fundamental surfactant research and characterization.
The equilibrium surface tension of anionic surfactant n-decyl sulfate (DS-) for various monovalent
(alkali) counterions was investigated. It was found that surface activity of surface chemically pure DS-
significantly increases with decreasing hydrated size of the counterion. We describe our experimental
results in terms of the previously developed adsorption model, which assumes that the counterions may
penetrate the Stern layer where the surfactant headgroups are adsorbed. The headgroups and counterions
have a finite size that leads to the surface exclusion effects in the adsorption isotherm. The model is
improved by explicitly taking into account the electric interactions between adsorbed ions in the adsorbed
layer. We obtain a good correlation between the relative counterion size in the Stern layer, the measure
of the area excluded by the adsorbed counterions, and the effective radius of the hydrated counterion in
the solution. The limiting areas per molecule at the critical micelle concentration for the adsorbed decyl
sulfate for various counterions are in good agreement with those measured by neutron scattering.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.