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The surface tension and adsorption kinetics of aqueous solutions of slightly volatile, organic amphiphiles are influenced by both liquid-and vapor-phase surfactant concentrations. Here we derive a new kinetic transfer equation, based on the classic Langmuir analysis, which can account for adsorption and desorption from both sides of the vapor/liquid interface during surface equilibration. The new transfer equation was tested against dynamic surface tension data from two normal alcohols (1-butanol and 1-hexanol) in aqueous solutions. The experimental data was collected at conditions where the dynamic surface tension is controlled by a combination liquid-and vapor-phase surfactant adsorption. The validity of the transfer equation was assessed based on its ability to model the experimental data accurately and generate suitable values for the kinetic rate constants. The theoretical predictions from the transfer equation fit well with the experimental data for both systems. However, variability was observed in the least-squares estimates of the rate constants. The variability is attributed to the limitations of empirical models that utilize adjustable fitting parameters to optimize the model predictions and the wide range of surfactant concentrations studied. Specific concentration regions were identified where the variability in the rate constants was minimal and, thus, where the model is most appropriate. The new transfer equation can be applied to volatile surfactant systems where the dynamic surface tension is influenced by surfactant adsorption and desorption from both sides of the vapor/liquid interface.

Tension at the surface is a most fundamental physicochemical property of a liquid surface. The concept of surface tension has widespread implications in numerous natural, engineering and biomedical processes. Research to date has been largely focused on the liquid side; little attention has been paid to the vapor—the other side of the surface, despite over 100 years of study. However, the question remains as to whether the vapor plays any role, and to what extent it affects the surface tension of the liquid. Here we show a systematic study of the effect of vapor on the surface tension and in particular, a surprising observation that the vapor, not the liquid, plays a dominant role in determining the surface tension of a range of common volatile organic solutions. This is in stark contrast to results of common surfactants where the concentration in the liquid plays the major role. We further confirmed our results with a modified adsorption isotherm and molecular dynamics simulations, where highly structured, hydrogen bonded networks, and in particular a solute depletion layer just beneath the Gibbs dividing surface, were revealed.

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