Adsorption Kinetics of Aqueous n-Alcohols: A New Kinetic Equation for Surfactant Transfer

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Abstract

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.

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