Effect of Organic Vapors on the Behavior of Air Bubbles in Solutions of Nonionic and Ionic Surfactants

We explored the physicochemical aspects of the problem of a rising air bubble in an aqueous surfactant solution, where saturated n-hexane vapor is present within the bubble. The rising velocity profiles of these bubbles were measured in pure water and salt-free solutions of a nonionic (n-octanol) or cationic (dodecyltrimethylammonium bromide, C12TAB) surfactant at various concentrations. They were compared with the results for corresponding hexane-free systems. Additionally, dynamic surface tension for stationary bubbles was measured using bubble profile analysis tensiometry. To support these experimental data, we conducted an investigation using molecular dynamics (MD) simulations. For pure water, both surface tension measurements and MD simulations confirmed the adsorption of n-hexane molecules from the vapor phase to the stationary water interface, which is consistent with the literature reports. However, the rising bubble velocity was not affected by n-hexane vapor. We discuss this intriguing finding within the context of hydrodynamic forces. In the surfactant systems, a strong effect of coadsorption of surfactant from the solution and n-hexane from the vapor phase was observed in all investigations. The surface tension isotherms were theoretically described using a modified Frumkin adsorption model, additionally accounting for the ionic nature of C12TAB and the coadsorption of n-hexane from the vapor. The free energy of adsorption exhibited a strong correlation with the free energy profiles at the interface, as determined by MD simulations. The rising bubble data were theoretically analyzed in terms of the drag coefficient and the extent of bubble deformation. However, studies of the bubble velocity profiles revealed some unusual features, particularly during the dynamic layer formation phase.