Coalescence of air bubbles in aqueous solutions of alcohols and nonionic surfactants

Coalescence of air bubbles in aqueous solutions of two aliphatic alcohols (viz. butanol and hexanol) and four nonionic surfactants (viz. Tween 20, Tween 40, Tween 60 and Tween 80) is reported in this work. Single-component alcohol and surfactant solutions as well as mixed binary surfactant–alcohol solutions were studied. Adsorption of the surface active compounds at air–water interface was studied by measuring the surface tension of the aqueous solutions. The critical micelle concentration and surface tension at this concentration were determined for the single and mixed surfactant–alcohol systems. The effect of concentration of surface active compounds on coalescence of air bubbles at flat air–water interface was studied. The role of electrostatic double layer, hydration and steric forces on coalescence was investigated. It was found that the stability of the thin aqueous films in mixed surfactant–alcohol systems depends on the subtle interplay of the intermolecular and surface forces in the film, which vary with the composition of the monolayer at the air–water interface. Stochastic distributions of coalescence time were observed in all systems. The coalescence time distributions were fitted by the stochastic model. The mean values of the distributions were compared with the predictions of seven film-drainage models.