Coalescence of Bubbles and Stability of Foams in Brij Surfactant Systems

This work presents coalescence of air bubbles and stability of foams in aqueous solutions of polyoxyethylene (4) lauryl ether (Brij 30), polyoxyethylene (23) lauryl ether (Brij 35), and polyoxyethylene (20) oleyl ether (Brij 98). Adsorption of these surfactants at the air-water interface was also studied. The surface tension profiles were fitted by the Szyskowski equation. It was found that the surface tensions at the critical micelle concentrations of these surfactants followed the sequence Brij 35 > Brij 98 > Brij 30. The air bubbles were most stable to coalescence in Brij 35 solution and least stable in Brij 30 solution. Coalescence in Brij 35 solution was hindered beyond a certain concentration of the surfactant. Stochastic distributions of coalescence time were observed in all surfactant systems. These distributions were fitted well by the stochastic model. Seven film-drainage models were used to predict the coalescence time. However, their predictions did not agree well with the mean values of the coalescence time distributions. Stability of foams was analyzed by the Ross-Miles test. The initial and residual foam heights were measured at each surfactant concentration. The foam heights followed the trends that are expected from the coalescence times of the bubbles.