Adsorption of nonionic surfactants at fluid–fluid interfaces: Importance in the coalescence of bubbles and drops

Major industrial applications of surfactants are related to the stability of emulsions and foams, which is directly dependent on the rate of coalescence of drops and bubbles. Surfactant molecules adsorb at the liquid–liquid and gas–liquid interfaces and prevent the drops and bubbles from coalescing with one another. Therefore, it is important to correlate the adsorption of surfactant with the time required for coalescence. In this work, we have studied the adsorption of three nonionic surfactants, Tween 20, Triton X-100 and Span 80 at air–water and water–toluene interfaces. The variation of surface and interfacial tension with the concentration of surfactant was studied and the data were fitted using a surface equation of state derived from the Langmuir adsorption isotherm. We have studied binary coalescence of water drops in toluene in presence of these three surfactants. Coalescence of air bubbles at flat air–water interface was studied in presence of the water-soluble surfactants, Tween 20 and Triton X-100. A stochastic model for coalescence was used to fit the coalescence time distributions. The significance of the model parameters was discussed.