Investigation of drop breakage and coalescence in the liquid–liquid system with nonionic surfactants Tween 20 and Tween 80

Breakage and coalescence of toluene drops in the aqueous Tween 20 and Tween 80 solutions in stirred tank were considered. Additionally, dynamic interfacial tension were measured with drop volume tensiometer and critical micelle concentration, surface coverage and surface elasticity values were determined. Results show that at studied surfactant concentration range (0.0012–0.006 mM) the coalescence rate was significantly reduced. Small droplets at higher surfactant concentrations behave like rigid spheres. Analysis of drop size distribution evolution at the same surfactant concentration shows that, generally, coalescence rate increases with the increase in impeller speed N, the steady drop size distribution is achieved in shorter time at higher N, and both breakage and coalescence take place mainly in the impeller zone. At the lowest surfactant concentration droplet surface remains partially mobile. Significant effect of the difference between dynamic interfacial tension for t→0 (equal to interfacial tension without surfactant) and equilibrium interfacial tension on drop breakage was observed (due to generation of extra stress which increases drop breakage rate). Modification of multifractal breakage model was proposed to describe this effect. It was observed that Tween 20 has better stabilizing properties than Tween 80.

► Transient toluene drop size distributions in stirred batch dispersion.
► Dynamic interfacial tension for toluene–Tween 20 solution and toluene–Tween 80 solution.
► Critical micelle concentrations in presence of second liquid phase.
► Higher coalescence rate for larger drops.
► Higher coalescence efficiency in impeller zone.