Adsorption of Ionic Surfactants at Liquid-Liquid Interfaces in the Presence of Salt: Application in Binary Coalescence of Drops

Binary coalescence of water drops in organic liquids has several commercial applications that are related to the stabilization of emulsions. The stability of the emulsion is dependent on adsorption of the surfactant molecules at the water/organic interface. In this work, the adsorption of ionic surfactants at water/organic interfaces, both in the presence and absence of salt, and its effect on binary coalescence were studied. The ionic surfactants used were sodium dodecyl sulfate and cetrimide. The organic phase was constituted from either toluene or carbon tetrachloride. Adsorption was studied by measuring the interfacial tension. The presence of salt had a strong effect on the interfacial tension in the presence of these surfactants. In the absence of surfactant, salt had little effect on interfacial tension. The data were fitted using a surface equation of state that was developed from the Gibbs adsorption equation and the Langmuir isotherm. Binary coalescence of water drops in toluene was studied in a specially designed coalescence apparatus. Coalescence time did not have a single value but a stochastic distribution of coalescence time was observed in each case. The addition of salt had a significant effect on coalescence time. A stochastic model was used to characterize the distributions.