Effects of binding of counterions on adsorption and coalescence

Abstract

In many industrial applications, ionic surfactants are used in aqueous media in the presence of a mixture of salts. The surface activity of a surfactant in such solutions depends strongly on the ions of different valence. In the present study, surface tensions of aqueous solutions of cetyltrimethylammonium bromide (CTAB) were measured in the presence of sodium bromide and potassium sulfate. The interfacial tension between water and toluene was also measured in the presence of CTAB and these salts. The effects of monovalent (bromide) and divalent (sulfate) counterions on the adsorption of CTAB at air–water and water–toluene interfaces were studied. Coalescence of air bubbles and toluene drops at flat interfaces was studied in these surfactant–salt systems. A stochastic model was used to fit the coalescence time distributions. The significance of the model parameters was discussed. The difference between air–water and water–hydrocarbon interfaces was analyzed in terms of the effects of counterion-binding on adsorption and coalescence. The mean values of the coalescence time distributions were compared with the predictions from seven film-drainage models.