Effects of counterions and co-ions on the drainage and stability of liquid films and foams

The influence of various electrolytes on the thinning of foam films and the decay of foams stabilized with sodium dodecyl sulfate (SDS) is investigated. The ionic strength was kept constant, 0.024 M, in all experiments. Sodium salts, Na2SO4, NaNO3, NaCl, Na2CO3, Na3PO4, and sodium citrate, as well as magnesium salts, MgSO4, Mg(NO3)(2), MgCl2, and Mg(OOCCH3)(2), are used. The lifetime and the critical thickness of rupture of the unstable films, as well as the equilibrium thickness of the stable films, are measured, The rate of drainage of the liquid from the foams and their decay time are also measured. The influence of the bivalent counterion, Mg2+, is related to two major effects influencing the film stability in the opposite directions: (i) increase of the surface elasticity and viscosity due to connections between neighboring surfactant headgroups produced by Mg2+ and (ii) enhancement of the ionic correlation attraction between the film surfaces. The data show also an unexpectedly large influence of the co-ions on the film and foam stability. Carbonates, phosphates, and citrates have the most significant stabilizing effect. At least two effects may be responsible for the larger stability of the films with co-ions of weak acids: (i) The dissolved carbonate and phosphate salts spontaneously adjust to pH 11.4 in the solution, which leads to hydrolysis of SDS to dodecanol. The latter increases the surface elasticity, viscosity, and kinetic stability of films and foams. (ii) The molecules of the carbonic, phosphoric, or citric acid can interconnect headgroups of neighboring SDS molecules in the surface adsorption monolayers by formation of multiple H-bonds. Such a linking also enhances the kinetic stability. These hypotheses agree well with the accumulated experimental data and can be helpful for a better understanding of the role of citrates, phosphates, and other electrolytes in body-care and household detergency. (C) 1998 Academic Press.