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 an d the decay of foams stabilized with sodium dodecyl sulfate (SDS) is i nvestigated. The ionic strength was kept constant, 0.024 M, in all exp eriments. Sodium salts, Na2SO4, NaNO3, NaCl, Na2CO3, Na3PO4, and sodiu m citrate, as well as magnesium salts, MgSO4, Mg(NO3)(2), MgCl2, and M g(OOCCH3)(2), are used. The lifetime and the critical thickness of rup ture of the unstable films, as well as the equilibrium thickness of th e 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 influencin g the film stability in the opposite directions: (i) increase of the s urface elasticity and viscosity due to connections between neighboring surfactant headgroups produced by Mg2+ and (ii) enhancement of the io nic correlation attraction between the film surfaces. The data show al so an unexpectedly large influence of the co-ions on the film and foam stability. Carbonates, phosphates, and citrates have the most signifi cant stabilizing effect. At least two effects may be responsible for t he larger stability of the films with co-ions of weak acids: (i) The d issolved carbonate and phosphate salts spontaneously adjust to pH 11.4 in the solution, which leads to hydrolysis of SDS to dodecanol. The l atter increases the surface elasticity, viscosity, and kinetic stabili ty of films and foams. (ii) The molecules of the carbonic, phosphoric, or citric acid can interconnect headgroups of neighboring SDS molecul es in the surface adsorption monolayers by formation of multiple H-bon ds. Such a linking also enhances the kinetic stability. These hypothes es agree well with the accumulated experimental data and can be helpfu l for a better understanding of the role of citrates, phosphates, and other electrolytes in body-care and household detergency. (C) 1998 Aca demic Press.