JOURNAL OF COLLOID AND INTERFACE SCIENCE 200, 31–45 (1998) ARTICLE NO. CS975334 Effects of Counterions and Co-ions on the Drainage and Stability of Liquid Films and Foams J. K. Angarska,* , ² K. D. Tachev,* , ² P. A. Kralchevsky,² ,1 A. Mehreteab,‡ and G. Broze§ * Faculty of Chemistry, University of Shoumen, 9700 Shoumen, Bulgaria; ² Laboratory of Thermodynamics and Physicochemical Hydrodynamics, Faculty of Chemistry, University of Sofia, 1126 Sofia, Bulgaria; ‡Colgate – Palmolive Technology Center, 909 River Road, Piscataway, New Jersey 08854-5596; and §Colgate – Palmolive Research and Development, Inc., Avenue Du Parc Industriel, B-4041 Milmort (Herstal), Belgium Received May 12, 1997; accepted November 24, 1997 1. INTRODUCTION The influence of various electrolytes on the thinning of foam The great importance of foams in various fields of practice films and the decay of foams stabilized with sodium dodecyl sulfate has stimulated studies of foam systems for many years (1). (SDS) is investigated. The ionic strength was kept constant, 0.024 The stability and the lifetime of foams are governed by the M, in all experiments. Sodium salts, Na 2 SO 4 , NaNO 3 , NaCl, Na 2 CO 3 , Na 3 PO 4 , and sodium citrate, as well as magnesium salts, properties of their structural units: thin liquid films (TLF) MgSO 4 , Mg(NO 3 ) 2 , MgCl 2 , and Mg(OOCCH 3 ) 2 , are used. The and Plateau borders (1–3). The latter are influenced by the lifetime and the critical thickness of rupture of the unstable films, rheological properties of the surfactant adsorption mono- as well as the equilibrium thickness of the stable films, are mea- layers and their interaction across the TLF. The electrostatic sured. The rate of drainage of the liquid from the foams and interaction, which gives rise to long-range double-layer their decay time are also measured. The influence of the bivalent forces (2) and to short-range hydration repulsion (4), plays counterion, Mg 2/ , is related to two major effects influencing the an important role in the stability of foam films. The effect film stability in the opposite directions: (i) increase of the surface of electrostatics was studied experimentally by variation of elasticity and viscosity due to connections between neighboring electrolyte concentration (2, 5, 6). Sharma and Khilar (7) surfactant headgroups produced by Mg 2/ and (ii) enhancement established that the addition of electrolyte affects the surface of the ionic correlation attraction between the film surfaces. The mobility. As a rule 1:1 electrolytes were used in these stud- data show also an unexpectedly large influence of the co-ions on the film and foam stability. Carbonates, phosphates, and citrates ies. There are only few works on the effect of bivalent coun- have the most significant stabilizing effect. At least two effects terions on the process of film thinning (8, 9). It was ob- may be responsible for the larger stability of the films with co-ions served (9) that the type of film rupture depends to some of weak acids: (i) The dissolved carbonate and phosphate salts extent also on the type of co-ions. spontaneously adjust to pH 11.4 in the solution, which leads to The influence of co-ions (monovalent or bivalent) has hydrolysis of SDS to dodecanol. The latter increases the surface been studied mainly with respect to the properties of sodium elasticity, viscosity, and kinetic stability of films and foams. (ii) dodecyl sulfate ( SDS ) adsorption monolayers (10–12). Ya- The molecules of the carbonic, phosphoric, or citric acid can in- manaka ( 13 ) investigated the properties of vertical films and terconnect headgroups of neighboring SDS molecules in the sur- foams containing sodium salts with various anions. These face adsorption monolayers by formation of multiple H-bonds. films and foams were stabilized by a nonionic surfactant Such a linking also enhances the kinetic stability. These hypotheses ( polyoxyethylene nonylphenyl ether ) instead of SDS. It was agree well with the accumulated experimental data and can be helpful for a better understanding of the role of citrates, phos- experimentally established that the stability of the studied phates, and other electrolytes in body-care and household de- foams depends on the type of the anion in the solution. tergency.  1998 Academic Press According to the DLVO theory (14), at a fixed ionic Key Words: detergency: effect of citrates, phosphates; foam strength of the solution the type of co-ion should not affect films, kinetic stability; foams, effect of co-ions and Mg 2/ ; H-bonds: the double layer interaction in so far as the co-ions are re- SDS with weak acids; hydrolysis of SDS; ionic correlations: effect pelled by the charged film surfaces and do not enter the Stern on film stability;liquid films, drainage and stability;surface elastic- layers. On the other hand, in our preliminary experiments we ity: effect of linkers; surface forces: ionic correlations; syneresis of observed that the drainage and stability of foam films depend foams, effect of electrolytes. strongly on the type of the co-ion at a given counterion and total ionic strength. This intriguing observation stimulated us to extend the study, whose results are the subject of the 1 To whom correspondence should be addressed. present article. 31 0021-9797/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.