Colloids and Surfaces A: Physicochemical and Engineering Aspects

g r a p h i c a l a b s t r a c t a b s t r a c t Recently we showed that triple surfactant mixtures, comprising the anionic surfactant SLES, zwitterionic surfactant CAPB and long-chain fatty acid (FAc), possess very high surface modulus which can be used to modify the dynamic properties of foams (Golemanov et al. Langmuir 2008, 24, 9956). In the current paper we perform a systematic experimental study of the effects of several factors on the surface tension, surface dilatational modulus, viscous friction inside sheared foam, foam-wall friction, and mean bubble size in sheared foams, for such triple surfactant mixtures. The factors studied are: (1) chain length of fatty acid; (2) fatty acid concentration; (3) pH; (4) presence of glycerol in the aqueous solution. Lauric (C12Ac), myristic (C14Ac) and palmitic (C16Ac) acids are studied. The concentrations of the main surfac- tants were fixed, 10 mM for SLES and 5 mM for CAPB. Varying the myristic acid (C14Ac) concentration, CC14Ac, we observed two ranges: (1) At CC14Ac 0.11 mM, C14Ac leads to significant decrease of surface tension (from 29 to 22 mN/m), increase of surface modulus (from 50 to 400 mN/m), decrease of mean bubble size in sheared foams (from 300 to 150 m), and strong increase of the inside-foam friction and foam-wall friction. Qualitatively similar results are obtained with the other FAc studied. Mass-balance estimates show that low surface ten- sion and high surface modulus are observed, when the molar fraction of FAc in the adsorption layer approaches ca. 30%. The main role of FAc is to induce a surface phase transition, leading to formation of surface condensed phase in the mixed adsorption layer. For all systems studied, the increase of pH above a certain transitional value leads to a sharp increase of surface tension and decrease of surface dilatational modulus, which is accompanied with a decrease in the inside-foam and foam-wall viscous