Surface and bulk properties of mixed anionic/cationic surfactant systems i. equilibrium surface tensions

The strong synergistic effect on surface pressure for mixed solutions of cationic and anionic surfactants has been studied quantitatively for the system sodium dodecyl sulfate—dodecyl trimethyl ammonium bromide. Upon mixing dilute solutions of these two surfactants the surface pressure can increase by more than 40 mN m−1. This is explained as mainly the result of strong additional adsorption of the double long-chain salt consequent on mixing. As a corollary the surface should contain an equimolar mixture of cationic and anionic surfactant for a wide range of bulk mixing ratios. This was confirmed by the experimental evidence, which showed the surface tension to be solely dependent on the product of the long-chain cation and anion concentration. Apart from the synergistic effect of additional adsorption there is a measurable, though smaller, effect of “specific interactions” in the mixed monolayers. At the solubility limit of the mixed system, micelles or crystals of equimolar composition are formed as well. When the overall bulk composition is not equimolar, the formation of an equimolar second phase will shift the solution composition in favor of the surfactant in excess. Experimental evidence shows that eventually the CMC of this surfactant can be reached and that at high concentration the phase of equimolar composition will be solubilized in the micelles of the surfactant in excess.