Effects of inorganic electrolytes and of pH on micelle formation of amphoteric-anionic mixed surfactant systems

Abstract

The effect of inorganic electrolytes on micelle formation of mixed amphoteric-anionic surfactant systems has been studied for various surface tensions, pH values, and relative viscosities. These systems are N α, N α-dimethyl- N ϵ-lauroyl lysine (DMLL)-sodium dodecyl sulfate (SDS), which include different inorganic electrolytes (NaCl, NaOH, or HCl). The critical micelle concentration (CMC) values of the surfactant, except that of DMLL, decrease with increasing concentration of NaCl. The ratio of decrease of CMC {[CMC 0 (CMC of salt-free system) — CMC s (CMC of added NaCl system)]/CMC 0} increases with increasing mole fraction of SDS in the mixed surfactant systems including NaCl. The pH value at every molar ratio decreases with increasing NaCl. The ratio of decrease of pH values { [a s , (activity of H + in added NaCl system) − a 0, (activity of H + in salt-free system) ] a s } for mixed surfactant systems is greater than that for each single surfactant system, and increases with increasing mole fraction of SDS in the mixed surfactant system including NaCl. The CMC values and relative viscosities increase monotonically with increasing mole fraction of DMLL in the mixed surfactant systems including NaOH. The CMC values of mixed surfactant systems including HCl are much smaller than those of systems including NaCl. Inorganic electrolytes appear to affect the magnitude of the hydrophilic-hydrophilic interaction between DMLL and SDS molecules in mixed micelles.