Light scattering and electrophoretic studies of mixed micelles of ionic and nonionic surfactants

Abstract

Light scattering and electrophoretic studies have been made of the mixed micelles formed in the systems ofn-dodecyl nonaoxyethylene ether/sodium dodecyl sulfate (NaC12S), andi-octylphenyl nonaoxyethylene ether/NaC12S as a function of the mole ratio of nonionic/ionic surfactants. In the former system the micellar molecular weight increases simply with increasing nonionic content, while in the latter system it rises abruptly when the nonionic content exceeds about 50% by mole. This behaviour would be interpreted by a difference in hydrocarbon-chain attraction between these two systems. The degree of ionic dissociation, α, of NaC12S in the mixed micelles increases as the content of the nonionic surfactant increases. This tendency is in accordance with the previous result obtained by pNa and vapor pressure depression data. The value of α is closely related to the charge density, σ, on the surface of the micelle; α increasing with decreasing σ. The micellar charge for NaC12S alone, estimated from electrophoretic data, is much larger than that calculated from light scattering data by using the equation derived byMysels. For this discrepancy, a plausible explanation would be made by the different surfaces of the micelle measured by these two techniques.