Effect of Hydrophobic Groups in Nonionic Surfactant on the Solution Property of SDS-Nonionic Surfactant Mixed Micelles

Abstract

The effects of alkyl chain length in a nonionic surfactant of the solution properties of anionic-nonionic mixed surfactant systems are described. The systems studied were sodium dodecyl sulfate (SDS) -alkyl polyoxyethylene ethers (C_mPOE_2O, m=6, 8, 10, 12, 14, 16, and 18). The degree of ionic dissociation of mixed micelle shows a minimum with an increase in the number of alkyl chains in the nonionic surfactant. As chain length increases, the electrical conductivities of the mixed surfactant solutions decrease, in spite of decreased activation energy for conduction. The radius of mixed micelle with the electric double layer is larger for a nonionic surfactant with longer alkyl chain length than one with short chain length. Two kinds of micelles (anionic surfactant rich and nonionic rich micelles) were found to form more easily with decreasing alkyl chain length in nonionic surfactants. Of particular significance is the fact that in a mixed system consisting of SDS and C₆POE₂₀ with extremely short alkyl chains, nonionic surfactant molecules may penetrate into the palisade of an SDS micelle.