Solution properties of mixed surfactant system: V. The effect of alkyl groups in nonionic surfactant on surface tension of anionic-nonionic surfactant systems

Abstract

The effect of alkyl groups in a nonionic surfactant on the surface tension of anionic-nonionic surfactant systems is described; these systems are sodium 3,6,9-trioxaicosanoate (ECL)-alkyl polyoxyethylene ethers (C m POE, m = 12, 14, 16, and 18). The surface tension of each mixed surfactant solution decreases with increasing mole fraction of anionic surfactant. However, these surface tension vs mole fraction curves are shifted from a monotonous curve to specific curves with increasing alkyl chain length in the nonionic surfactant. Furthermore, in the case of the mixed systems including the nonionic surfactant which has shorter alkyl chain length, the surface tension vs total concentration curve shows two breakpoints in the vicinity of the CMC for ECL alone and for C m POE alone. In the case of systems including a nonionic surfactant which has longer alkyl chain length, that curve shows only one breakpoint. This may be attributed to the fact that the mixed micelle is formed more easily by a nonionic surfactant including long alkyl chain length than by one having shorter alkyl chain length. Then, in the mixed surfactant systems, there are two kinds of micelles coexisting (one rich in anionic surfactant and the other rich in nonionic surfactant).