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

Abstract

The effect of oxyethylene groups in a nonionic surfactant on the surface tension of anionic-nonionic systems is described; these systems are sodium 3,6,9-trioxaicosanoate (ECL)-alkyl polyoxyethylene ethers (POE n , n = 10, 20, 30, and 40). 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 specific curves to monotonous curve with increasing polyoxyethylene chain length in nonionic surfactant. Moreover, in the case of the mixed systems including a nonionic surfactant which has shorter polyoxyethylene chain length, the surface tension vs total concentration curve shows only one breakpoint. In the case of systems including the nonionic surfactant which has longer polyoxyethylene chain length, that curve shows two breakpoints in the vicinity of the CMC for ECL alone and that for POE n alone. This may be attributed to the fact that the mixed micelle is formed more easily by a nonionic surfactant including shorter polyoxyethylene chain length than by one having long. Then, in the mixed surfactant systems, there are two kinds of micelles coexisting (one rich in nonionic surfactant and the other rich in nonionic one).