The Effect of Mixing of Surfactants on Solubilization in a Microemulsion System

Abstract

The effect of mixing homogeneous polyethylene glycol alkyl ethers (R m EO n ) on their maximum solubilizing power was investigated at the hydrophile-lipophile balance (HLB) temperature at which a microemulsion (surfactant) containing equal weights of water and oil is one phase of a three-phase body. The solubilization was compared at a fixed temperature equal to the HLB temperature for a single R 12EO 5 system. It was found that the minimum weight fraction of surfactant required to solubilize equal weights of water and oil into a single phase ( X b ) is decreased by mixing R 12EO 2 and R 12EO 8 or R 14EO 5and R 10EO 5. However, X b for the R 12OH-R 12EO 8 system is larger than for the R 12EO 2-R 12EO 8 system. Namely, the solubilization apparently reaches its maximum in a mixed surfactant system in which the hydrophilic-lipophilic properties of the surfactants are not very different. Nonionic surfactant is mainly distributed between oil microdomains and the interface between oil and water microdomains inside the microemulsion phase. Assuming that the monomeric solubility of each surfactant component in oil is the same as in the oil microdomain of the microemulsion, the weight fraction of each surfactant at the interface in the microemulsion ( C or C 1 ) was obtained. C or C 1 + C 2 indicates the real solubilizing power, and it is dramatically decreased when surfactants are mixed. The more the HLBs of the surfactants are separated, the more the real solubilizing power is increased. It was also found that the mixing ratio of surfactants at the water-oil interface inside the microemulsion phase is directly related to the HLB temperature.