THE EFFECT OF MIXING OF SURFACTANTS ON SOLUBILIZATION IN A MICROEMULSION SYSTEM

The effect of mixing homogeneous polyethylene glycol alkyl ethers (R(m )EO(n)) on their maximum solubilizing power was investigated at the hy drophile-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 temper ature equal to the HLB temperature for a single R(12)EO(5) system. It was found that the minimum weight fraction of surfactant required to s olubilize equal weights of water and oil into a single phase (X(b)) is decreased by mixing R(12)EO(2) and R(12)EO(8) or R(14)EO(5) and R(10) EO(5). However, X(b) for the R(12)OH-R(12)EO(8) system is larger than for the R(12)EO(2)-R(12)EO(8) system. Namely, the solubilization appar ently reaches its maximum in a mixed surfactant system in which the hy drophilic-lipophilic properties of the surfactants are not very differ ent. Nonionic surfactant is mainly distributed between oil microdomain s and the interface between oil and water microdomains inside the micr oemulsion phase, Assuming that the monomeric solubility of each surfac tant component in oil is the same as in the oil microdomain of the mic roemulsion, the weight fraction of each surfactant at the interface in the microemulsion (C or C-i) was obtained. C or C-1 + C-2 indicates t he real solubilizing power, and it is dramatically decreased when surf actants 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. ( C) 1995 Academic Press, Inc.