Solubilization by different-sized surfactant mixtures

The solubilization phenomenon was investigated in mixed surfactant systems. The solubilization power of a mixed surfactant reaches its maximum at a particular temperature at each mixing ratio of surfactants. When the mole fraction of C4E1 in the total surfactant (w1 value) was varied in a water/C12E5/C4E1/decane system, the minimum mole fraction of total surfactant in the system necessary to obtain a single microemulsion phase (ξ value) was almost unchanged for w1<0.3, whereas it increased remarkably for w1>0.8. The molar solubilization capacity (Cs=(1−ξ)/ξ) of the mixed surfactant decreased remarkably for w1<0.3, whereas it decreased gradually for w1>0.8. The result |dξ/dw1|w1<0.3<|dξ/dw1|w1>0.8 is due largely to the characteristic of the function ξ(Cs)=1/(1+Cs), specifically, |dξ/dCs|w1<0.3<|dξ/dCs|w1>0.8, where dξ/dw1=(dξ/dCs)(dCs/dw1). The partial molar solubilization capacity (C¯s) of C4E1 was negative at almost all w1, but the C¯s value of C12E5 went through a maximum on the addition of C4E1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C12E6/propanol/heptane system. In the water/C12E5/C12E7/decane system, the C¯s value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The Cs and ξ values were close to molar additivity for each mixing ratio.