Abstract: Butanediyl-α, ω-bis-(dialkyldodecyl-ammonium bromides) (alkyl = methyl, ethyl, and propyl), which referred to as C12(n)-4-C12(n)·2Br (n=1, 2, 3), had been synthesized. The results studied by dilution method showed that a part of n-hexanol in n-heptane together with surfactant constructed a mixed film, which improved the molecular geometry of the mixed components and resulted in the formation of W/O microemulsion. The size of the water pool enlarged with increasing water content W0, which decreased the surface curvature of the water pool and therefore reduced the requirement for n-hexanol in the mixed film. With increasing the head-group size of C12(n)-4-C12(n)·2Br, i.e. n from 1 to 2, the mole ratio of n-hexanol at the film to surfactant a increased, and then decreased with further increasing n from 2 to 3. At W0=10 and 20, the radius of water pool RW reduced with increasing n, but at W0=30, RW increased considerably with increasing n from 2 to 3. These results indicated that the propyl groups linked with quaternary ammonium behaved differently from linked methyl or ethyl groups. The propyl groups may bend toward the bulk n-heptane. This unfurled the volume of the alkyl chain region of surfactant and thus decreased the requirement for n-hexanol in the film, which favored to form large water pool.