Solubilization by different-sized surfactant mixtures

Abstract

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 C 4E 1 in the total surfactant ( w 1 value) was varied in a water/C 12E 5/C 4E 1/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 w 1 < 0.3 , whereas it increased remarkably for w 1 > 0.8 . The molar solubilization capacity ( C s = ( 1 − ξ ) / ξ ) of the mixed surfactant decreased remarkably for w 1 < 0.3 , whereas it decreased gradually for w 1 > 0.8 . The result | d ξ / d w 1 | w 1 < 0.3 < | d ξ / d w 1 | w 1 > 0.8 is due largely to the characteristic of the function ξ ( C s ) = 1 / ( 1 + C s ) , specifically, | d ξ / d C s | w 1 < 0.3 < | d ξ / d C s | w 1 > 0.8 , where d ξ / d w 1 = ( d ξ / d C s ) ( d C s / d w 1 ) . The partial molar solubilization capacity ( C ¯ s ) of C 4E 1 was negative at almost all w 1 , but the C ¯ s value of C 12E 5 went through a maximum on the addition of C 4E 1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C 12E 6/propanol/heptane system. In the water/C 12E 5/C 12E 7/decane system, the C ¯ s value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The C s and ξ values were close to molar additivity for each mixing ratio.