Surface properties in binary mixtures of aerosol OT and nonionic surfactants

The surface tensions of the mixed systems consisting of sodium di-2-ethylhexyl sulfosuccinate (Aerosol OT) and homogeneous polyoxyethylene glycol-n-dodecyl ether (nED; n=5 to 8) were measured in the absence and presence of sodium chloride (NaCl) by a modified Wilhelmy plate method. In the case of the system containing nED and Aerosol OT, the nED concentrations were fixed at various amounts, and all the surface tension curves approached 32 or 33 mN/m as the concentration of Aerosol OT approached 1 × 10⁻³ mol/l. On the other hand, when the Aerosol OT concentration was fixed at 1 × 10⁻³ mol/l, the surface tension curves for the mixed system showed a flat portion in the range where mixed micelle in the solution and the two dimensional ones on the water surface were formed. Further, molecular interaction parameter B was calculated by the Rosen extension of the regular solution treatment of Rubingh. The average values of B in the nED-AOT systems increased as the value of n increased. The values of B increased with increasing activity of AOT. In the case of the system containing 6ED, AOT and NaCl, the surface tension curves showed inflection points shifted to higher concentrations of AOT as the concentration of nED increased.