PHASE-BEHAVIOR OF THE QUATERNARY SYSTEM H2O N-OCTANE/C(8)E(5)/N-OCTANOL - ROLE OF THE ALCOHOL IN MICROEMULSIONS/

We examine the effect of n-octanol (C(8)E(0)) on the phase behavior of the ternary microemulsion system;H2O/n-octane/pentaethylene glycol mo no-rt-octyl ether (C(8)E(5)) at constant 1:1 water-to-oil volume ratio . The addition of C(8)E(0) shifts the three-phase regime to lower temp eratures and distorts it. The shift of the three-phase regime can be e xplained by assuming that in the microemulsion a mixed amphiphilic fil m is formed, which becomes more hydrophobic as the alcohol concentrati on is increased. The distortion of the three-phase body is caused by a larger solubility of C(8)E(0) in n-octane than that of the C(8)E(5) m onomers. From a quantitative analysis taking this effect into account, one obtains the relative proportions of C(8)E(5) and C(8)E(0) in the mixed film. Furthermore, the amphiphilic mixture (C(8)E(5) + C(8)E(0)) displays an increased efficiency compared to the pure components, whi ch is attributed to synergism. Measurements of the interfacial tension between bulk water- and oil-rich phases of the quaternary microemulsi on system support this interpretation. The interfacial tension passes through a minimum both on either changing the temperature or the C(8)E (0) content when the curvature of the film is tuned through zero. Inte restingly, the synergism lowers the absolute value of the minimum inte rfacial tension of the mixed amphiphiles below that for an equivalent pure component.