ROLE OF MEDIUM-CHAIN ALCOHOLS IN INTERFACIAL FILMS OF NONIONIC MICROEMULSIONS

In this paper we report on experiments aimed at elucidating the role of medium-chain alcohols in bicontinuous microemulsions. We have determined isothermal sections through the phase tetrahedra of quaternary systems H2O-n-octane-C(n)E0-C8E5 at equal amounts of water and oil. From these measurements we obtained the compositions of the middle-phase microemulsions as well as estimates for the alcohol concentrations in the bulk excess phases. The efficiency of the amphiphile combination (C(n)E0 + C8E5) increases with the alcohol chain length (n = 4, 5, 6, 8, 10). Furthermore, we obtained from the phase diagrams estimates for the ratio of alcohol to surfactant molecules in the interfacial films. In order to independently determine the fraction of alcohol effectively present in the film, we used perdeuterated C4E0 in a small-angle neutron scattering (SANS) experiment utilizing the technique of contrast-matching. From absolutely scaled intensities we found that the film contained 2.1 C4E0 molecules per surfactant molecule in sufficient agreement with the estimate (2.6) from the phase diagram. Apparently, the surfactant monolayer acts as a two-dimensional third phase (Schulman's "interphase") into which the medium-chain alcohols preferentially dissolve. They do so because of the intermediate polarity of the films rather than by true surface action. Accordingly, one has to discuss to what extent the action of the alcohols is that of "cosolvents" as opposed to "cosurfactants". Finally, the influence of alcohol addition on the bending properties of the film is briefly discussed.