On understanding microemulsions : I. Interfacial tensions and adsorptions of SDS and pentanol at the cyclohexane/water interface

Abstract

We measured interfacial tensions using the spinning drop technique in two-phase oil/water (O/W) systems containing sodium dodecyl sulfate (SDS), n-pentanol, NaCl, cyclohexane, and water. The systems contained only small amounts of SDS (mostly <CMC). Surfactant activities were obtained from measured distribution data. Addition of pentanol decreases the adsorption of SDS, whereas the saturation adsorption of pentanol is almost not affected by the presence of SDS. Under conditions that also exist in W/O microemulsions a molar ratio pentanol/SDS of 2.4 at the interface is found from interfacial tension measurements. Similar values are obtained for the interface in microemulsions from dilution experiments. In the mixed monolayer, the total adsorption (pentanol + SDS) corresponds to approximately 25 Å2/ molecule, whereas for pure SDS and pentanol the saturation adsorptions correspond respectively to a coverage of 54 and 36 Å2/molecule.