WATER IN OIL MICROEMULSIONS AS MOBILE PHASE IN LIQUID-CHROMATOGRAPHY

L2 microemulsions are water in oil systems. The mass phase diagram of water-sodium diethylhexyl sulfosuccinate or Aerosol OT (AOT)-heptane was determined. L2 liquid systems containing up to 45% water and 42% AOT were used as mobile phase In an unbonded silica column. The capacity factors, k', of two test solutes, 4-nitrobenzoic acid and 4-nitrophenol, were measured for about 50 different L2 compositions. It was found that the k' evolutions were very dependent upon the physicochemical microemulsion structure described by using the parameter MW/AOT, the water/AOT molar ratio. It was shown that drastic changes in chromatographic parameter occurred at MW/AOT < 10, conditions under which most of the water molecules are tightly bound to the surfactant polar head and sodium ion. Up to 50% decrease of the column dead-volume was observed when the L2 system water content increased. The column permeability was changing at low water content mobile phase. These phenomena were related to AOT adsorption onto the silica stationary phase. At Mw/Aot> 10, the AOT adsorption and the column permeability were constant and dependent upon the AOT concentration. However the test solute retention were still evolving significantly. © 1990, American Chemical Society. All rights reserved.