EXPERIMENTAL CONDITIONS FOR HPLC ANALYSIS OF ETHOXYLATED ALKYL PHENOLSURFACTANTS IN MICROEMULSION SYSTEMS .2. GRADIENT MODE FOR EXTENDED EON RANGE AS FOUND IN THE ANALYSIS OF OLIGOMER FRACTIONATION

The different oligomers of ethoxylated nonylphenols present in commerc ial surfactants can be separated by HPLC. While low ethylene oxide num ber (EON) species may be separated by isocratic HPLC on silica, interm ediate EONs require gradient elution HPLC. The separation of higher ol igomers can be carried out with an intermediate polarity column of the NH2 type and a solvent gradient. The suggested experimental condition s allow a satisfactory single run separation of a complex mixture with EONs ranging from 1 to 25 by using a linear gradient from 100% less p olar solvent (n-heptane-chloroform-methanol 90/5/5) to its mixture wit h up to 20% of a more polar solvent made of chloroform-methanol (50/50 ). This method is used to analyze the EON distribution resulting from the mixing of different commercial surfactants. When two surfactants w ith very different EON distributions are mixed to produce a formulatio n scan, the occurrence of three-phase behavior at the optimum formulat ion of the surfactant-oil-water system is found to correlate with the absence of a gap in the overall EON distribution. Both the fractionati on of low EON species into the oil phase and the fractionation of high EON oligomers into the water phase result in a depletion of the micro emulsion phase.