QUANTITATIVE TRACE ANALYSIS OF SURFACTANT MIXTURES BY REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY WITH REFRACTOMETRIC DETECTION

Reversed-phase partition liquid chromatography on an octyl column allo wed the separation of complex non-ionic poly(ethylene oxide)-type (PEG ) surfactant mixtures resulting from the condensation of ethylene oxid e with saturated fatty alcohols. As these compounds have no chromophor ic group, they were detected by differential refractometry. Accurate q uantitation of each oligomer (C(m)E(n)) allowed the main characteristi cs of each non-ionic surfactant, i.e., the nature and percentage of th e different alkyl chains (with m = number of carbons) and the average number of ethylene oxide units ((n) over bar) to be obtained in one an alysis. Preparative liquid chromatography was used to isolate pure oli gomers with a higher degree of ethoxylation (n = 10, 11, 12, 14 and mo re) than the commercially available standards, in order to determine a wide range of refractometric response factors.' It appeared that they are constant as a function of alkyl chain length (C-10-C-16 range) bu t that they vary significantly and non-linearly as a function of the d egree of ethoxylation, n. It was found that neglecting the variation o f response factors can result in a distortion of the average ethoxylat ion number and in an unsatisfactory quantitative analysis. This chroma tographic method, involving a quantitative and reproducible trace enri chment procedure with liquid-solid extraction, allowed the analysis of very dilute PEO mixtures in water. The components of complex PEO mixt ures in water were determined at concentrations as low as 0.5 mg l(-1) , without any distortion of the distribution, the detection limit bein g 0.25 mu g l(-1) for the less abundant oligomers.