Determination of ethylene oxide oligomer distributions in alcohol ethoxylates by HPLC using a rotating disc-flame ionization detector

A high performance liquid chromatographic (HPLC) method has been developed for the quantitative determination of ethylene oxide (EO) oligomer distributions (% wt) in acetylated alcohol ethoxylates, R(OCH₂CH₂)_nOH, from n=0, 1 to n=30 using a rotating disc-flame ionization detector. Both single carbon number and mixed carbon number alcohol-based (NEODOL® ethoxylates) samples have been analyzed by gradient elution with 2 different solvent systems on a Waters μ-Porasil column. With both solvent systems, 95% hexane is the initial solvent but with one system, 100% acetone is the final solvent and with the other, 10% methanol/90% acetone is used. The latter solvent elutes the higher ethoxylates from n=21 to n=30 quantitatively from the μ-Porasil column which the 100% acetone solvent fails to do. The 100% acetone solvent separates n=2 and n=3 from n=0,1 which the methanol-containing solvent does not do. Response factors for n=3 and n=8 have been experimentally determined and the response factors for the other EO units have been calculated from these 2 results. The corrected EO oligomer distributions for both NEODOL® 25-9 and NEODOL® 23-6.5 determined by HPLC are in good agreement with those determined earlier by circular thin layer chromatography (up to n=16 can be determined by this method). The average EO numbers determined by the HPLC method and by a wet chemical (phthalic anhydride) method are in excellent agreement for the above 2 samples and a sample of NEODOL® 23-7.5. The results are discussed in terms of Snyder’s theory for gradient elution in HPLC using the gradient steepness parameter.