Purification of ethoxylated anionic surfactants by preparative high-performance liquid chromatography

Abstract

Alkyl and alkylarylalkoxyethanesulphonates are well-known surfactants which have been suggested for a variety of applications. Recent examples that have appeared in the patent literature include the application of such sulphonates to the treatment of inorganic mineral scale’, as emulsion polymerisation surfactants2 and as surfactant systems for the enhanced recovery of crude oi13. The alkylphenylethoxyethanesulphonates [RPh(OCH2CH2)nS03Na] were originally prepared and patented by Rohm and Haas and this company also explored their surfactant propertiesJ and still markets them under the “Triton” trademark in North America. The selection of an alkylphenylethoxyethanesulphonate of correct structure for a given application depends upon a detailed structure-activity study and this can require the provision of substantial quantities of purified anionic surfactant of welldefined structure. The synthesis of such surfactants generally involves the conversion of alkylphenylethoxyalcohol in a oneor two-step process into the sulphonate, so that crude sulphonate products generally contain unreacted nonionic precursors together with anionic reaction by-products. The separation of anionic surfactant sulphonate from this crude product is a formidable task by conventional means. For example, ion-exchange chromatography is unable to separate sulphonate surfactant from anionic impurities and because alkylphenylethoxyethanesulphonates are in themselves complex mixtures containing a distribution of molecules of different ethoxy chain lengths, normal-phase gravity column chromatography results in the separation of these anionic ethoxylate oligomers and consequent overlap with nonionics. It has now been demonstrated that pure anionic alkylphenylethoxyethanesulphonate may be rapidly separated from crude commercial products by preparative high-performance liquid chromatography (HPLC) using a reversed-phase system. In addition, the molar ratio of anionic surfactant to non-ionic ethoxyalcohol may also be approximated by a similar system in the analytical mode and the purity of sulphonate fractions from the HPLC system rapidly monitored.