Journal of Chromatography A, 910 (2001) 207–216 www.elsevier.com / locate / chroma Liquid exclusion–adsorption chromatography: new technique for isocratic separation of nonionic surfactants I. Retention behaviour of fatty alcohol ethoxylates a, b * Bernd Trathnigg , Alexei Gorbunov a Institute of Chemistry, Karl-Franzens-University, A-8010 Graz, Austria b Institute for Highly Pure Biopreparations, Pudozhskaya 7, 197110 St. Petersburg, Russia Received 3 August 2000; received in revised form 21 November 2000; accepted 28 November 2000 Abstract A new technique of liquid chromatography is described, which allows a baseline separation of fatty alcohol ethoxylates with 15–20 ethylene oxide units under isocratic conditions. The new method is based on a combination of two different chromatographic modes for the individual structural units: size exclusion for the poly(oxyethylene) chain and adsorption interaction for the hydrophobic end fragments. A theory is provided for this mixed exclusion–adsorption mode of liquid chromatography. Chromatographic data are found to be in a good agreement with this theory.  2001 Elsevier Science B.V. All rights reserved. Keywords: Liquid exclusion–adsorption chromatography; Fatty alcohols ethoxylates; Non-ionic surfactants 1. Introduction niques can be applied, which differ in the stationary and mobile phases used, and thus in the separation Ethoxylates of fatty alcohols, fatty acid methyl mechanism. esters and alkylphenols can be used in many fields: • Gas chromatography (GC) is limited to the lowest according to the hydrophilic nature of the polyoxy- oligomers [1]. ethylene chain, they may be used as nonionic • Supercritical fluid chromatography (SFC) [1–3] is surfactants, the most important of which are fatty suitable in the analysis of polymers and oligo- alcohol ethoxylates (FAEs). These products often mers, but it is not — or not yet — a common consist of different polymer homologous series routine technique, despite the fact, that its ef- (based on the purity of the starting materials). Hence ficiency is often considerably higher than that of their full characterization requires the independent liquid chromatography. A special case is chroma- determination of two distributions: molar mass tography in subcritical or enhanced fluidity mo- (MMD) and type of functionality (FTD). bile phases [4,5]. In such phases, similar sepa- Basically, the following chromatographic tech- ration mechanisms can be observed as in liquid chromatography [3,5–10]. • Liquid chromatography (LC, HPLC) is the most *Corresponding author. E-mail address: trathnig@kfunigraz.ac.at (B. Trathnigg). frequently used technique in the separation of 0021-9673 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0021-9673(00)01215-2