Theory of liquid adsorption chromatography of macromolecules applied to polyethylene glycol and fatty alcohol ethoxylates

Liquid adsorption chromatography retention modes of poly(ethylene glycol) (PEG), its mono- and dimethyl ethers (MMEs and DMEs) and also of fatty alcohol ethoxylates (FAEs) are studied both experimentally and theoretically. The experimental system under investigation was a Spherisorb S5W 80 Angstrom column and isopropanol-water as the mobile phase. At various compositions df the mobile phase (in the range of 75 to 87% of isopropanol) chromatograms exhibiting good peak resolution were obtained and the dependencies of the elution volume on the number of repeating EO units for both PEG and FAE samples evaluated on the basis of a closer inspection of the chromatographic data. The experiments did not reveal any differences between the chromatographic behavior of PEGs, MMEs and DMEs, while the FAEs gave substantially smaller values of elution volume at all mobile phase compositions. These data were interpreted by using a molecular-statistic theory of homopolymers (to describe both PEG, MME and DME) and two-block copolymers (for FAEs) based on a continuum Gaussian chain model of macromolecules and a slit-like model of pores of stationary phase, wide pore approximation and the adsorption chromatography mode for PEG molecules were assumed in the development of this theory. This theory described very well the experimental data obtained, and two thermodynamic parameters characterizing interactions of EO units of PEG and both EO and (CH2)(n) chains of FAE molecules with the adsorbent pore walls have been determined from the comparison of the theory and the experiments, Although the mean thickness of adsorbed oxyethylene chain, H, was estimated as being equal to about 3.5-4.5 Angstrom, H proved to be slightly decreasing with increasing isopropanol content in mobile phase. Chromatograms visualizing the adsorption of PEG and FAE molecules are presented, and the correspondence between the theoretical approach and the experimental situation under investigation is discussed. (C) 1997 Elsevier Science B.V.