Reversed-phase high-performance liquid chromatography of polyethers - Comparison with a theory for flexible-chain macromolecules

Reversed-phase adsorption chromatography retention modes of polyethylene glycol (PEG), its mono and dimethyl ethers (MME and DME) and polypropylene glycol (PPG) are studied both experimentally and theoretically. The experimental conditions were: narrow-and wide-pore Spherisorb C-18 adsorbents and two mixed-solvent systems (methanol-water and acetone-water) as mobile phases. At varying compositions of the mobile phase, fully-resolved chromatograms of polyether samples were obtained, in which all peaks could be identified, and the dependencies of the distribution coefficient on the degree of polymerization for PEG and PPG molecules were extracted by processing these chromatograms. The data were interpreted by using a theory of homopolymers based on a continuum Gaussian chain model of flexible macromolecules and a slit-like model of pens of the stationary phase. Two regimes of an adsorption chromatography of macromolecules are examined in relation to the well-known Martin rule, the role of pore size and the end-group effects being discussed. The theory proved to describe the experimental data on both polymers well, in the whole range of molecular masses studied, and thermodynamic parameters characterizing interactions of ethylene oxide and propylene oxide (PO) repeating units in polyether molecules with the adsorbent pore walls have been determined from a comparison of the theory with the experimental data. The conditions corresponding to the critical chromatography mode are estimated from the thermodynamic data for PEG and methanol-water system as being at about 76% of methanol, while for the PPG and acetone-water system, it is shown that such critical conditions at the given temperature are not attainable. Additionally, the mean thickness H of adsorbed PEG and PPG macromolecules was estimated as being equal to about 3-10 Angstrom. The parameter H proved to be a decreasing function of the water content in the mixed solvent. The same estimate of thickness H approximate to 2.6 Angstrom is obtained for both polymers in purl water. (C) 1998 Elsevier Science B.V.