Liquid adsorption chromatography of polyethers - Theory and experiment

The chromatographic behavior of low-molecular-mass poly(ethylene glycol) (PEG), and of its mono- and dimethyl ethers (PEG-MME and PEG-DME, respectively) has been studied in the isocratic mode of reversed-phase liquid adsorption chromatography (RP-LAC) on C-18-modified silica (ODS2) column packings with pore diameters of 8 nm and 30 nm, using methanol-water of different compositions as mobile phase. For all three types of oligomers completely resolved chromatograms were obtained, in which all peaks could be identified. Precise and detailed data on the molar mass dependence of distribution coefficients resulted from these experiments. The experimental data were interpreted according to simple molecular statistical theory based on the model of a rigid-rod molecule, which is capable of being adsorbed inside the homogeneous layers near the pore walls. This approach allowed us to determine the free energies of transfer of the repeating unit (oxyethylene group) and of the two types of the end groups (methoxy and hydroxy groups) from a solution (in mixed eluents of different composition) into a bonded C-18 phase. In addition, the effective thickness of the bonded layer was estimated as well as the critical point of adsorption for polyoxyethylene, which is of importance for separation of macromolecules by using the critical-condition mode of liquid chromatography.