α-Cyclodextrin (α-CD) and urea (U) hosts were used in order to separate a mixture of poly(ethylene glycols) (PEGs) with two different molecular weights ( M w = 600 [PEG 600 ] and M w = 20 000 [PEG 20000 ]) by forming the respective inclusion compounds (ICs). Differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD) have been used to verify the formation and successfully characterize all inclusion compounds. The high melting point of the urea inclusion compound (PEG*-U IC) formed from the solution containing both polymers was the first evidence that urea preferentially included poly(ethylene glycol) with the higher molecular weight. The PEG*-U IC and PEG 20000 -U IC X-ray diffraction patterns and FTIR spectra were very similar to each other, confirming that urea preferentially complexes PEG 20000 in their mutual solution. Since the above-mentioned techniques were not as useful in the case of α-CD ICs, viscosity measurements were made in order to demonstrate the molecular weight selectivity of α-CD host. The efflux time of PEG*-α-CD IC solution was much closer to that of a PEG 20000 -α-CD IC solution, which suggested that α-CD also included poly(ethylene glycol) having a high molecular weight rather than the low molecular weight PEG 600 . The efflux time of PEG*-U IC solution is almost identical with that of the PEG 20000 -U IC solution indicating that urea has better molecular weight selective properties than α-CD.
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Rusa et al. (2000) studied this question.