Partitioning and diffusion of solutes in hydrogels of poly(ethylene oxide)

Hydrogels were created by electron beam irradiation of aqueous solutions of poly(ethylene oxide) (PEO) having a nominal molecular weight of 35,000. The molecular weight between cross-links Mc varied from 3000 to 15,000, and the equilibrium volume fractions of polymer V2,s from 0.01 to 0.08. These hydrogels were exposed to aqueous solutions of solutes: tricyclic antidepressants, cyanocobalamin, four globular proteins and three linear species of PEO. Partition coefficients and diffusion coefficients were determined. For each solute the ratio diffusion coefficient in hydrogel/diffusion coefficient in free solution was determined, and related to the hydrogel parameters Mc and v2,s and to the solute effective radius rE (Einstein radius). The diffusion coefficient ratio is greater for the flexible random coiling PEO than for the 'rigid' solutes at a given set of Mc, v2s and rE, and the disparity increases rapidly as rE increases. Among the globular proteins the diffusion coefficient ratio decreases by orders of magnitude with small changes in rE (20.6-27.6 A) and was found to be nearly zero for albumin (rE = 36.1 A). The tricyclic antidepressants had partition coefficients of around 2, whereas the other solutes had partition coefficients of about unity. By reason of the partition coefficient of around 2, the diffusion coefficient ratio of a tricyclic antidepressant having a value of rE = 5.5 A is half that of the larger cyanocobalamin, for which rE = 8.5 A.