PARTITIONING AND DIFFUSION OF SOLUTES IN HYDROGELS OF POLY(ETHYLENE OXIDE)

Hydrogels were created by electron beam irradiation of aqueous solutio ns of poly(ethylene oxide) (PEO) having a nominal molecular weight of 35000. The molecular weight between cross-links M(c) varied from 3000 to 15000, and the equilibrium volume fractions of polymer v2,s from 0. 01 to 0.08. These hydrogels were exposed to aqueous solutions of solut es: 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 coef ficient in hydrogel/diffusion coefficient in free solution was determi ned, and related to the hydrogel parameters M(c) and v2,s and to the s olute effective radius r(E) (Einstein radius). The diffusion coefficie nt ratio is greater for the flexible random coiling PEO than for the ' rigid' solutes at a given set of M(c), v2s and r(E), and the disparity increases rapidly as r(E) increases. Among the globular proteins the diffusion coefficient ratio decreases by orders of magnitude with smal l changes in r(E) (20.6-27.6 angstrom) and was found to be nearly zero for albumin (r(E) = 36.1 angstrom). The tricyclic antidepressants had partition coefficients of around 2, whereas the other solutes had par tition coefficients of about unity. By reason of the partition coeffic ient of around 2, the diffusion coefficient ratio of a tricyclic antid epressant having a value of r(E) = 5.5 angstrom is half that of the la rger cyanocobalamin, for which r(E) = 8.5 angstrom.