Using two-photon confocal microscopy, equilibrium partition coefficients, k, were measured for aqueous Na-fluorescein, fluorescently labeled dextrans with molecular masses ranging from 4 to 20 kDa, two fluorescently labeled proteins with opposite charges, anionic bovine serum albumin (BSA), and cationic avidin in anionic 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) gels saturated with aqueous phosphate buffer solution. Cross-linking density with ethylene glycol–dimethacrylate (EGDMA) ranged from 0 to 1 wt %. All partition coefficients, except for avidin, were considerably less than unity and diminished strongly with increasing Stokes–Einstein diameter of the free aqueous solute. The average mesh size of the wet gels, obtained from the zero-frequency oscillatory shear-storage gel modulus, ranged from 3.6 to 8.3 nm over the cross-link ratios studied. Except for Na-fluorescein, solute hydrodynamic diameters were larger than the smallest average gel mesh size. Yet, all solutes permeated the gels but with small partition coefficients less than about 0.001 for the largest diameter solutes in the small mesh size gels. To express deviation from ideal partitioning, we define an enhancement (or exclusion) factor, E ≡ k/(1 – φ), where φ is the polymer volume fraction in the gel and E is unity for point solutes. A hard-sphere excluded-volume Ogston mesh size distribution is adopted to predict a priori the measured enhancement factors as a function of average gel mesh size for those solutes that do not interact specifically with the anionic gel (i.e., for solutes with E < 1). Agreement between the extended Ogston distribution and experiment is qualitative for both enhancement factors and water content of the gels. The cationic protein, Fl-avidin, exhibits a large enhancement factor in the anionic gels due to strong specific interaction with the charged carboxylate groups of MAA. In this case, consideration must be given to both hard-sphere size exclusion and specific complexation with the polymer strands.

Transient solute absorption and desorption concentration profiles were measured in a 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) anionic hydrogel using two-photon confocal microscopy. Dilute aqueous solutes included fluorescently labeled dextrans with molecular masses of 4, 10, and 20 kDa, and fluorescently labeled cationic avidin protein. Cross-linking densities with ethylene glycol dimethacrylate (EGDMA) varied from 0 to 1 wt % with polymer volume fractions increasing from 0.15 to 0.25. Average gel mesh sizes, determined from zero-frequency oscillatory shear storage moduli, ranged from about 3.6 to 8.4 nm over the cross-link ratios studied. All solutes exhibit Stokes−Einstein hydrodynamic radii obtained from measured free diffusion coefficients, D o , comparable to or larger than the average gel mesh size. In spite of considerable size exclusion, the studied solutes penetrate the gels indicating a range of mesh sizes available for transport. Transient uptake and release concentration profiles for FITC-dextrans follow simple diffusion theory with diffusion coefficients, D, essentially independent of loading or release characteristic of reversible absorption. Although strongly sizeexcluded, these solutes do not interact specifically with the polymer network. Diffusivities are accordingly predicted from a largepore effective-medium (LPEM) model developed to account for solute size, hydrodynamic drag, and distribution of mesh sizes available for transport in the polymer network. For this class of solute, and using no adjustable parameters, diffusivities predicted from the new effective-medium model demonstrate good agreement with experiment. For the specific-interacting cationic protein, avidin, gel loading is 3 orders of magnitude slower than that of dextran of similar hydrodynamic radius. Desorption of avidin is not complete even after 2 weeks of extraction. On the basis of size alone, avidin is strongly size-excluded, yet it exhibits a partition coefficient of over 20. For the positively charged protein, we observed specific ion binding on the negatively charged carboxylate groups of MAA-decorated polymer strands in the larger mesh spaces. Simple linear sorption kinetics gives an adsorption time constant of 5 min and a desorption time constant of about 20 days, suggesting nearly irreversible uptake of cationic avidin on the anionic gel matrix.