The effect of solute concentration on the equilibrium partitioning and hindered gradient diffusion of colloids in polymeric hydrogels

The behavior of colloidal solutes in fibrous media is important in the design of controlled release, extraction and separations techniques used in many industries, e.g. pharmaceuticals, food science and agriculture. In this study, the effect of solute concentration on partitioning and hindered gradient diffusion of colloids in hydrogels was investigated. The partition coefficient was determined theoretically as a function of solute and gel fiber volume fractions. Experiments were done to measure the partition coefficient of two proteins, bovine serum albumin and alpha-lactalbumin, in agarose gel, for a range of solute and gel fiber concentrations. The theory predicts that for hard-sphere and/or electrostatic repulsive solute-solute interactions, solute partitions away from the gel phase, but partitioning into the gel increases with increasing solute concentration. These effects are enhanced for larger solutes, increased fiber volume fraction, or stronger electrostatic repulsion. Our theoretical predictions are consistent with the measured concentration dependence of the partition coefficient for BSA. Experimental results for ALA indicate the presence of attractive interactions between the protein molecules, in addition to hard-sphere and electrostatic interactions. A theory to describe the effect of solute concentration on hindered gradient diffusion in polymeric gels was also derived. This theory shows that the gel fibers enhance the thermodynamic driving force for gradient diffusion while simultaneously decreasing the effect of concentration on the hydrodynamic drag experienced by a globular solute. To compare the theoretical predictions with experimental data, holographic interferometry was used to measure rates of diffusion for C12E 8 surfactant micelles and for BSA as a function of solute and fiber volume fractions. If the solutes are modeled as spheres with short-range repulsive interactions, the measured concentration dependence of the diffusivities is in good agreement with the theoretical results. This is the first report of experimentally measured partition coefficients and gradient diffusion coefficients of solutes in gels as a function of solute concentration. The theories developed here for partitioning and gradient diffusion of spheres with short-range interactions in fibrous media, which do not depend on adjustable parameters, provide useful insight to the physical interactions that affect the behavior of colloids in fibrous and other microporous media.