The effect of solute concentration on the hindered diffusion of dextran and polyethylene glycol in porous membranes

In this study, the effect of solute concentration on hindered diffusion has been examined by measuring macromolecular diffusion rates of dextran and polyethylene glycol (PEG) in porous membranes. Diffusion experiments were performed using track-etched polycarbonate membranes. The pore density and pore length of the membranes were determined using scanning electron microscopy, while the pore radius was measured by independent solvent flow and solute diffusion experiments. Characterization experiments revealed that the membranes contain parallel uniform cylindrical pores with a relatively narrow pore size distribution. Commercial dextrans were fractionated to tighten the molecular weight distribution using gel permeation chromatography. PEG was used as received.; At infinite dilution concentrations, measured effective diffusion coefficients were found to be larger than those predicted for either solid, spherical solutes or random coil polymers without long-range interactions. Experiments confirmed that dextran adsorption effects as well as electrostatic interactions between dextran and the pore wall were negligible. PEG adsorption effects were also found to be negligible. The experimental results of both dextran and PEG were found to be in good agreement with a model that incorporates attractive van der Waals interactions between a solid spherical solute and the pore wall.; At finite concentrations, a model that expresses the concentration dependence of both the equilibrium partition coefficient as well as the intrapore diffusivity with a first virial expansion in concentration was proposed. Measured effective diffusion coefficients of dextran increase as solute concentration increases. Experimental values of the first virial coefficient for partitioning and the first virial coefficient for intrapore diffusivity were found to be internally consistent with different solute concentrations and molecular weight fractions. Measured values of the first order virial coefficient for equilibrium partitioning were found to be in agreement with the proposed model that includes attractive van der Waals interactions between dextran and the pore wall. Measured effective diffusion coefficients of PEG were observed to decrease as solute concentration increases, an observation that may be due to chain entanglement that occurs as PEG concentration increases, resulting in an increase in the effective size of the diffusing solute.