Diffusion and interaction in gels and solutions

Abstract

The diffusion of monodisperse polyethylene glycols (PEGs) in gels and solutions of the polymers κ-carrageenan and polystyrene sulfonate (NaPSS) has been studied experimentally and theoretically. It was found that the diffusion quotient D/D ₀ (ratio between diffusion coefficients of the PEGs in systems with and without polymer) was higher in gels than in solutions of κ-carrageenan at the same total polymer volume fraction. Also, D/D ₀ of a PEG was lower in NaPSS solutions at low ionic strengths compared to the diffusion in solutions where NaPSS had a higher degree of flexibility (induced by added salt, which had no influence on D ₀). Further, it was found that D/D ₀ for each PEG was a convex declining function of the total polymer volume fraction, while at constant volume fraction the D/D ₀ quotient was a concave function of the radius of the PEGs. — The results are discussed by means of a novel theory. The basic concepts and equations of this theory are outlined in the paper, and it is shown that the main hindrance to the diffusion of molecules like PEG is the result of sterical obstruction due to the presence of the polymer chains. The crucial parameters determining the diffusion are the size of the diffusing molecule, the polymer radius, the persistence length of the polymer, and the total polymer volume fraction.