A conformational interpretation for the peak of reduced viscosity for polyelectrolytes at low ionic strength

Abstract

Several articles over the past decades have reported a peak in the curve of reduced viscosity η r vs polyelectrolyte concentration Cp when diluted with pure water or a fixed, low ionic strength solution. The current article shows that the large changes in conformation which accompany changes in ionic strength C s , for long, linear, semiflexible polyelectrolytes, and which enter in the first, quadratic interaction term of the solutionviscosity, are sufficient to account for the existence, relative magnitudes, and positions of peaks in such viscosity profiles. This is illustrated for hyaluronic acid (HA), for which the variations of radius of gyration with ionic strength have been previously determined experimentally. The conformational interpretation predicts that in the coil limit, the position of the viscosity vs Cp peak is independent of mass, but that its height should be roughly proportional to M at fixed Cs, and to roughly CpCs −1.5 for a fixed M. The explanation is meant as a limiting case for long, linear polyelectrolytes and posits that steric, or hard, interactions between separate polymers dominate over electrostatic interactions when the polyelectrolyte dimensions are much larger than κ−1 the electrostatic screening length. The other extreme, short, chain polyelectrolytes and other inextensible objects, such as spheres, are mentioned in light of previously elaborated theories based on interacting electrostatic atmospheres when the dimensions of such objects are smaller than κ−1.