ELECTRICAL-CONDUCTIVITY BEHAVIOR OF POLY(L-GLUTAMIC ACID) IN NACL ELECTROLYTE-SOLUTIONS - EFFECT OF POLYION CONFORMATION

The electrical conductivity of aqueous solutions of poly(L-glutamic acid) of three different degrees of polymerization has been measured as a function of polymer content at different NaCl concentrations, from 10-4 to 3 × 10-2 mol/L. The equivalent conductance of the polymer has been estimated from the slope in the plots of conductivity against the polyion concentration. The results are compared to the conductivity deduced from a simple additive relationship whose main contribution derives from the ion distribution in a spherical polyelectrolyte cell, on the basis of the mean-field Poisson-Boltzmann equation. This theory provides a quantitative description of the conductivity behavior of these polymer solutions in a wide range of salt concentrations and polyion densities. As the electrostatic interactions of small ions in the neighborhood of the polyion become stronger and the polymer conformation deviates from the spherical random coil structure, the transition to counterion condensation occurs, providing support for the concept of an effective charge in linear polyions in solution, smaller than their titration charges. In this context, the conductivity of the polyion of lowest molecular weight is found to agree reasonably well with the predictions of the Manning theory. © 1990 American Chemical Society.