Apparent Molar Mass of a Polyelectrolyte in an Organic Solvent in the Low Ionic Strength Limit As Revealed by Light Scattering

Abstract

The apparent molar mass of a partially quaternized poly-2-vinylpyridines (degree of quaternization 4.3% < <i>Q</i> < 35%) in 1-propanol is measured by light scattering at low ionic strength (10^–6 M < <i>c</i>_s < 10^–3 M) as a function of polyion repeat unit concentration (7 × 10^–5 monomol/L < <i>c</i>_m,p < 4 × 10^–2 monomol/L). No dialysis was applied prior to measurements. The apparent molar mass under “salt-free” conditions is smaller than the true molar mass by an order of magnitude, while approaching the true value at higher ionic strength. Concomitant data on the dependence of scattering intensity on scattering wave vector show that the dilute polyelectrolyte solutions are strongly correlated. A recent theory by Muthukumar for light scattering of dilute polyelectrolyte solutions, developed for correlated multicomponent systems, accounts for interchain electrostatic correlations and regularization of polymer charge by counterion binding isotherm. The experimental results on the relation between the apparent and true molar masses as a function of salt concentration are compared with the predictions of Muthukumar’s counterion adsorption theory. Taking binding equilibrium constant as a single fitting parameter, this theory is demonstrated to describe the experimental data as long as the chains do not interact significantly