Solution Structure and Counterion Condensation of Carboxymethyl Cellulose in Organic Solvents

We report scattering (SANS and SAXS) and electrical conductivity data for aqueous and organic solutions of carboxymethyl cellulose with tetrabutylammonium counterions. For SANS in deuterated solvents, the contrast is heavily dominated by the hydrogen-rich counterions, while for SAXS the polymer backbone has a more significant contribution to the scattering signal. The correlation length calculated from the SAXS measurements follows a scaling law of ξ ∝ c–1/2, while that measured by SANS displays a weaker exponent at higher concentrations for solvents of high dielectric constants. These results indicate a decoupling in the characteristic length scales of fluctuations of the polymer backbone and counterions at high polyelectrolyte concentrations. The stretching parameter calculated from the correlation length indicates a highly stretched local conformation for TBACMC in water and several high dielectric constant solvents, consistent with the semiflexible nature of the cellulose backbone. In solvents of lower dielectric permittivity, the chains display a higher degree of local coiling. Combining electrical conductivity and scattering data, we find that the fraction of monomers bearing a dissociated charge is nearly independent of concentration and approximately proportional to the reciprocal of the Bjerrum length of the solvent, as expected by the Oosawa-Manning model.