Influence of counterion type on the scattering of a semiflexible polyelectrolyte

Understanding the influence of counterion and backbone solvation on the conformational and thermodynamic properties of polyelectrolytes in solution is one of the main open challenges in polyelectrolyte science. To address this problem, we study the scattering from semidilute solutions of a semiflexible polyelectrolyte, carboxymethyl cellulose (CMC) with alkaline and tetra-alkyl-ammonium (TAA) counterions in aqueous media using small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS), which allow us to probe concentration fluctuations of the polymer backbone and counterions. In SAXS, the calculated contrast arises primarily from the polymer backbone for both alkaline and TAA salts of CMC. In SANS, however, the contrast is dominated by the counterions for the TAA salts and the polymer backbone for the alkaline salts. Solutions are found to display a correlation peak in their scattering function, which at low concentrations is independent of counterion type. At moderate salt concentrations (c ≳ 0.1 M), the peak positions obtained from SANS and SAXS for the CMC salts with the TAA counterions differ. This divergence suggests a decoupling in the lengthscale over which the couterions and the polymer fluctuate. Upturns in the scattering intensity in the low-q region signal the presence of long-ranged compositional inhomogeneities in the solutions. The strength of these decreases with increasing counterion-solvent interaction strength, as measured by the viscosity B coefficient, and are strongest for the corresponding sodium salt of CMC.