We report a small angle neutron scattering (SANS) study of semi-dilute aqueous solutions of sodium carboxymethyl cellulose (NaCMC), in the presence of mono-(Na +) and divalent salts (Mg 2+ , Ca 2+ , Zn 2+ , Ba 2+). A degree of substitution of 1.3 is selected to ensure that, in salt-free solution, the polymer is molecularly dissolved with conformation well described by polyelectrolyte scaling theory in semi-dilute regime. We find that Na + and Mg 2+ salt addition yield H-type phase behaviour, while Ca 2+ , Zn 2+ , and Ba 2+ instead yield L-type behaviour, in decreasing order of the salt concentration associated with demixing. Charge screening by addition of Na + induces the progressive disappearance of the characteristic polyelectrolyte correlation peak and eventually yields scattering profiles with a q −1 dependence over nearly three decades in wavenumber q. By fitting a descriptive model to data with excess Na + , we obtain a correlation length ξ'= 1030c −0.72 p [Å] with polymer concentration c p [g L −1 ]. Addition of Mg 2+ , which does not interact specifically with carboxylate groups, yields an analogous screening behaviour to that of Na + , albeit at lower salt concentrations, in line with its higher ionic strength. At low salt concentration, addition of specifically interacting Ca 2+ , Zn 2+ , and Ba 2+ yield a comparatively greater screening of the polyelectrolyte screening and, at concentrations above the phase boundary, results in excess scattering at low q, compatible with the formation of clusters of 10s nm in size. This behaviour is interpreted as due to the reduction in charge density along the chain, promoting association, clustering and eventual phase separation. Overall, drawing analogies with NaCMC at lower degree of substitution, where hydrophobic association takes place, our findings provide a framework to describe the solution conformation and phase behaviour of NaCMC in salt-free and salt solutions.
