Hydrophilicity and salt tolerance of water soluble (charged and uncharged) polymers
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Abstract
The dependence of the viscosity of dilute aqueous solutions of polyelectrolytes and uncharged macromolecules on their concentration and on the salinity of the solvent provides information concerning their interactions with water and low-molecular salts. The central parameters are salt tolerance and hydrophilicity. For polyelectrolytes the salt tolerance τ (logarithm of the external salt concentration at which the intrinsic viscosity changes most) is at least one order of magnitude less than for uncharged polymers. This finding is attributed to the fact that low molecular weight ions bind water molecules in their hydration shell that would be required for the solvation of the macro-ions. The hydrophilicity parameter λ100 (characterizing the equilibrium of water inside isolated polymer chains and the surrounding pure water) is for polyelectrolytes composed of contributions stemming from the main chain and the attached macro-ions. In the absence of extra salt λ100 is at least one order of magnitude larger than for the uncharged polymers due to the high solvation tendency of the macroions. External salt invariably reduces λ100 due to water's preferential interaction with free ions compared to macro-ions. Limiting values are reached once micro-ions fully establish their hydration shells. For uncharged polymers, their solubility is determined solely by the backbones, and the hydrophilicity is generally much lower than for polyelectrolytes. Specific polymer/salt interactions can increase or decrease λ100. This approach uses simple experiments and provides quantitative information about the interaction between inorganic salts and polymers, as well as their salting-in and salting-out properties.