Electrostatically‐Driven Collapse of Polyelectrolytes: The Role of the Solvent's Dielectric Constant

We experimentally confirm a longstanding theoretical prediction of counterion-induced polyelectrolyte collapse in low dielectric media. The scattering behavior of polystyrene sulfonate in different solvents with dielectric permittivities in the range of ε ≃ 12 − 180 is investigated. For high and intermediate ε media, typical polyelectrolyte behavior is observed: the correlation length (ξ) scales with concentration (c) as ξ ∼ c−1∕2, as predicted by various theories. When the dielectric constant of the solvent decreases below ≃ 22, a scaling of ξ ∼ c−1∕3, characteristic of partially collapsed polyelectrolytes, is observed. For these solvents, the correlation peak disappears at high concentrations. Interestingly, polyelectrolyte collapse is observed under both solvophilic and solvophobic conditions, supporting the existence of attractive electrostatic interactions. These results are in qualitative agreement with theoretical predictions which expect chain collapse in low dielectric media due to the influence of condensed counterions, either via dipolar attraction and/or charge-correlation-induced attractions.