Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions

TL;DR

This study uses small angle neutron scattering to investigate the screening lengths and osmotic properties of flexible polyelectrolytes in salt solutions, revealing deviations from existing models and providing insights into electrostatic interactions.

Contribution

It provides experimental data on polyelectrolyte behavior in salt solutions, challenging the predictions of the scaling model and RPA, and proposes a modified electrostatic interaction model.

Findings

Polymer contribution to osmotic pressure is much smaller than predicted.

RPA fails at low salt concentrations ($c_S < 2$ M).

Electrostatic interactions follow a $1/c_S^{1/2}$ dependence, not $1/c_S$.

Abstract

We report results of small angle neutron scattering measurements made on sodium polystyrene sulfonate in aqueous salt solutions. The correlation length and osmotic compressibility are measured as a function of polymer (c) and added salt ($c_S$) concentrations, and the results are compared with scaling predictions and the random-phase approximation (RPA). In Dobrynin et al's scaling model the osmotic pressure consists of a counter-ion contribution and a polymer contribution. The polymer contribution is found to be two orders of magnitude smaller than expected from the scaling model, in agreement with earlier observations made on neutral polymers in good solvent condition. RPA allows the determination of single-chain dimensions in semidilute solutions at high polymer and added salt concentrations, but fails for $c_S < 2$ M. The \chi parameter can be modelled as the sum of an intrinsic contribution and an electrostatic term: $\chi \simeq \chi0+K/c_S^{1/2}$, where $\chi_0 > 0.5$ is consistent with the hydrophobic nature of the backbone of NaPSS. The dependence of $\chi_{elec} \simeq 1/c_S^{1/2}$ disagrees with the random-phase approximation ($\chi_{elec} \simeq 1/c_S$), but agrees with the light scattering results in dilute solution and Dobrynin et al's scaling treatment of electrostatic excluded volume.