Tuning the collapse transition of weakly charged polymers by ion-specific screening and adsorption

Abstract

The experimentally observed swelling and collapse response of weakly charged polymers to the addition of specific salts displays quite convoluted behavior that is not easy to categorize. Here we use a minimalistic implicit solvent / explicit salt simulation model with a focus on ion-specific interactions between ions and a single weakly charged polyelectrolyte to qualitatively explain the observed effects. In particular, we demonstrate ion-specific screening and bridging effects cause collapse at low salt concentrations whereas the same strong ion-specific direct interactions drive re-entrant swelling at high concentrations. Consistently with experiments, a distinct salt concentration at which the salting-out power of anions inverts from the reverse to direct Hofmeister series is observed. At this, so called isospheric point, the ion-specific effects vanish. Furthermore, with additional simplifying assumptions, an ion-specific mean-field model is developed for the collapse transition which quantitatively agrees with the simulations. Our work demonstrates the sensitivity of the structural behavior of charged polymers to the addition of specific salt and shall be useful for further guidance of experiments.

Theoretical and Computational Insight into Solvent and Specific Ion Effects for Polyelectrolytes: The Importance of Local Molecular Interactions

“…In addition, certain weakly charged polymers also show transitions from swollen to collapsed conformations and vice versa. At low salt concentrations, ion-screening and bridging effects prefer collapsed conformations whereas ion-specific direct interactions induce swollen conformations at high salt concentrations [99]. Noteworthy, specifically multivalent salt solutions are known to favor collapsed polyelectrolyte conformations.…”

Antibacterial and Degradable Thioimidazolium Poly(ionic liquid)

“…When NaI or NaHClO 4 was added to a D 2 O solution of the polymer, immediate precipitation was observed. This can be explained using the Hofmeister series, which dictates that proteins and neutral polymers should have enhanced solubility in the presence of some cations when compared to others in the order of SO 4 2– > HPO 4 2– > HCO 3 – > Cl – > Br – > I – > ClO 4 – . − For charged polymers, this order is switched and salting out is observed for I – and ClO 4 – . It is also interesting in the case of PBS and H 2 SO 4 that no decomposition is observed over the 21 day time span.…”

Competitive sorption of mono- versus di-valent ions by highly charged globular macromolecules

“…The inclusion of ∆µ int, i has been considered in previous work, for example, as a term reflecting the steric ion-ion packing effects in a Donnan model for ion binding by polyelectrolytes or charged hydrogels. 99,110,111 Eq. ( 6) with the help of Eq.…”

“…We continue with the meanfield Poisson-Boltzmann (PB) model, widely used in colloidal science and electrochemistry, 3,[100][101][102][103] and with the limitations well known and discussed, in particular the neglect of electrostatic and steric correlations, 92,[104][105][106][107] or ion-specific sorption effects. [108][109][110][111][112][113][114][115][116][117] The PB model has also been implemented to address the problem of competitive counterion binding in a mixed salt for the cases of linear polyelectrolytes such as DNA [118][119][120][121][122] and planar geometries. 123 We also devise a twostate approximation model for an ion condensation around a charged globule.…”