Polyampholyte model of ion clusters: double-layer interactions in the presence of dissociated simple salt

TL;DR

This work investigates how ion clusters modeled as polyampholytes modify double-layer forces between equally charged surfaces in salt solutions. Using polymer-density functional theory with an implicit solvent, it compares alternating-charge and block-charge architectures, exploring both neutral and monovalent charged polymers across varying cluster sizes and salt contents. The key finding is that neutral clusters can amplify repulsion via dielectric response, while block-charge architectures produce strong, long-range forces that scale with cluster length and polarisation, even at high salt. The results imply new strategies for tuning colloidal stability with synthetic polyampholytes and highlight the importance of cluster internal structure beyond simple ion counting.

Abstract

We explore interactions between equally charged surfaces, in the presence of simple salt and either neutral or monovalently charged polyampholytes. We consider the possibility of using these charged polymers as crude models of ion clusters. The latter have been hypothesised to form in concentrated aqueous salt solutions, and are possibly related to anomalous underscreening. This phenomenon usually manifests itself by unexpectedly strong and long-ranged effective forces at very high ionic strengths. If ion clusters are formed, they are expected to carry at most a weak net charge. Keeping this in mind, we investigate how polyampholyte chains mediate interactions between charged surfaces. A significant amount of simple salt is also present, in most cases. We highlight that if the charges of the polyampholytes are unevenly distributed, there is a polarisation response that in turn can generate very strong and long-ranged surface forces, even at rather high concentrations of simple salt. Aside from their possible relevance to ion clusters and underscreening phenomena, these results also suggest the possibility of tailoring synthetic polyampholytes, in order to regulate colloidal stability.

Figures (6)

• Figure 1: An illustration of a (monovalent) 11:11 polyampholyte plus 1:1 simple salt. a The alternating charge model. b The block charge model.
• Figure 2: a The dependence of $\varepsilon/\varepsilon_w$ with zwitterion concentration, for two different choices of $b$. Calculated data are indicated by symbols. The lines are linear regression fits. b Force per radius values between equally charged surfaces, in the presence of 1900 mM zwitterions and 100 mM simple 1:1 salt (no excluded volume). c The corresponding logarithm of the net pressure.
• Figure 3: Interactions between equally charged surfaces, in the presence of neutral $r$-mer polyampholytes, and 100 mM simple 1:1 salt (including excluded volume). For a given polymer length $r$, the polyampholyte concentration is chosen in a way that is commensurate with clustering of a 2 M simple salt solution. The case of a pure 100 mM simple salt solution is shown as reference. The insert presents the logarithm of the $F/R$ data. a The chains are composed of monomers with an alternating charge structure. b The chains are composed of monomers with a block charge structure.
• Figure 4: a Surface forces, in the presence of 100 mM pure 1:1 salt solutions, as well as in mixtures of 100 mM simple 1:1 plus added monovalent 25:25 (40 mM) or 1001:1001 (1 mM) salt. The concentrations of the polyampholyte salt are commensurate with an overall monomer concentration of 2000 mM.The alternating charge model has been adopted for the polyampholyte chains. b Same as a, but for 40 mM monovalent 25:25 salt using the block charge structure. c Density profiles, at $h=400$ Å, in solutions containing 100 mM simple 1:1 salt and 40 mM 25:25. Full lines denote the cations, while the dashed lines denote the anions.
• Figure 5: Surface forces, in the presence of pure 1:1 salt solutions, and in mixtures of simple 1:1 and monovalent 25:25 salt. In the latter cases, the block charge model has been adopted. With one exception, the total concentration of positive, as well as negative, charged spheres in the bulk is 2 M. The case of a pure 100 mM simple salt solution is shown as reference.