Strong attractions and repulsions mediated by monovalent salts

Abstract

Controlling interactions between proteins and nanoparticles in electrolyte solutions is crucial for advancing biological sciences and biotechnology. The assembly of charged nanoparticles (NPs) and proteins in aqueous solutions can be directed by modifying the salt concentration. High concentrations of monovalent salt can induce the solubilization or crystallization of NPs and proteins. By using a multiscale coarse-grained molecular dynamics approach, we show that, due to ionic correlations in the electrolyte, NPs pairs at high monovalent salt concentrations interact via remarkably strong long-range attractions or repulsions, which can be split into three regimes depending on the surface charge densities of the NPs. NPs with zero-to-low surface charge densities interact via a long-range attraction that is stronger and has a similar range to the depletion attraction induced by polymers with radius of gyrations comparable to the NP diameter. On the other hand, moderately charged NPs with smooth surfaces as well as DNA-functionalized NPs with no possibility of hybridization between them interact via a strong repulsion of range and strength larger than the repulsion predicted by models that neglect ionic correlations, including the Derjaguin-Landau-Vervey-Overbeek (DLVO) model. Interactions between strongly charged NPs (>2 e/nm), both types smooth and DNA-functionalized NPs, show an attractive potential well at intermediate-to-high salt concentrations, which demonstrates that electrolytes can induce aggregation of strongly charged NPs. Our work provides an improved understanding of the role of ionic correlations in NP assembly and design rules to utilize the salting-out process to crystallize NPs.

The Importance of Salt-Enhanced Electrostatic Repulsion in Colloidal Crystal Engineering with DNA

“…The impact of salt concentration on the interaction potential between a pair of PAEs was studied by calculating the potential energies as a function of interparticle distance (i.e., core-to-core distance) ranging from 30 to 40 nm. Previous studies on nanoparticles with noncomplementary DNA strands exhibit repulsion forces between nanoparticles that are longer range than the repulsion predicted by DLVO theory when the salt concentration increases above 0.3 M, 25 which is in agreement with experiments on repulsion between charged surfaces at high monovalent salt concentrations. 26,27 This electrostatic interaction depends on multiple factors, including grafting density, temperature, and ionic strength, and can even lead to salting out.…”

“…27 Furthermore, simulations have shown that the strength of interparticle interactions depends on the surface charge density of nanoparticles; at high salt concentration, weakly charged nanoparticles have a depletion-type attraction, whereas nanoparticles with moderate to high surface charge densities experience long-range repulsion attributed to ionic correlations. 25 In particular, for DNA-mediated colloidal assemblies, high salt concentration has been shown to induce phase transitions 28 and nanoparticle aggregation in the absence of hybridization interactions. 29 However, it is difficult with the current tools to gain a full understanding of the mechanism of how surface grafting densities and electrostatic energies cause salting out and affect crystal formation in systems with complementary linkers.…”

Single-crystal Winterbottom constructions of nanoparticle superlattices

“…Because DNA is negatively charged, increasing the amount of counterions in solution strongly modifies the interactions between individual PAEs and between PAEs and the substrate. These interactions have been shown to be more complex than simple charge screening [22][23][24] , meaning that fully understanding the complexity of this effect may require significant modeling of PAE assembly. However, prior work with the PAE design used here 20 has shown that at [NaCl] below 1.5 M, the combined result of these complex interactions resulted in increased surface coverage with increasing salt concentration.…”

Screening Lengths in Ionic Fluids

“…The existence of multiple coexisting species-dependent decay lengths implies that addressable wetting could be achieved. Tuning the asymptotic correlations may also be used to control colloidal dispersions, for instance to prevent aggregation [6] and to switch effective potentials by tuning the salt concentration [45]. It might be promising to construct complex interactions to achieve a rich crossover structure, for instance in complex plasmas [46], colloid-polymer mixtures [47], and colloidal fluids [48].…”