Large Counterions Boost the Solubility and Renormalized Charge of Suspended Nanoparticles

Abstract

Colloidal particles are ubiquitous in biology and in everyday products such as milk, cosmetics, lubricants, paints, or drugs. The stability and aggregation of colloidal suspensions are of paramount importance in nature and in diverse nanotechnological applications, including the fabrication of photonic materials and scaffolds for biological assemblies, gene therapy, diagnostics, targeted drug delivery, and molecular labeling. Electrolyte solutions have been extensively used to stabilize and direct the assembly of colloidal particles. In electrolytes, the effective electrostatic interactions among the suspended colloids can be changed over various length scales by tuning the ionic concentration. However, a major limitation is gelation or flocculation at high salt concentrations. This is explained by classical theories, which show that the electrostatic repulsion among charged colloids is significantly reduced at high electrolyte concentrations. As a result, these screened colloidal particles are expected to aggregate due to short-range attractive interactions or dispersion forces as the salt concentration increases. We discuss here a robust, tunable mechanism for colloidal stability by which large counterions prevent highly charged nanoparticles from aggregating in salt solutions with concentrations up to 1 M. Large counterions are shown to generate a thicker ionic cloud in the proximity of each charged colloid, which strengthens short-range repulsions among colloidal particles and also increases the corresponding renormalized colloidal charge perceived at larger separation distances. These effects thus provide a reliable stabilization mechanism in a broad range of biological and synthetic colloidal suspensions.

Nonadditivity of nanoparticle interactions

“…For instance, the size of a counterion strongly affects particle interactions, contrary to the DLVO predictions (48,50). As confirmed by experiments and simulations, a large counterion such as tetrabutylammonium hydroxide can provide a repulsive barrier, even at vanishing electrostatic potential (Fig.…”

“…2C). Interparticle gaps d < 2.5 nm were found to be especially problematic, regardless of a specific NP diameter (48,55,56).…”

The role of natural processes and surface energy of inhaled engineered nanoparticles on aggregation and corona formation

“…With ever-faster computers such simulations will become routine, and promise to revolutionize our ability to predict nanoparticle interactions. MD simulations have been shown to be capable of predicting nanoparticle interactions at least qualitatively; for example the unusual stability of charged nanoparticles in the presence of large counterions, which has long been observed experimentally, has recently been predicted by molecular simulations (Guerrero-García et al, 2013). There is now a large literature demonstrating that molecular dynamics simulations can accurately predict a wide variety of association phenomena, if the force field is chosen carefully (see for example, Tang et al 2014 ).…”

Opposite counter-ion effects on condensed bundles of highly charged supramolecular nanotubes in water

“…[17][18][19] Experiments on colloidal and biological science have revealed some unique phenomena such as counter-ions aggregation on the macro-ions which even result in overcharging of the macro-ions and the counter-ions induced like-charge attraction. 23,24 In particular, profound specific ion effects are expected when highly charged poly-electrolytes form concentrated aggregates in water. 23,24 In particular, profound specific ion effects are expected when highly charged poly-electrolytes form concentrated aggregates in water.…”