Dispersion state phase diagram of citrate-coated metallic nanoparticles in saline solutions

Abstract

The fundamental interactions underlying citrate-mediated chemical stability of metal nanoparticles, and their surface characteristics dictating particle dispersion/aggregation in aqueous solutions, are largely unclear. Here, we developed a theoretical model to estimate the stoichiometry of small, charged ligands (like citrate) chemisorbed onto spherical metallic nanoparticles and coupled it with atomistic molecular dynamics simulations to define the uncovered solvent-accessible surface area of the nanoparticle. Then, we integrated coarse-grained molecular dynamics simulations and two-body free energy calculations to define dispersion state phase diagrams for charged metal nanoparticles in a range of medium’s ionic strength, a known trigger for aggregation. Ultraviolet-visible spectroscopy experiments of citrate-capped nanocolloids validated our predictions and extended our results to nanoparticles up to 35 nm. Altogether, our results disclose a complex interplay between the particle size, its surface charge density, and the ionic strength of the medium, which ultimately clarifies how these variables impact colloidal stability.

Study of the Stability of Citrate Capped AgNPs in Several Environmental Water Matrices by Asymmetrical Flow Field Flow Fractionation

“…Consequently, the stability of the system depends on the resultant force of both, whose magnitude determines the kinetic energy that two NPs must have when colliding to overcome the energy barrier that prevents their aggregation. However, DLVO lacks an atomistic description of the electrical double layer at the surface of NPs, which excludes the examination of the molecular properties of the solvent electrolytes, and eventually aggregating onto the NP [ 37 ]. Franco-Ulloa et al [ 37 ] developed a theoretical framework that determines the surface coverage of charged ligands onto spherical NPs and its application permits to describe at the molecular level the dispersion state of citrate-capped gold nanocolloids.…”

“…The aggregation is more severe for O1 and O2 batches than that obtained for M1 and M2 batches in BW1 and TW waters, although their ʒ-potentials were somewhat greater in absolute values for O than for M batches. The electrostatic interactions between a charged NP and an electrolytic solvent fall within one of three regimes as proposed in [ 37 ] for citrate capped AuNPs, according to the net charge of the sphere: (i) depolarized (σ = surface charge density < ~0.6 e nm −2 ), where the thermal motion of the sodium counterions overcomes the electrostatic attraction toward the NP; (ii) mildly polarized (~0.6 < σ < ~4.1 e nm −2 ), in which the Coulomb forces attract enough ions into the Stern layer to screen out the charge load of the NP, and (iii) hyperpolarized (σ > ~4.1 e nm −2 ), the situation at which the counterions’ steric and electrostatic hindrance, as well as their loss of translational degrees of freedom, limit their binding onto the NP. The chemisorption of citrate onto the assembled metallic surfaces critically depends on variables such as the particle size, the surface charge density, and the ionic strength of the medium in which the NPs are dispersed [ 37 ].…”

“…All of this seems to show a trend of size transformation of AgNPs with the ionic strength of the medium. NP aggregation can be induced by increasing the salt concentration of the medium, in [ 37 ] it is found that alterations in the aggregation state of the NPs arise mostly from the screening of the NP charge by the sodium counterions in solution. These ions can rest at the interface of the metallic bodies and form salt bridges that stabilize NP–NP dimer.…”

“…However, DLVO lacks an atomistic description of the electrical double layer at the surface of NPs, which excludes the examination of the molecular properties of the solvent electrolytes, and eventually aggregating onto the NP [37]. Franco-Ulloa et al [37] developed a theoretical framework that determines the surface coverage of charged ligands onto spherical NPs and its application permits to describe at the molecular level the dispersion state of citrate-capped gold nanocolloids. The fundamental interactions underlying citrate-mediated chemical stability of metal nanoparticles, and their surface characteristics dictating particle dispersion/aggregation in aqueous solutions, are largely unclear [37].…”

“…Franco-Ulloa et al [37] developed a theoretical framework that determines the surface coverage of charged ligands onto spherical NPs and its application permits to describe at the molecular level the dispersion state of citrate-capped gold nanocolloids. The fundamental interactions underlying citrate-mediated chemical stability of metal nanoparticles, and their surface characteristics dictating particle dispersion/aggregation in aqueous solutions, are largely unclear [37]. Moreover, dilution-induced changes, mainly on the surface interactions that may affect the performance of AgNPs.…”