From Fresh to Salty: How Ions Modulate Solvent-Mediated Interactions between Grafted Silica Nanoparticles in Water

Abstract

Nanoparticles (NPs) are fundamental building blocks for engineering functional soft materials, where precise control over the solvent-mediated inter-particle effective interaction (Ueff) is essential for tailoring bulk structure and properties. These solvent-mediated interactions are strongly influenced by NP's surface chemistry, solvent properties, and thermodynamic conditions such as temperature (T) and pressure (P). However, despite considerable progress, a general predictive framework for tuning Ueff and guiding self-assembly remains lacking. In this work, using all-atom molecular dynamics simulations, we investigated the alteration of Ueff between silica nanoparticles (Si-NPs) functionalized with polyethylene (PE) and polyethylene glycol (PEG) by salt (sodium chloride) across a range of thermodynamic conditions. At ambient thermodynamic conditions, bare (not functionalized) Si-NPs exhibit minimal variation in Ueff even at high salt concentrations. In contrast, PE-grafted Si-NPs display strong salt-induced attractions, while PEG-grafted Si-NPs show an intermediate, more gradual response. To asses the transferability of these salt-induced effects on effective interactions, we further examined the effects of salt on Ueff under different (T,P) conditions. Our results indicate that the salt-induced modulation of Ueff between both bare and grafted Si-NPs is largely invariant across the explored (T,P) conditions. Molecular-level analysis reveals that salt promotes solvent depletion within the interparticle cavity for both hydrophobic PE and hydrophilic PEG grafts, with the strongest effect observed in the PE case. In general, this study highlights the coupled roles of surface chemistry, ion-polymer interactions, and solvent structuring in the regulation of Ueff, and provides important insights into the predictable control of interparticle interactions for soft material engineering.