Water-Orchestrated Hydrogen-Bond Networks Modulate the Coalescence Pathway of Carboxyl-Functionalized Gold Nanoparticles

The coalescence of carboxyl-functionalized gold nanoparticles (NPs) in aqueous solution is widely assumed to be governed by ligand interdigitation, which creates a steric barrier that must be overcome for coalescence to occur. However, this purely ligand-centric view overlooks the potential role of solvent molecules that coexist with ligands at the nanoparticle interface. Here, using molecular dynamics simulations, we reveal a possible water-mediated bottleneck that can emerge during the coalescence of not fully coated, carboxyl-functionalized Au NPs in water. In this pathway, interstitial water molecules form persistent hydrogen-bonded networks with the carboxyl groups of surface ligands, stabilizing a metastable paired state that persists for tens of nanoseconds prior to coalescence. Beyond simply occupying the interparticle gap, this hydrogen-bonded network locally hinders direct gap-center closure and biases coalescence toward rearrangement-mediated pathways that avoid direct closure through the water-occupied gap center. Conditional/local potential of mean force (PMF) calculations performed within a metastable pairwise geometry show that the trapped interstitial water is strongly stabilized and that direct migration along the selected lateral coordinate is locally unfavorable. This local water-mediated stabilization provides a molecular picture for how solvent-ligand interactions can contribute to heterogeneous precoalescence states and pathway variability in ligand-modified NPs. Furthermore, we demonstrate that the availability of strongly bound water on NP surfaces is modulated by ligand density and alkyl chain length, thereby defining the interfacial conditions under which this water-mediated bottleneck emerges and suggesting possible design guidelines for controlling NP coalescence through surface engineering. These findings provide an atomistic route by which solvent-ligand interactions can contribute to NP coalescence pathway selection and suggest possible strategies for tuning nanocrystal growth through interfacial engineering.