One-dimensional assemblies of charged nanoparticles in water: A simulation study

While the template-free one-dimensional assembly of nanoparticles, e.g., in chains, has been widely observed experimentally, the formation mechanism is still not well known. Therefore, the homogeneous nucleation in a model system of charged nanoparticles in water is investigated using Brownian dynamics simulations. The interaction between the particles is described by a sum of steric repulsion, screened Coulomb potential, and van der Waals attraction. A systematic study is carried out by varying the effective charge and the counterion concentration. The accuracy of the Brownian dynamics results is verified by the comparison with Monte Carlo simulations. At low particle charge and low counterion concentrations, a thermodynamically stable phase of clusters with few particles is observed. An increase in the ion concentration at low particle charge leads to anisotropic assemblies of the small clusters. In contrast at high particle charge, large spherical nuclei are observed, which assemble to form larger aggregates. These simulation results are interpreted with the help of recent theoretical work using similar interaction potentials. Only when the rearrangements of the initial clusters are blocked using constraint dynamics are one-dimensional assemblies of particles observed in simulations, in good agreement with the experiments. The experimental conditions of linear nanoparticle assembly in water are discussed. The comparison with the simulation results leads to proposing a mechanism for one-dimensional nanoparticle assemblies: The nanoparticles form chains by a diffusion-limited aggregation at low particle charge and the rearrangement of the chains in compact structures is hindered due to attractive spots at the particle surface created by the desorption of coating molecules.