Modelling of Multi-Scale Phenomena in Nanoparticle Suspensions

Self- or driven-assembly of nanoparticles (NPs) into mesoscopic ordered structures play a crucial role in a large variety of applications including energy, pharmaceutical, food, drug delivery, immunology and technological. On the one hand, trying to prevent and avoid the self-organization of nanoparticles has traditionally been the main issue to stabilizing nanosuspensions, foams, and emulsions. On the other hand, the aggregation of building-blocks into mesoscopic structures has allowed to explore new materials with desired functionalities and properties. In the latter context, a proper attention has been devoted to the dominant role of aggregation in altering the thermal properties of nanosuspensions. However, due to the challenges of controlling the inter-particle interactions and the process of aggregation, clear guidelines for a rational design of tailored suspensions is still missing. Accurately modelling heat and mass transport phenomena across many different length scales is essential to optimize the self-assembly and stability of colloidal suspensions. In this thesis, Molecular Dynamics (MD) simulations, Coarse-Grained (CG) techniques, Brownian Dynamics (BD) and theoretical modelling studies are combined to understand how the interfacial phenomena influence the mechanisms of building-block interactions and hence how to predict the shapes of assembled clusters and their related macroscopic properties. First, the behaviour of nanoconfined water and the adsorption of ionic surfactants at the solid-liquid nanoscale interface are investigated. Second, atomistic Potentials of Mean Force (PMFs) are evaluated between couples of NPs dispersed in aqueous solutions. A sensitivity analysis is carried out by altering the hydrophilicity of the nanoparticles, their surface charge and the salt concentration of the bulk solution. Moreover, the role of anionic (Sodium Dodecyl Sulphate -SDS-) and cationic (Dodecyl Trimethyl Ammonium -DTAB-) surfactants is included in the evaluation of the PMF. All the study cases are then compared with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and remarkable discrepancies have emerged, underling the limits of a continuum theory to model the colloidal interactions at the nanoscale. In particular, the results highlight that the assumption of a uniform and continuum media and the hypothesis of homogeneous particles present in the DLVO theory break down at the solid-liquid nanoscale interface and by considering patchy NPs after surfactant adsorption. Thus, MD simulations offer the best alternatives to capture all coupled phenomena included in NP interactions. Subsequently, the atomistic PMFs are implemented in a multi-scale model, where MD simulations and Brownian dynamics are integrated offering a detailed picture of the kinetic of NP aggregation. The qualitative agreement with the experimental observations validates the novel multi-scale platform, able to connect the nanoscale features to the size of aggregates and related macroscopic properties of colloidal suspensions. Finally, with a coarse-grained technique, a force field for heterogeneous NPs is also provided. Thus, in the present work, powerful tools and multi-scale modelling approaches are developed to describe some of the multi-scale phenomena occurring in NP suspensions. Clear guidelines to perform multi-scale simulations of the self-assembly processes are proposed, and the first step towards a rational design of NP suspensions is presented.