Towards a mesoscale rheology model for aqueous particulate suspensions

Particulate suspensions are ubiquitous and diverse; pharmaceutical formulations, biological fluids, magma and foodstuffs are just few of numerous examples. In many cases, the flow behaviour (rheology) of the suspension is critical to its function. A key rheological property is viscosity; a measure of a substance’s resistance to flow. This work aims to understand molecular-level mechanisms responsible for determining flow behaviour in moderately dense suspensions; 35% particles by volume (i.e., volume fraction 0.35). The industrial application of interest to this thesis is catalysis; namely, the ‘washcoat’, a key component in the performance of catalytic converters. A typical washcoat formulation is an aqueous suspension, comprising a high surface-area support powder, an active catalyst material, together with organic additives and certain salts used to optimise properties of the washcoat; including its flow behaviour. Of these components, this work investigates ‘salt-specific effects’; i.e. the influence of differing salt-types. Investigation is conducted at molecular and macroscopic resolution via simulations and experiments, respectively. The research approach probes the constituents of a suspension: the aqueous phase, the particle-aqueous phase interface, and particle interactions. Molecular dynamics simulations are employed as the foundation of this analysis, with experiments - rheology, nuclear magnetic resonance and dynamic light scattering - utilised alongside. A final set of rheology experiments is conducted on particulate suspensions of 35% volume fraction, in pure water and the aqueous salt solutions of interest. At all stages of analysis, results suggest that macroscopic behaviours are a cumulative manifestation of phenomena at molecular resolution. However, such phenomena are varied; the challenge lies in identifying which mechanisms are relevant to the behaviour of interest, how they work together, and how they manifest cumulatively. Towards a mesoscale rheology model for aqueous particulate suspensions, results are discussed in terms of input for such a model, which would predict rheology as a function of particle loading, ionic strength and possibly other factors, in future work

Hydration Structures on γ-Alumina Surfaces With and Without Electrolytes Probed by Atomistic Molecular Dynamics Simulations

A wide range of systems, both engineered and natural, feature aqueous electrolyte solutions at interfaces. In this study, the structure and dynamics of water at the two prevalent crystallographic terminations of gamma-alumina, [110] and [100], and the influence of salts─sodium chloride, ammonium acetate, barium acetate, and barium nitrate on such properties─were investigated using equilibrium molecular dynamics simulations. The resulting interfacial phenomena were quantified from simulation trajectories via atomic density profiles, angle probability distributions, residence times, 2-D density distributions within the hydration layers, and hydrogen bond density profiles. Analysis and interpretation of the results are supported by simulation snapshots. Taken together, our results show stronger interaction and closer association of water with the [110] surface, compared to [100], while ion-induced disruption of interfacial water structure was more prevalent at the [100] surface. For the latter, a stronger association of cations is observed, namely sodium and ammonium, and ion adsorption appears determined by their size. The differences in surface-water interactions between the two terminations are linked to their respective surface features and distributions of surface groups, with atomistic-scale roughness of the [110] surface promoting closer association of interfacial water. The results highlight the fundamental role of surface characteristics in determining surface-water interactions, and the resulting effects on ion-surface and ion-water interactions. Since the two terminations of gamma-alumina considered represent interfaces of significance to numerous industrial applications, the results provide insights relevant for catalyst preparation and adsorption-based water treatment, among other applications

Interactions between γ-alumina surfaces in water and aqueous salt solutions

Particle agglomeration is relevant to numerous industrial applications and consumer products. The present work explores interactions between and agglomeration of gamma (γ)-alumina nanoparticles in pure water and dilute aqueous salt solutions. To characterize surface- and salt-specific effects, potential of mean force (PMF) profiles between γ-alumina surfaces ([110] and [100] facets) are extracted using classical molecular dynamics (MD) simulations. Supporting experiments are conducted using dynamic light scattering (DLS) to investigate agglomeration at the macroscale. The ion pairs considered are sodium chloride, ammonium acetate, barium nitrate, and barium acetate; sampling a broad range of the Hofmeister series. As particle surfaces approach contact, free-energy fluctuations of the PMF profiles reflect structural adjustments of the intervening aqueous phase. We extract values for the cohesive energy from the MD results, and parse the resultant effective pair interactions into van der Waals and electrostatic contributions. Molecular scale findings from simulations correlate with hydrodynamic radii of γ-alumina nanoparticles, obtained from DLS experiments. The results highlight the applicability of molecular simulations to identify the origins of macroscale observables