Contact Forces between TiO2 Nanoparticles Governed by an Interplay of Adsorbed Water Layers and Roughness.

Interparticle forces govern the mechanical behavior of granular matter and direct the hierarchical assembling of nanoparticles into supramolecular structures. Understanding how these forces change under different ambient conditions would directly benefit industrial-scale nanoparticle processing units such as filtering and fluidization. Here we rationalize and quantify the contributions of dispersion, capillary, and solvation forces between hydrophilic TiO2 nanoparticles with sub-10 nm diameter and show that the humidity dependence of the interparticle forces is governed by a delicate interplay between the structure of adsorbed water layers and the surface roughness. All-atom molecular dynamics modeling supported by force-spectroscopy experiments reveals an unexpected decrease in the contact forces at increasing humidity for nearly spherical particles, while the forces between rough particles are insensitive to strong humidity changes. Our results also frame the limits of applicability of discrete solvation and continuum capillary theories in a regime where interparticle forces are dominated by the molecular nature of surface adsorbates.

Adhesion mechanisms of the contact interface of TiO2 nanoparticles in films and aggregates.

Fundamental knowledge about the mechanisms of adhesion between oxide particles with diameters of few nanometers is impeded by the difficulties associated with direct measurements of contact forces at such a small size scale. Here we develop a strategy based on AFM force spectroscopy combined with all-atom molecular dynamics simulations to quantify and explain the nature of the contact forces between 10 nm small TiO(2) nanoparticles. The method is based on the statistical analysis of the force peaks measured in repeated approaching/retracting loops of an AFM cantilever into a film of nanoparticle agglomerates and relies on the in-situ imaging of the film stretching behavior in an AFM/TEM setup. Sliding and rolling events first lead to local rearrangements in the film structure when subjected to tensile load, prior to its final rupture caused by the reversible detaching of individual nanoparticles. The associated contact force of about 2.5 nN is in quantitative agreement with the results of molecular dynamics simulations of the particle-particle detachment. We reveal that the contact forces are dominated by the structure of water layers adsorbed on the particles' surfaces at ambient conditions. This leads to nonmonotonous force-displacement curves that can be explained only in part by classical capillary effects and highlights the importance of considering explicitly the molecular nature of the adsorbates.

Electrostatic assembly of zwitterionic and amphiphilic supraparticles.

We demonstrate the electrostatic assembly of oppositely charged silica particles into an ensemble of well-defined core-satellite supraparticles, which are a type of patchy particle. To achieve controlled heteroaggregation, we used oppositely charged silica particles with different sizes ranging from 5nm to 150nm at several concentrations. The assembly works best with larger particles, resulting in a fairly low polydispersity and a low amount of bridging between the individual clusters. Using smaller particles produces high polydispersity, large clusters and uncontrolled aggregation and bridging. Furthermore, even with controlled aggregation into well-defined clusters in the case of bigger particles, we observe an uneven covering of the central particles with around 1-6 satellite particles adsorbed to the central particle. This behavior is not predicted by simple pairwise DLVO potentials which would anticipate an even spacing of the satellite particles on the core. We explain these observations by taking into account the interactions of the adsorbing particles within the ionic cloud of the central particle. We hypothesize that when the adsorbing satellite particles are small compared to the diameter of the ion cloud of the core particle, they aggregate within the ion cloud and therefore do not create a well-defined monolayer on the surface of the core particle, instead forming small agglomerates during adsorption. Finally, both the assembled zwitterionic clusters and clusters that were partially hydrophobized were tested for their capabilities as Pickering emulsifiers. The zwitterionic clusters showed a strongly increased surface activity compared to the individual particles, while the hydrophobized particles changed the emulsion type from w/o to o/w. Interfacial dilatational rheological tests supported the observations from the emulsion tests. With this, we demonstrate that a relatively unordered ensemble of supraparticles is able to show well-defined functionality at a higher hierarchical level as Pickering emulsifiers.