The phase behavior of colloids that interact via competing interactions - short-range attraction and long-range repulsion - is studied by computer simulation. In particular, for a fixed strength and range of repulsion, the effect of the strength of an attractive interaction (ε) on the phase behavior is investigated at various colloid densities (ρ). A thermodynamically stable equilibrium colloidal cluster phase, consisting of compact crystalline clusters, is found below the fluid-solid coexistence line in the ε-ρ parameter space. The mean cluster size is found to linearly increase with the colloid density. At large ε and low densities, and at small ε and high densities, a non-equilibrium cluster phase, consisting of elongated Bernal spiral-like clusters, is observed. Although gelation can be induced either by increasing ε at constant density or vice versa, the gelation mechanism is different in either route. While in the ρ route gelation occurs via a glass transition of compact clusters, gelation in the ε route is characterized by percolation of elongated clusters. This study both provides the location of equilibrium and non-equilibrium cluster phases with respect to the fluid-solid coexistence, and reveals the dependencies of the gelation mechanism on the preparation route.

Several kinds of small molecules and nanoparticles (NP) are known for effective inhibition of amyloid fibrillation, which is known as a precursor for many neurodegenerative diseases. We address the role of the surface charge of NP in this process from a systematic study using charged NP and surfactants. The fibrillation kinetics is investigated using time-resolved Thioflavin T fluorescence and transmission electron microscopy. It is found that if the protein residues corresponding to the β-sheet (key secondary structure for the formation of fibrils) are charged, addition of oppositely charged NP will inhibit the fibrillation process, irrespective of the material composition of the NP. Molecular dynamics simulations show that electrostatic interaction between the β-sheet forming residues and the NP is responsible for this effect. The finding is further validated by using oppositely charged surfactants: like-charged surfactants and NP do not affect the fibrillation kinetics. Further, even the preformed fibrils are dissociated into protein aggregates upon adding oppositely charged NP/surfactants. These results suggest that NP can be used as therapeutic agents even after the fibrils are formed and that they can be dissolved into soluble aggregates. The inhibitory effect is essentially decided by the charge of the inhibitor, irrespective of the size, shape, and material composition. The study provides new insight for the rational design of nanoparticle-based therapeutics, with appropriate surface charge, to inhibit the onset of amyloid β fibrillation and to dissociate the preformed fibrils.

In this study we explore the fundamental aspects of Pickering emulsions stabilized by oppositely charged particles. Using oppositely charged latex particles as a model system, Pickering emulsions with good long-term stability can be obtained without the need for any electrolyte. The effects of parameters like oil to water ratio, mixed particle composition, and pH on emulsion type and stability are explored and linked to the behavior of the aqueous particle dispersion prior to emulsification. The particle composition is found to affect the formation of emulsions, viz., stable emulsions were obtained close to a particle number ratio of 1:1, and no emulsion was formed with either positively or negatively charged particles alone. The emulsions in particle mixtures exhibited phase inversion from oil-in-water to water-in-oil beyond an oil volume fraction of 0.8. Morphological features of emulsion droplets in terms of particle arrangement on the droplets are discussed.

The adsorption of particles to the fluid-fluid interface is a key factor for the stabilization of fluid-fluid interfaces such as those found in emulsions, foams and bijels. However, for the formation of stable particle-laden interfaces, the particles must migrate to the interface from the bulk. Recent studies show that the adsorption of particles to the interface formed during emulsification is influenced by the surface charge of the particles. To further investigate this phenomenon, we study the effect of the surface charge of the particle on the adsorption kinetics of particles to the oil-water interface. By suspending a drop of aqueous dispersion of charge stabilized nanoparticles in decane, the adsorption dynamics of particles to the decane-water interface is studied using the dynamic surface tension measurements. When the particles are highly charged (low salt), a negligible change in the interface tension is observed indicating that almost no particles are adsorbed. These results show that the charged particles experience an energy barrier when they approach the interface. But when the particle surface charge is screened by the addition of monovalent salt, a significant reduction in surface tension is observed indicating the migration and adsorption of particles to the decane-water interface. We estimate the effective diffusivity of particles to the interface by analyzing the initial decay in the measured surface tension by considering particle laden drops containing different amounts of salt using the modified Ward and Tordai theory. This effective diffusivity is used to calculate the energy barrier for the adsorption of particles to the interface. The energy barrier from the analysis of dynamic surface tension data agrees well with the concept of image charge repulsion which inhibits the adsorption of highly charged particles to the interface. By considering various types of relevant interactions, we derive an analytical expression that qualitatively captures the effect of the surface charge on the equilibrium surface coverage of particles at the drop surface.