Charged Colloidal Systems Research Articles

In search of a precursor for crystal nucleation of hard and charged colloids.

The interplay between crystal nucleation and the structure of the metastable fluid has been a topic of significant debate over recent years. In particular, it has been suggested that even in simple model systems such as hard or charged colloids, crystal nucleation might be foreshadowed by significant fluctuations in local structure around the location where the nucleus first arises. We investigate this using computer simulations of spontaneous nucleation events in both hard and charged colloidal systems. To detect local structural variations, we use both standard and unsupervised machine learning methods capable of finding hidden structures in the metastable fluid phase. We track numerous nucleation events for the face-centered cubic and body-centered cubic crystals on a local level and demonstrate that all signs of crystallinity emerge simultaneously from the very start of the nucleation process. We thus conclude that we observe no precursor for the crystal nucleation of hard and charged colloids.

Exploiting kaolinite-alumina heteroaggregation in Pickering emulsion stabilisation and porous mullite fabrication

Clay particles have gained considerable attention as a potential option for stabilising Pickering emulsions in recent years, which are promising templates for the development of porous ceramics intended for high temperature applications. However, the development of porous ceramics using this approach requires control over a number of factors such as stability, microstructure, rheology, and processability of the Pickering emulsions. In this work, we discuss a versatile approach for formulating porous mullite ceramics from emulsions stabilised by oppositely charged kaolinite and alumina particles. In this process, we exploit the electrostatic heteroaggregation phenomena in the oppositely charged mixed colloidal particle system. The aggregates formed by electrostatic interaction readily adsorb to the interface created during emulsification and favor the formation of highly stable Pickering emulsions. The influence of parameters such as concentration of each particle in the mixture, pH, and total particle concentration on the microstructure of Pickering emulsions are studied. We further show that processable Pickering emulsions can be engineered by in-situ surface modification of the hetero-aggregates using a short-chain amphiphile. Porous mullite ceramic is obtained by drying and sintering the optimised emulsion formulation. The phase evolution, microstructure, and porosity of the resulting ceramic are characterised. The macroporous mullite developed through this approach can be used as thermal insulator due to low thermal conductivity and tailored microstructure.

Amorphous photonic structure in a charged colloidal system showing angle-independent uniform color

Structural colors derived from the scattering or reflection of light by submicron-sized ordered structures are interesting due to their non-fading and tunable colors. Herein we show a colored angle-independent amorphous phase of monodisperse colloids between the crystalline and random phases in a charged colloidal system. The amorphous photonic structures with low packing density display uniform color over large areas; this is easily altered by adjusting the particle volume fraction. The transmission spectra of amorphous phases display an intersection point at the Bragg wavelength, allowing design of the transmission color. We demonstrate that the aqueous suspended colloidal amorphous structures can be immobilized in hydrogel films, while preserving their spectral properties. The sensitivity of the gel to ethanol allows the color to be tuned by changing the ethanol concentration. This angle-independent uniform color with easy tunability has great potential for application in advanced color materials and sensors.

Revealing thermally-activated nucleation pathways of diffusionless solid-to-solid transition

Solid-to-solid transitions usually occur via athermal nucleation pathways on pre-existing defects due to immense strain energy. However, the extent to which athermal nucleation persists under low strain energy comparable to the interface energy, and whether thermally-activated nucleation is still possible are mostly unknown. To address these questions, the microscopic observation of the transformation dynamics is a prerequisite. Using a charged colloidal system that allows the triggering of an fcc-to-bcc transition while enabling in-situ single-particle-level observation, we experimentally find both athermal and thermally-activated pathways controlled by the softness of the parent crystal. In particular, we reveal three new transition pathways: ingrain homogeneous nucleation driven by spontaneous dislocation generation, heterogeneous nucleation assisted by premelting grain boundaries, and wall-assisted growth. Our findings reveal the physical principles behind the system-dependent pathway selection and shed light on the control of solid-to-solid transitions through the parent phase’s softness and defect landscape.

Fast crystal growth at ultra-low temperatures

It is believed that the slow liquid diffusion and geometric frustration brought by a rapid, deep quench inhibit fast crystallization and promote vitrification. Here we report fast crystal growth in charged colloidal systems under deep supercooling, where liquid diffusion is extremely low. By combining experiments and simulations, we show that this process occurs via wall-induced barrierless ordering consisting of two coupled steps: the step-like advancement of the rough interface that disintegrates frustration, followed by defect repairing inside the newly formed solid phase. The former is a diffusionless collective process, whereas the latter controls crystal quality. We further show that the intrinsic mechanical instability of a disordered glassy state subject to the crystal growth front allows for domino-like fast crystal growth even at ultra-low temperatures. These findings contribute to a deeper understanding of fast crystal growth and may be useful for applications related to vitrification prevention and crystal-quality control.

Colloidal Tornadoes in a Vial under Gravitational Sedimentation

ollective motion in living matter is highly intriguing but can also be observed in charged colloidal systems. Collective motion observed in colloidal systems requires extensive material synthesis, external stimuli, and advanced characterization methods that might be highly costly to be employed for teaching. Besides that, colloidal systems can inherently possess dynamic movements. Generally, colloidal sedimentation is pictured as a linear movement, but under certain conditions autonomously activated sedimentation can be attained. In this demonstration, a simple formation of colloidal tornadoes will be explored by gravitational force as the sole stimulus. This demonstration is ideal for the general public for colloid chemistry learning, and these findings can be coupled to colloidal stability lectures.

Optimized Large Hyperuniform Binary Colloidal Suspensions in Two Dimensions.

The creation of disordered hyperuniform materials with extraordinary optical properties (e.g., large complete photonic band gaps) requires a capacity to synthesize large samples that are effectively hyperuniform down to the nanoscale. Motivated by this challenge, we propose a feasible equilibrium fabrication protocol using binary paramagnetic colloidal particles confined in a 2D plane. The strong and long-ranged dipolar interaction induced by a tunable magnetic field is free from screening effects that attenuate long-ranged electrostatic interactions in charged colloidal systems. Specifically, we numerically find a family of optimal size ratios that makes the two-phase system effectively hyperuniform. We show that hyperuniformity is a general consequence of low isothermal compressibilities, which makes our protocol suitable to treat more general systems with other long-ranged interactions, dimensionalities, and/or polydispersity. Our methodology paves the way to synthesize large photonic hyperuniform materials that function in the visible to infrared range and hence may accelerate the discovery of novel photonic materials.

Revealing roles of competing local structural orderings in crystallization of polymorphic systems.

Most systems have more than two stable crystalline states in the phase diagram, which is known as polymorphism. Crystallization in such a system is often under strong influence of competing orderings linked to those crystals. However, how such competition affects crystal nucleation and ordering toward the final crystalline state is largely unknown. This is primarily because the competition takes place locally and thus is masked by large positional fluctuations. We develop a unique method to correctly identify local symmetries by removing their distortions due to positional fluctuations. This allows us to experimentally access the spatiotemporal fluctuations of local symmetries at a single-particle level in crystallization of a charged colloidal system near the body-centered cubic-face-centered cubic border. Thus, we successfully reveal the crucial roles of competing ordering in the initial selection of polymorphs and the final grain boundary motion toward the most stable state from a microscopic perspective.

Ion exchange selectivity in clay is controlled by nanoscale chemical–mechanical coupling

Ion exchange in nanoporous clay-rich media plays an integral role in water, nutrient, and contaminant storage and transport. In montmorillonite (MMT), a common clay mineral in soils, sediments, and muds, the swelling and collapse of clay particles through the addition or removal of discrete molecular layers of water alters cation exchange selectivities in a poorly understood way. Here, we show that ion exchange is coupled to the dynamic delamination and restacking of clay layers, which creates a feedback between the hydration state of the exchanging cation and the composition of the clay interlayer. Particles with different hydration states are distinct phases with unique binding selectivities. Surprisingly, equilibrium achieved through thermal fluctuations in cation concentration and hydration state leads to the exchange of both ions and individual MMT layers between particles, a process we image directly with high-resolution transmission electron microscopy at cryogenic conditions (cryo-TEM). We introduce an exchange model that accounts for the binding selectivities of different phases, which is likely applicable to many charged colloidal or macromolecular systems in which the structural conformation is correlated with the activities of water and counterions within spatially confined compartments.

Individually isolated single wall carbon nanotubes with controlled surface charge density

Individually isolated single wall carbon nanotubes (SWNTs) with controlled surface charge density in water have been developed. First, the polymerizable cationic surfactant, cetyltrimethylammonium 4-vinylbenzoate (CTVB), is self-assembled on the hydrophobic surface of SWNTs in the presence of different amount of polymerizable co-monomer, sodium 4-styrenesulfonate (NaSS). Second, the self-assembled CTVB/NaSS monolayer is permanently fixed by in situ polymerization, making individually isolated SWNT dispersion in water. The surface charge density of functionalized SWNTs is controlled by the amount of NaSS relative to that of CTVB. The zeta potential of the functionalized SWNTs is systematically varied from positive to negative (+12.3 ∼ −47.8 mV) as the concentration of NaSS is increased from 0 to 60 mol % relative to the CTVB concentration. The charge variation is understood in terms of an overcharging effect in the charged colloidal system. The small angle neutron scattering (SANS) measurements show that electrostatic interaction between the functionalized SWNTs is strongly dependent on the NaSS concentration, which is consistent with the zeta potential measurements. The SANS measurements also show that the thickness of polymerized CTVB/NaSS monolayer adsorbed on SWNTs is almost constant at 1.89 nm regardless of the NaSS concentration, and the microstructure and surface charge are well maintained at high temperature (60 °C) as well as even after harsh condition such as freeze-drying.

Interactions between charged particles with bathing multivalent counterions: experiments vs. dressed ion theory.

We compare the recent experimentally measured forces between charged colloidal particles, as well as their effective surface potentials (surface charge) in the presence of multivalent counterions in a bathing monovalent salt solution, with the predictions of the dressed ion theory of strongly charged colloidal systems. The benchmark for comparison is provided by the DLVO theory and the deviations from its predictions at small separations are taken as an indication of the additional non-DLVO attractions that can be fitted by an additional phenomenological exponential term. The parameters characterizing this non-DLVO exponential term as well as the dependencies of the effective potential on the counterion concentration and valency predicted by the dressed ion theory are well within the experimental values. This suggests that the deviations from the DLVO theory are probably caused by ion correlations as formalized within the dressed ion theory.

Size effects of solvent molecules on the phase behavior and effective interaction of colloidal systems with the bridging attraction

There has been much recent research interest towards understanding the phase behavior of colloidal systems interacting with a bridging attraction, where the small solvent particles and large solute colloidal particles can be reversibly associated with each other. These systems show interesting phase behavior compared to the more widely studied depletion attraction systems. Here, we use Baxter’s two-component sticky hard sphere model with a Percus–Yevick closure to solve the Ornstein–Zernike equation and study the size effect on colloidal systems with bridging attractions. The spinodal decomposition regions, percolation transition boundaries and binodal regions are systematically investigated as a function of the relative size of the small solvent and large solute particles as well as the attraction strength between the small and large particles. In the phase space determined by the concentrations of small and large particles, the spinodal and binodal regions form isolated islands. The locations and shapes of the spinodal and binodal regions sensitively depend on the relative size of the small and large particles and the attraction strength between them. The percolation region shrinks by decreasing the size ratio, while the binodal region slightly expands with the decrease of the size ratio. Our results are very important in understanding the phase behavior for a bridging attraction colloidal system, a model system that provides insight into oppositely charged colloidal systems, protein phase behavior, and colloidal gelation mechanisms.

Asymmetrical phase separation and gelation in binary mixtures of oppositely charged colloids.

Two types of colloidal particles, which are nearly the same in chemical composition but carry opposite surface charges, are mixed in water. Depending on the relative proportion of the oppositely charged particles, the process of aggregation leads to the formation of discrete clusters of various sizes in dilute dispersions, and to the development of particle gel networks in more concentrated systems. Due to the significant difference in the absolute values of surface charges (negative particle: -48 mV, positive particle: +24 mV), the phase separation and the gelation behaviors are asymmetric with respect to the mixing ratio. Mixtures with excess negative particles are more stable, while mixtures with excess positive particles are easily affected by phase separation. The hetero-aggregation triggered by the addition of microscopically large macro-ions is similar to what is often observed in a mono-component charged colloidal system, i.e., phase separation occurs through addition of small electrolyte ions. Within the concentration region investigated here, it is clear that the gel line is buried inside the phase separation region. Gelation occurs only when the number and size of the clusters are large and big enough to connect up into a space-spanning network. Our results indicate that, in this binary mixture of oppositely charged colloids, although the interaction between unlike species is attractive and that between like species is repulsive, the onset of gelation is in fact governed by the equilibrium phase separation, as in the case of purely attractive systems with short-range isotropic interaction.

Charge regulation: A generalized boundary condition?

The three most commonly used boundary conditions for charged colloidal systems are constant charge (insulator), constant potential (conducting electrode) and charge regulation (ionizable groups at the surface). It is usually believed that the charge regulation is a generalized boundary condition that reduces in some specific limits to either constant-charge or constant-potential boundary conditions. By computing the disjoining pressure between two symmetric planes for these three boundary conditions, both numerically (for all inter-plate separations) and analytically (for small inter-plate separations), we show that this is not, in general, the case. In fact, the limit of charge regulation is a separate boundary condition, yielding a disjoining pressure with a different characteristic separation scaling. Our findings are supported by several examples demonstrating that the disjoining pressure at small separations for the charge regulation boundary condition depends on the details of the dissociation/association process.

Kolmogorov flow in two dimensional strongly coupled Yukawa liquid: A molecular dynamics study

The transition from laminar to turbulent flows in liquids remains a problem of great interest despite decades of intensive research. Here, we report an atomistic study of this transition in a model Yukawa liquid using molecular dynamics simulations. Starting from an thermally equilibrated Yukawa liquid, for a given value of coupling parameter Γ (defined as ratio of potential energy to kinetic energy per particle) and screening length κ, a subsonic flow of magnitude U0 is superposed and transition to an unstable regime is observed eventually leading to turbulent flow at sufficiently high Reynolds numbers. We have performed a parametric study for a range of Reynolds number R and found that the flow is neutrally stable for R<Rc(Γ), while a transition from laminar to turbulent flow occurs for R>Rc(Γ), where Rc is the critical value of Reynolds number. Strong molecular shear heating is observed in all cases studied here. It is found that the coupling parameter Γ decreases because of molecular shear heating on a time scale comparable to the instability time scale. Irrespective of the initial value of coupling parameter Γ, the average heating rate is found to be sensitive to the ratio of equilibrium flow speed to the thermal speed, say, α=U0vth, where vth=2Γ. Our results reported here are expected to be generic and should apply to a wide variety of strongly coupled systems such as laboratory dusty plasma, molten salts, and charged colloidal systems.

Inverse patchy colloids with small patches: fluid structure and dynamical slowing down

Inverse patchy colloids (IPCs) differ from conventional patchy particles because their patches repel (rather than attract) each other and attract (rather than repel) the part of the colloidal surface that is free of patches. These particular features occur, e.g. in heterogeneously charged colloidal systems. Here we consider overall neutral IPCs carrying two, relatively small, polar patches. Previous studies of the same model under planar confinement have evidenced the formation of branched, disordered aggregates composed of ring-like structures. We investigate here the bulk behavior of the system via molecular dynamics simulations, focusing on both the structure and the dynamics of the fluid phase in a wide region of the phase diagram. Additionally, the simulation results for the static observables are compared to the Associative Percus Yevick solution of an integral equation approach based on the multi-density Ornstein–Zernike theory. A good agreement between theoretical and numerical quantities is observed even in the region of the phase diagram where the slowing down of the dynamics occurs.

Experimental investigation on the use of highly charged nanoparticles to improve the stability of weakly charged colloidal system

The present work highlighted on the implementation of a unique concept for stabilizing colloids at their incipiently low charge potential. A highly charged nanoparticle was introduced within a coagulated prone colloidal system, serving as stabilizer to resist otherwise rapid flocculation and sedimentation process. A low size asymmetry of nanoparticle/colloid serves as the new topic of investigation in addition to the well-established large size ratio nanoparticle/microparticle study. Highly charged Al2O3 nanoparticles were used within the present research context to stabilize TiO2 and Fe3O4 based colloids via the formation of composite structures. It was believed, based on the experimental evidence, that Al2O3 nanoparticle interact with the weakly charged TiO2 and Fe3O4 colloids within the binary system via absorption and/or haloing modes to increase the overall charge potential of the respective colloids, thus preventing further surface contact via van der Waal’s attraction. Series of experimental results strongly suggest the presence of weakly charged colloids in the studied bimodal system where, in the absence of highly charged nanoparticle, experience rapid instability. Absorbance measurement indicated that the colloidal stability drops in accordance to the highly charged nanoparticle sedimentation rate, suggesting the dominant influence of nanoparticles to attain a well-dispersed binary system. Further, it was found that the level of colloidal stability was enhanced with increasing nanoparticle fraction within the mixture. Rheological observation revealed that each hybrid complexes demonstrated behavior reminiscence to water with negligible increase in viscosity which serves as highly favorable condition particularly in thermal transport applications.

Micelle-induced depletion interaction and resultant structure in charged colloidal nanoparticle system

The evolution of the interaction and the resultant structure in the mixed system of anionic silica nanoparticles (Ludox LS30) and non-ionic surfactant decaethylene glycol monododecylether (C12E10), undergoing phase separation, have been studied using small-angle neutron scattering and dynamic light scattering. The measurements have been carried out for a fixed concentration of nanoparticle (1 wt. %) with varying concentration of surfactant (0 to 1 wt. %), in the absence and presence of an electrolyte. It is found that the micelles of non-ionic surfactant adsorb on the nanoparticle in the absence of electrolyte (form stable system), whereas these micelles become non-adsorbing in the presence of electrolyte (show phase separation). The phase separation arises because of C12E10 micelles, causing depletion interaction between nanoparticles and leading to their aggregation. The interaction is modeled by double Yukawa potential accounting for attractive depletion as well as repulsive electrostatic forces. Both the interactions (attraction and repulsion) are found to be of long-range. The nanoparticle aggregation (phase separation) is governed by the increase in the magnitude and the range of the depletion attraction with the increase in the surfactant concentration. The nanoparticle aggregates formed are quite large in size (order of micron) and are characterized by the surface fractal having simple cubic packing of nanoparticles within the aggregates.

Multi-region to single region shear thinning transitions in drying PEDOT:PSS dispersions: contributions from charge density fluctuations.

PSS), in water during the drying process is explored. PSS dispersions in water can be non-stoichiometric polyelectrolyte complexes (PECs) with free polyanion (PSS) chains resulting in scrambled egg conformations. They also behave as charged colloidal particles. The behaviour of PSS dispersions as PECs and charged colloidal systems during drying is analyzed using rheology and TEM, zeta potential, conductivity, pH, dynamic light scattering (DLS) and rheo-small angle light scattering (rheo-SALS) information. The rheological behaviour is found to have significant contributions from conformational changes in the PSS PEC due to charge density fluctuations affecting the electrostatic persistence length, le. A low concentration regime with multiple shear thinning behaviour and a high concentration regime with single shear thinning behaviour were observed. The PEC behavior of PSS is further explored by charge neutralizing the dispersions with salts.

Coatings of Fe/Al Hydroxides Inhibited Acidification of Kaolinite and an Alfisol Subsoil Through Electrical Double-Layer Interaction and Physical Blocking

Variably charged soils confer a different pattern of soil chemical properties compared with a homogeneously charged colloidal system. The relative contents of soil mineralogical constituents and their interaction play important roles in the variation of charge characteristics and other chemical properties of the soils. In this study, coating and simple mixing of iron (Fe)/aluminum (Al) hydroxides with kaolinite and an Alfisol subsoil were made for comparative study of their effect on the electrochemical and acidity properties of kaolinite and Alfisol subsoil after electrodialysis. Transmission electron microscopy and X-ray diffraction analyses indicated that Fe hydroxide coating was more effective than their mixing in physically blocking the kaolinite surface and decreasing the peak intensity of XRD for kaolinite. Coated Fe hydroxides were also more effective than the mixed ones in shifting zeta potential in a positive value direction, decreasing effective cation exchange capacity, and inhibiting acidification of kaolinite and Alfisol subsoil, whereas there was little difference between coated and mixed amorphous aluminum hydroxide (Al(OH)3). The effect of coated Fe/Al hydroxides on effective cation exchange capacity of electrodialyzed kaolinite and Alfisol subsoil varied irreversibly with ionic strength of the bathing solutions. All the results demonstrated that coatings of Fe/Al hydroxides could decrease effective negative charge and inhibit acidification of kaolinite and Alfisol subsoil mainly through two mechanisms: diffuse double-layer overlapping between oppositely charged particles and physical blocking of kaolinite and Alfisol subsoil during electrodialysis. Therefore, Fe/Al hydroxides can act as antiacidification agents in variably charged soils.