Nanoparticles adsorbed at the water/oil interface: coverage and composition effects on structure and diffusion

Enhanced Biphasic Reactions in Amphiphilic Silica Mesopores

In this study, we investigated the effect of the pore volume and mesopore size of surface-active catalytic organosilicas on the genesis of particle-stabilized (Pickering) emulsions for the dodecanal/ethylene glycol system and their reactivity for the acid-catalyzed biphasic acetalization reaction. To this aim, we functionalized a series of fumed silica superparticles (size 100-300 nm) displaying an average mesopore size in the range of 11-14 nm and variable mesopore volume, with a similar surface density of octyl and propylsulfonic acid groups. The modified silica superparticles were characterized in detail using different techniques, including acid-base titration, thermogravimetric analysis, TEM, and dynamic light scattering. The pore volume of the particles impacts their self-assembly and coverage at the dodecanal/ethylene glycol (DA/EG) interface. This affects the stability and the average droplet size of emulsions and conditions of the available interfacial surface area for reaction. The maximum DA-EG productivity is observed for A200 super-SiNPs with a pore volume of 0.39 cm3·g-1 with an interfacial coverage by particles lower than 1 (i.e., submonolayer). Using dissipative particle dynamics and all-atom grand canonical Monte Carlo simulations, we unveil a stabilizing role of the pore volume of porous silica superparticles for generating emulsions and local micromixing of immiscible dodecanal and ethylene glycol, allowing fast and efficient solvent-free acetalization in the presence of Pickering emulsions. The micromixing level is interrelated to the adsorption energy of self-assembled particles at the DA/EG interface.

Criteria of Distribution Transitions in Dispersed Multiphase Systems Based on an Extended Lattice Model

Dispersed multiphase systems are ubiquitous in biological systems, energy industries, and medical science. The distribution transition of the dispersed phase is critical to the properties and functions of the multiphase systems, among which the agglomeration, adsorption, and extraction processes are of most significance due to their impact on the colloidal stability, interface modification, and particle synthesis. To reveal fundamental correlations between the macroscopic particle distributions and the microscopic interactions, general thermodynamic models of the dispersed multiphase systems are needed. Here, based on Meyer's model, which is restricted to binary isotropic mixtures, we propose a novel extended lattice model that can be applied to multicomponent anisotropic mixtures with interfaces considered. For agglomeration, adsorption, and extraction processes, the approximate free energy differences between the initial distribution and the final distribution are obtained. Based on the minimum free energy principle, the above free energy differences are used to derive three criteria for the prediction of the preferable distribution of the system with given interparticle interaction potentials. While the quasi-uniform number density assumption is still required for all the previous lattice models, the long-range interactions neglected by previous lattice models are incorporated. The validity of our model and criteria is verified by many-body dissipative particle dynamics (mDPD) simulations. By tuning the interaction coefficients between mDPD particles, the simulated distribution transitions for all the agglomeration, adsorption, and extraction cases perfectly match the predictions from the three criteria. The good agreement between the theoretical predictions and the mDPD simulation results shows the great potential of our model for applications in various dispersed multiphase systems.

Evaporation-driven assembly of colloidal nanoparticles into clusters: A dissipative particle dynamics study

In this work we consider a simulation strategy for assembling Janus nanoparticles in oil-in-water emulsion droplets by evaporation based on the dissipative particle dynamics method. Our simple method reproduces all the observed cluster configurations that have been explored experimentally. In addition, the kinetic process of cluster formation is systematically investigated. We observe a structural transition from spherical packings to minimal second-moment configurations via visual inspection and a simple angle parameter. We reveal that the critical volume at which the transition occurs is a cubic function of the number of particles, N. Our approach also allows us to anticipate higher-order clusters, overcoming the limitations of the standard methods in the literature. Similarly to small N values, we find that for each N in the range of 16-39, all final clusters have a unique configuration.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Tracking of Nanoparticle Diffusion at a Liquid-Liquid Interface Adsorbed by Nonionic Surfactants

Emulsions stabilized by both nanoparticles and surfactants often display longer shelf life than those stabilized by nanoparticles or surfactants alone. Although numerous works have been conducted to understand the effect of nanoparticles and surfactants on the variation of interfacial tension, little is known about interfacial diffusion when both nanoparticles and surfactants are present at interfaces. In this work, we used single-particle fluorescence tracking to study the lateral diffusion of individual hydrophobic nanoparticles at hexane-glycerol interfaces adsorbed by different amounts of nonionic surfactants. When the surfactant concentration is over a threshold, we found that the nanoparticle diffusion exhibits a two-regime behavior involving short-time Brownian and the emergence of subdiffusive, non-Gaussian, and dynamically anticorrelated diffusion in the long lag time regime. A stepwise analysis rationalized diffusion in different lag time regimes, leading to a mechanistic interpretation regarding the two-regime behavior. These results could provide insight into the understanding of the synergistic effect for the surfactant-assistant Pickering emulsion.

Lattice Model of Multilayer Adsorption of Particles with Orientation Dependent Interactions at Solid Surfaces

A simple lattice model has been used to study the formation of multilayer films by fluids with orientation-dependent interactions on solid surfaces. The particles, composed of two halves (A and B) were allowed to take on one of six different orientations. The interaction between a pair of differently oriented neighboring particles was assumed to depend on the degrees to which their A and B parts overlap. Here, we have assumed that the AA interaction was strongly attractive, the AB interaction was set to zero, while the BB interaction was varied between 0 and -1.0. The ground state properties of the model have been determined for the systems being in contact with non-selective and selective walls over the entire range of BB interaction energies between 0 and -1.0. It has been demonstrated that the structure of multilayer films depends on the strengths of surface potential felt by differently oriented particles and the interaction between the B halves of fluid particles. Finite temperature behavior has been studied by Monte Carlo simulation methods. It has been shown that the bulk phase phase diagram is qualitatively independent of the BB interaction energy, and has the swan neck shape, since the high stability of the dense ordered phase suppresses the possibility of the formation of disordered liquid-like phase. Only one class of non-uniform systems with the BB interaction set to zero has been considered. The results have been found to be consistent with the predictions stemming form the ground state considerations. In particular, we have found that a complete wetting occurs at any temperature, down to zero. Furthermore, the sequences of layering transitions, and the structure of multilayer films, have been found to be the same as observed in the ground state.

Armored Droplets as Soft Nanocarriers for Encapsulation and Release under Flow Conditions

Technical challenges in precision medicine and environmental remediation create an increasing demand for smart materials that can select and deliver a probe load to targets with high precision. In this context, soft nanomaterials have attracted considerable attention due to their ability to simultaneously adapt their morphology and functionality to complex ambients. Two major challenges are to precisely control this adaptability under dynamic conditions and provide predesigned functionalities that can be manipulated by external stimuli. Here, we report on the computational design of a distinctive class of soft nanocarriers, built from armored nanodroplets, able to selectively encapsulate or release a probe load under specific flow conditions. First, we describe in detail the mechanisms at play in the formation of pocket-like structures in armored nanodroplets and their stability under external flow. Then we use that knowledge to test the capacity of these pockets to yield flow-assisted encapsulation or expulsion of a probe load. Finally, the rheological properties of these nanocarriers are put into perspective with those of delivery systems employed in pharmaceutical and cosmetic technology.

Revealing the dynamic adsorption and diffusion of peptide amphiphile on supported lipid bilayer by single molecule experiment and simulation

Dynamic adsorption and diffusion of peptide amphiphiles (PAs) with different numbers of alkyl tails on supported lipid bilayers (SLBs) were investigated by single molecule tracking experiment and molecule dynamic simulation. Experimental results show two distinct populations of PAs with different residence time. Residence time of adsorbed PAs increases with the increase of the alkyl tails, whereas diffusion coefficient monotonically decreases with rising the number of alkyl tails and also decreases with increasing the mobility of SLBs. All-atom molecule dynamic simulation results prove that the adsorption and diffusion of PAs on SLB surface are mainly determined by interactions between PAs and SLBs and also the intrinsic mobility of PAs in aqueous solution. The electrostatic interactions and the mobility of PAs are two dominated but contradictory coefficients for the adsorption and diffusion of PAs. By increasing the alkyl tails, the mobility of PAs decreases while the electrostatic property does not change significantly, resulting in the increase of residence time and decrease of diffusivity of PAs. Additionally, for the PAs with large number of alkyl tails (≥ 3 alkyl tails), steric hindrance of alkyl tails leads to the decrease of adsorption probability of PAs on SLB surface. These findings lay the groundwork for guiding the design of PAs with high cell affinity, potentially useful for efficient drug delivery.

Self-assembly of mono- and poly-dispersed nanoparticles on emulsion droplets: antagonistic vs. synergistic effects as a function of particle size

In this work, using Dissipative Particle Dynamics simulations, we provide fundamental insights into the self-assembly of nanoparticles (NPs) on droplet surfaces in an oil-in-water emulsion. We highlight the effect of particle size on the arrangement of NPs for different interparticle interactions. NPs of two different sizes were considered. In general, when the NP-NP interaction is changed from repulsive to attractive, a transition in the NP arrangement occurs from weekly-connected networks to clusters of NPs separated by particle-free domains. When NP-NP interactions are strongly attractive, NPs yield small 3D aggregates on the droplet surface. These arrangements seem to agree with experimental observations reported in the literature. In addition, our simulations suggest that small NPs are able to diffuse more easily on the droplet surface, which leads to prompt self-organisation, while large NPs are more likely to form metastable structures, perhaps because of slow mobility and strong adsorption to the interface. Our analysis suggests that thermal fluctuations could provide the activation energy for the small NPs to escape local minima in the free energy landscape. The results obtained for systems containing NPs of two sizes provide evidence of size segregation on the droplet surface, which could be useful when NP self-assemblies are used, for example, to template supra-molecular materials. However, analysis of the simulated trajectories suggests that the results depend strongly on the initial configuration, as the larger NPs seem to impose barriers for the small NPs to adsorb and diffuse on the droplet surface.

Brownian Diffusion of Individual Janus Nanoparticles at Water/Oil Interfaces

Janus nanoparticles could exhibit a higher interfacial activity and adsorb stronger to fluid interfaces than homogeneous nanoparticles of similar sizes. However, little is known about the interfacial diffusion of Janus nanoparticles and how it compares to that of homogeneous ones. Here, we employed fluorescence correlation spectroscopy to study the lateral diffusion of ligand-grafted Janus nanoparticles adsorbed at water/oil interfaces. We found that the diffusion was significantly slower than that of homogeneous nanoparticles. We carried out dissipative particle dynamic simulations to study the mechanism of interfacial slowdown. Good agreement between experimental and simulation results has been obtained only provided that the flexibility of ligands grafted on the nanoparticle surface was taken into account. The polymeric ligands were deformed and oriented at an interface so that the effective radius of Janus nanoparticles is larger than the nominal one obtained by measuring the diffusion in bulk solution. These findings highlight further the critical importance of the ligands grafted on Janus nanoparticles for applications involving nanoparticle adsorption at an interface, such as oil recovery or two-dimensional self-assembly.

Compression and Ordering of Microgels in Monolayers Formed at Liquid-Liquid Interfaces: Computer Simulation Studies

Monolayers of polymer microgels adsorbed at the liquid interfaces were studied by dissipative particle dynamics simulations. The results demonstrated that the compressibility of the monolayers can be widely tuned by varying the cross-linking density of the microgels and their (in)compatibility with the immiscible liquids. In particular, the compression of the monolayers (increase of 2D concentration of the microgels) leads to the decrease of their lateral size. Herewith, the shape of the individual soft particles gradually changes from oblate (diluted 2D system) to nearly spherical (compressed monolayer). The polymer concentration profiles plotted along the normal to the interface reveal a nonmonotonous shape with a sharp maximum at the interface. This is a consequence of the shielding effect: saturation of the interface by monomer units of the subchains is driven by minimization of unfavorable contacts between the immiscible liquids and is opposed by elasticity of the network. The decrease of the interfacial tension upon concentration (compression) of the monolayer is quantified. It has been demonstrated that the interfacial tension significantly differs if the solubility of the polymer chains of the microgel network in the liquids changes. These results correlate well with experimental data. The examination of the microgels' crystalline ordering in monolayers demonstrated a nonmonotonous dependency on the compression degree (microgel concentration). Finally, the worsening of the solvent quality leads to the collapse of the microgels in monolayer and nonmonotonous behavior of the interfacial tension.

Interfacial aggregation of Janus rods in binary polymer blends and their effect on phase separation

Janus particles interfacially self-assemble into different structures when incorporated into multiphase systems. Dissipative particle dynamics simulations are employed herein to investigate the interplay between aggregation mechanisms and phase separation in polymer blends. Shorter rods with a standing configuration become increasingly "caged" or trapped in larger aggregates as weight fraction increases, which is reflected in the way that their diffusion is coupled to their aggregation rates. Janus rods of higher aspect ratios that are tilted at the interface aggregate side-by-side and are able to hinder phase separation kinetics. This is due to a combination of individual Janus rod conformations at the interface, their intrinsic aggregation mechanisms, aggregate fractal dimension, and aggregation rates, and can also be traced back to the scaling of the diffusion coefficient of aggregates with their size. Findings presented provide insight into the mechanisms governing two dimensionally growing colloidal aggregates at fluid interfaces, more specifically, those associated with Janus particles, and shed light on the potential of these systems in paving the way for designing new functional materials.

Quantification of Ostwald Ripening in Emulsions via Coarse-Grained Simulations

Ostwald ripening is a diffusional mass transfer process that occurs in polydisperse emulsions, often with the result of threatening the emulsion stability. In this work, we design a simulation protocol that is capable of quantifying the process of Ostwald ripening at the molecular level. To achieve experimentally relevant time scales, the dissipative particle dynamics (DPD) simulation protocol is implemented. The simulation parameters are tuned to represent two benzene droplets dispersed in water. The coalescence between the two droplets is prevented via the introduction of membranes, which allow diffusion of benzene from one droplet to the other. The simulation results are quantified in terms of the changes in the droplet volume as a function of time. The results are in qualitative agreement with experiments. The agreement with the Lifshitz-Slyozov-Wagner theory becomes quantitative when the simulated solubility and diffusion coefficient of benzene-in-water are considered. The effect of two different surfactants was also investigated. In agreement with both experimental observations and theory, the addition of surfactants at moderate concentrations decreased the Ostwald ripening rate because of the reduction in the interfacial tension between benzene and water; as the surfactant film becomes dense, other phenomena are likely to further delay the Ostwald ripening. In fact, the results suggest that the surfactant that yields higher density at the benzene-water interface delayed more effectively Ostwald ripening. The formation of micelles can also affect the ripening rate, in qualitative agreement with experiments, although our simulations are not conclusive on such effects. Our simulations show that the coarse-grained DPD formalism is able to capture the molecular phenomena related to Ostwald ripening and reveal molecular level features that could help to understand experimental observations. The results could be useful for predicting and eventually controlling the long-term stability of emulsions.

Nanoparticles Actively Fragment Armored Droplets

Understanding the complexity of fragmentation processes is essential for regulating intercellular communication in mechanistic biology and developing bottom-up approaches in a large range of multiphase flow processes. In this context, self-fragmentation proceeds without any external mechanical energy input, allowing one to create efficiently micro- and nanodroplets. Here we examine self-fragmentation in emulsion nanodroplets stabilized by solid particles with different surface features. Mesoscopic modeling and accelerated dynamics simulations allow us to overcome the limitations of atomistic simulations and offer detailed insight into the interplay between the evolution of the droplet shape and the particle finite-size effects at the interface. We show that finite-size nanoparticles play an active role in the necking breakup, behaving like nanoscale razors, and affect strongly the thermodynamic properties of the system. The role played by the particles during self-fragmentation might be of relevance to multifunctional biomaterial design and tuning of signaling pathways in mechanistic biology.

Interfacial Rheology of Charged Anisotropic Cellulose Nanocrystals at the Air-Water Interface

Cellulose nanocrystals (CNCs) have received attention as a biological alternative for the stabilization of fluid interfaces, yielding biocompatible and sustainable emulsions, foams, and aerogels. The interfacial behavior of nanoparticles with shape anisotropy and surface charge like CNCs is still poorly understood, although it ultimately dictates the mechanical properties and stability of the macroscopic colloidal material. Here, we report on the linear and nonlinear interfacial dilatational and shear rheology of CNCs at the air-water interface. We observed the formation of viscoelastic CNC layers at comparably low surface coverage, which was attributed to the shape anisotropy of CNCs. Further, the interfacial elasticity of CNC layers can be modulated by salt-induced charge screening, thereby shifting the interplay of repulsive and attractive CNC interactions. CNC layers had a viscous character without salt, followed by increasing viscoelasticity upon salt addition. CNC layers display strain hardening during compression and show a yield stress followed by flow under shear. The observed interfacial behavior is discussed in the context of CNC-stabilized foam and emulsion properties. We conclude that understanding the CNC interfacial behavior may help improve the performance of CNC-stabilized colloidal materials.

Controlling the stability of Pickering emulsions by pH-responsive nanoparticles

Electrostatic dissipative particle dynamics simulations were conducted to model the interactions between emulsion droplets stabilized by pH-sensitive polyelectrolyte-grafted nanoparticles. Using a steered molecular dynamics approach, a mechanistic study of forced coalescence was performed to probe the resistance between two particle-covered droplets. The degree of ionization of the grafted polyelectrolytes was adjusted to capture the pH responsiveness. The maximal resistance forces were measured to quantitatively discriminate the efficacy of particles in stabilizing emulsions at different degrees of ionization. Through analyzing droplet dynamics, resistance force variation, and electric field, we discovered that the resistance is attributed to direct electrostatic repulsion, the image charge effect near the water-oil interface, and steric hindrance among extended polymers. When the particle density on the droplet surface is relatively low, the increasing resistance forces at higher degrees of ionization can effectively prevent droplet coalescence. Oppositely, the ionization compromises emulsion stability when the particle surface coverage is high. Substantial desorption of particles from the interface was triggered as the degree of ionization increases. This in turn reduces resistance force and facilitates coalescence. Moreover, the nanoparticles prevent coalescence at high surface coverages by forming dense layers at individual interfaces, while the particle bridges straddling two interfaces were found at low surface coverages, which can also keep the droplets apart.

Nanomixing Effects in Glycerol/Dodecanol Pickering Emulsions for Interfacial Catalysis

Pickering emulsions offer a promising platform for conducting interfacial reactions between immiscible reagents. Despite the significant progress in the engineering of amphiphilic catalysts for such reactions, the mechanism behind their enhanced activity is still poorly understood. Herein, using the glycerol/dodecanol system as a case study, we conducted a combined meso- and microscale study of Pickering emulsions stabilized by amphiphilic silica nanoparticles bearing acid centers by marrying dissipative particle dynamics simulations with emulsification experiments. The optimal surface properties of the silica particles in terms of length and density of alkyl chains were identified, matching the experimental results. The local distribution of glycerol and dodecanol near the acid centers was ascertained, unraveling potential reactivity zones near the catalytic acid centers due to an enhanced nanomixing between glycerol and dodecanol.

The Effect and Associate Mechanism of Nano SiO₂ Particles on the Diffusion Behavior of Water in Insulating Oil

Moisture has a significant effect on the internal insulation performance of transformers, and is closely related to the breakdown voltage of transformer insulating oil. In the present work, we studied the effect of nano-SiO₂ particles on the diffusion of water in insulating naphthenic mineral oil using molecular dynamics simulation. Six models were established, three of which contained nano-SiO₂ particles together with water concentration of 1 wt.%, 2 wt.%, or 3 wt.%. For each model variations in free volume, mean square displacement, and interaction energy were assessed. The addition of nano SiO₂ particles was found to reduce the free volume fraction of the model and as well as the free motion of water molecules in the oil. These particles also increased the interaction between the oil and water molecules, indicating that insulating oil containing nano-particles has a greater binding effect on water. The diffusion coefficient of water in oil containing nano-SiO₂ particles was reduced, such that water molecules were less likely to diffuse. The results also show that these particles adsorb water molecules in the oil and to reduce diffusion. Consequently, the addition nano-scale SiO₂ particles could potentially improve the breakdown voltage of the insulating oil.

Nanoparticle-induced ion-sensitive reduction in decane-water interfacial tension

The synergistic effect of ions and nanoparticles on the interfacial tension is of great significance for extensive applications in interface-related industrial processes. However, its mechanisms are still unclear owing to a lack of understanding on the interaction between nanoparticles/ions at the interface. Here, we employ the molecular dynamics method to explore the synergistic effect of ions and nanoparticles on reducing the decane-water interfacial tension and reveal the dominant role of the three-phase contact angle and the interaction between nanoparticles. The results show that the reduction of interfacial tension is sensitive to cation species and temperature. The stronger hydration of cations induces an increased three-phase contact angle, weakening the interaction between nanoparticles and water molecules at the interface. Hence, the virial term of interfacial tension decreases. Meanwhile, the potential of mean force between nanoparticles at the interface indicates that the order of interaction strength between nanoparticles for different cations is Ca2+ > Mg2+ > Na+. The strong interaction between nanoparticles restricts the motion of nanoparticles and water molecules at the interface, inducing a reduced kinetic energy term of interfacial tension. Therefore, the interfacial tension decreases after adding the nanoparticles. Besides, as temperature rises, the difference in the adsorption ability of nanoparticles on water molecules causes a falling interfacial tension with a characteristic stage.

Nanoparticle adsorption dynamics at fluid interfaces

Understanding the dynamic adsorption of nanoparticles (NPs) at fluid interfaces is important for stabilizing emulsions and for the preparation of 2D NP-based materials. Here we show that the Ward-Tordai equations commonly employed to describe the dynamics of surfactant adsorption at a fluid interface combined with a Frumkin adsorption isotherm can be employed to model the diffusion-limited adsorption of NPs onto a fluid interface. In contrast to surfactants, an additional wetting equation of state (EOS) must be incorporated to characterize the dynamic interfacial tension during the adsorption of NPs to the oil-water interface. Our results show agreement between the model and experiments with NP area fractions <0.3. Slower dynamics are observed at larger area fractions, which are speculated to arise from polydispersity or re-organization at the interface. We show the model can be extended to the competitive adsorption between the NPs and a surface active species.

Structure and Dynamics of Stimuli-Responsive Nanoparticle Monolayers at Fluid Interfaces

Stimuli-responsive nanoparticles at fluid interfaces offer great potential for realizing on-demand and controllable self-assembly that can benefit various applications. Here, we conducted electrostatic dissipative particle dynamics simulations to provide a fundamental understanding of the microstructure and interfacial dynamics of responsive nanoparticle monolayers at a water-oil interface. The model nanoparticle is functionalized with polyelectrolytes to render the pH sensitivity, which permits further manipulation of the monolayer properties. The monolayer structure was analyzed in great detail through the density and electric field distributions, structure factor, and Voronoi tessellation. Even at a low surface coverage, a continuous disorder-to-order phase transition was observed when the particle's degree of ionization increases in response to pH changes. The six-neighbor particle fraction and bond orientation order parameter quantitatively characterize the structural transition induced by long-range electrostatic interactions. Adding salt can screen the electrostatic interactions and offer additional control on the monolayer structure. The detailed dynamics of the monolayer in different states was revealed by analyzing mean-squared displacements, in which different diffusion regimes were identified. The self-diffusion of individual particles and the collective dynamics of the whole monolayer were probed and correlated with the structural transition. Our results provide deeper insight into the dynamic behavior of responsive nanoparticle surfactants and lay the groundwork for bottom-up synthesis of novel nanomaterials, responsive emulsions, and microdroplet reactors.

Dynamic Organization of Ligand-Grafted Nanoparticles during Adsorption and Surface Compression at Fluid-Fluid Interfaces

Monolayers of ligand-grafted nanoparticles at fluid interfaces exhibit a complex response to deformation due to an interplay of particle rearrangements within the monolayer, and molecular rearrangements of the ligand brush on the surface of the particles. We use grazing-incidence small-angle X-ray scattering (GISAXS) combined with pendant drop tensiometry to probe in situ the dynamic organization of ligand-grafted nanoparticles upon adsorption at a fluid-fluid interface, and during monolayer compression. Through the simultaneous measurements of interparticle distance, obtained from GISAXS, and of surface pressure, obtained from pendant drop tensiometry, we link the interfacial stress to the monolayer microstructure. The results indicate that, during adsorption, the nanoparticles form rafts that grow while the interparticle distance remains constant. For small-amplitude, slow compression of the monolayer, the evolution of the interparticle distance bears a signature of ligand rearrangements leading to a local decrease in thickness of the ligand brush. For large-amplitude compression, the surface pressure is found to be strongly dependent on the rate of compression. Two-dimensional Brownian dynamics simulations show that the rate-dependent features are not due to jamming of the monolayer, and suggest that they may be due to out-of-plane reorganization of the particles (for instance expulsion or buckling). The corresponding GISAXS patterns are also consistent with out-of-plane reorganization of the nanoparticles.

Electrokinetic motion of a spherical micro particle at an oil-water interface in microchannel

The electrokinetic motion of a negatively charged spherical particle at an oil-water interface in a microchannel is numerically investigated and analyzed in this paper. A three-dimensional (3D) transient numerical model is developed to simulate the particle electrokinetic motion. The channel wall, the surface of the particle and the oil-water interface are all considered negatively charged. The effects of the direct current (DC) electric field, the zeta potentials of the particle-water interface and the oil-water interface, and the dynamic viscosity ratio of oil to water on the velocity of the particle are studied in this paper. In addition, the influences of the particle size are also discussed. The simulation results show that the micro-particle with a small value of negative zeta potential moves in the same direction of the external electric field. However, if the zeta potential value of the particle-water interface is large enough, the moving direction of the particle is opposite to that of the electric field. The velocity of the particle at the interface increases with the increase in the electric field strength and the particle size, but decreases with the increase in the dynamic viscosity ratio of oil to water, and the absolute value of the negative zeta potentials of both the particle-water interface and the oil-water interface. This work is the first numerical study of the electrokinetic motion of a charged particle at an oil-water interface in a microchannel.

Buckling in armored droplets

The buckling mechanism in droplets stabilized by solid particles (armored droplets) is tackled at a mesoscopic level using dissipative particle dynamics simulations. We consider one spherical water droplet in a decane solvent coated with nanoparticle monolayers of two different types: Janus (particles whose surface shows two regions with different wetting properties) and homogeneous. The chosen particles yield comparable initial three-phase contact angles, selected to maximize the adsorption energy at the interface. We study the interplay between the evolution of droplet shape, layering of the particles, and their distribution at the interface when the volume of the droplets is reduced. We show that Janus particles affect strongly the shape of the droplet with the formation of a crater-like depression. This evolution is actively controlled by a close-packed particle monolayer at the curved interface. In contrast, homogeneous particles follow passively the volume reduction of the droplet, whose shape does not deviate too much from spherical, even when a nanoparticle monolayer/bilayer transition is detected at the interface. We discuss how these buckled armored droplets might be of relevance in various applications including potential drug delivery systems and biomimetic design of functional surfaces.

Amphiphilic nanosheet self-assembly at the water/oil interface: computer simulations

The self-assembly of amphiphilic Janus triangular-plates at the water/oil interface is simulated for the first time.

Dynamics of Pickering Emulsions in the Presence of an Interfacial Reaction: A Simulation Study

Pickering emulsions combining surface-active and catalytic properties offer a promising platform for conducting interfacial reactions between immiscible reagents. Despite the significant progress in the design of Pickering interfacial catalysts for a broad panel of reactions, the dynamics of Pickering emulsions under reaction conditions is still poorly understood. Herein, using benzene hydroxylation with aqueous H₂O₂ as a model system, we explored the dynamics of benzene/water Pickering emulsions during reaction by dissipative particle dynamics. Our study points out that the surface wettability of the silica nanoparticles is affected to a higher extent by the degree of polymer grafting rather than an increase of the chain length of hydrophobic polymer moieties. A remarkable decline of the oil-in-water (O/W) interfacial tension was observed when increasing the yield of the reaction product (phenol), affecting the emulsion stability. However, phenol did not alter to an important extent the distribution of immiscible reagents around the nanoparticles sitting at the benzene/water interface. A synergistic effect between phenol and silica nanoparticles on the O/W interfacial tension of the biphasic system could be ascertained.

Simulations of graphitic nanoparticles at air-water interfaces

The free energy associated with transferring a set of fullerene particles through a finite water layer is calculated using explicit solvent molecular dynamic simulations. Each fullerene particle is a carbon network of one or more spheroidal shells of graphitic carbon, and include single-shell (single-wall) or nested multi-shelled (nano-onions) structures ranging from 6 to 28 Å in radius. Corresponding changes in energy suggest a stronger affinity of carbon nano-onions for water compared to their single-shelled analogues. In the case of multi-shelled structures, the free energy profiles display a global minimum only in the bulk liquid indicating a high affinity of multi-shelled fullerene for complete hydration. Single-wall particles however, display a minimum at the air-water interface and for particles larger than 2 nm this minimum is a global minimum possessing a lower energy compared to the particle's state of complete hydration. While the propensity for single-shell particles to adsorb to the air-interface may increase with increasing particle size, there is an indication based on line tension calculations that larger single-shell particles may actually exhibit enhanced wetting compared to their smaller analogues.

Nanoparticle Assemblies into Luminescent Dendrites in Shrinking Microdroplets

The self-assembly of nanoparticles (NPs) is essential for emerging dispersion-based energy-conscious technologies. Of particular interest are micro- and macro-scale self-organizing superstructures that can bridge 2D/3D processing scales. Here we report the spontaneous assembly of CdTe NPs within an aqueous microdroplet suspended in soybean oil. The gradual diffusion of the water into the surrounding medium results in shrinking of the microdroplet, and a concomitant formation of branched assemblies from CdTe NPs that evolve in size from ∼50 μm to ∼1000 μm. The fractal dimension of NP assemblies increases from ∼1.7 to ∼1.9 during the assembly process. We found that constituents of the soybean oil enter the aqueous solution across the microdroplet interface and affect NP assembly. The obtained NP dendrites can be further altered morphologically by illumination with light that results in the disassembly of the NP dendrites. The use of this microheterogeneous dispersion platform with partially soluble hydrophilic and hydrophobic solvents highlights the sensitivity of the NP assembly process to environment and presents an opportunity to explore droplet-confined NP assembly.

Knowledge gaps between nanotoxicological research and nanomaterial safety

With the wide research and application of nanomaterials in various fields, the safety of nanomaterials attracts much attention. An increasing number of reports in the literature have shown the adverse effects of nanomaterials, representing the quick development of nanotoxicology. However, many studies in nanotoxicology have not reflected the real nanomaterial safety, and the knowledge gaps between nanotoxicological research and nanomaterial safety remain large. Considering the remarkable influence of biological or environmental matrices (e.g., biological corona) on nanotoxicity, the situation of performing nanotoxicological experiments should be relevant to the environment and humans. Given the possibility of long-term and low-concentration exposure of nanomaterials, the reversibility of and adaptation to nanotoxicity, and the transgenerational effects should not be ignored. Different from common pollutants, the specific analysis methodology for nanotoxicology need development and exploration furthermore. High-throughput assay integrating with omics was highlighted in the present review to globally investigate nanotoxicity. In addition, the biological responses beyond individual levels, special mechanisms and control of nanotoxicity deserve more attention. The progress of nanotoxicology has been reviewed by previous articles. This review focuses on the blind spots in nanotoxicological research and provides insight into what we should do in future work to support the healthy development of nanotechnology and the evaluation of real nanomaterial safety.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Numerical analysis of Pickering emulsion stability: insights from ABMD simulations

The issue of the stability of Pickering emulsions is tackled at a mesoscopic level using dissipative particle dynamics simulations within the Adiabatic Biased Molecular Dynamics framework. We consider the early stage of the coalescence process between two spherical water droplets in a decane solvent. The droplets are stabilized by Janus nanoparticles of different shapes (spherical and ellipsoidal) with different three-phase contact angles. Given a sufficiently dense layer of particles on the droplets, we show that the stabilization mechanism strongly depends on the collision speed. This is consistent with a coalescence mechanism governed by the rheology of the interfacial region. When the system is forced to coalesce sufficiently slowly, we investigate at a mesoscopic level how the ability of the nanoparticles to stabilize Pickering emulsions is discriminated by nanoparticle mobility and the associated caging effect. These properties are both related to the interparticle interaction and the hydrodynamic resistance in the liquid film between the approaching interfaces.

Structural properties of polymer-brush-grafted gold nanoparticles at the oil-water interface: insights from coarse-grained simulations

In this work, the structural properties of amphiphilic polymer-brush-grafted gold nanoparticles (AuNPs) at the oil-water interface were investigated by coarse-grained simulations. The effects of grafting architecture (diblock, mixed and Janus brush-grafted AuNPs) and hydrophilicity of polymer brushes are discussed. Simulation results indicate that functionalized AuNPs present abundant morphologies including typical core-shell, Janus-type, jellyfish-like, etc., in a water or oil-water solvent environment. It is found that hydrophobic/weak hydrophilic polymer-brush-grafted AuNPs have better phase transfer performance, especially for AuNPs modified with hydrophobic chains as outer blocks and weak hydrophilic chains as inner blocks. This kind of AuNP can cross the interface region and move into the oil phase completely. For hydrophobic/strong hydrophilic polymer-brush-grafted AuNPs, they are trapped in the interface region instead of moving into any phase. The mechanism of phase transfer is ascribed to the flexibility and mobility of outer blocks. Besides, we study the desorption energy by PMF analysis. The results demonstrate that Janus brush-grafted AuNPs show the highest interfacial stability and activity, which can be further strengthened by increasing the hydrophilicity of grafted polymer brushes. This work will promote the industrial applications of polymer-brush-grafted NPs such as phase transfer catalysis and Pickering emulsion catalysis.

Direct calculation of anisotropic surface stresses during deformation of a particle-covered drop

The modification of the surface tension and the surface shear elasticity by particles in particle-covered drops can be attributed to a particle-induced surface stress. This stress represents at the macroscopic, continuum level the microscopic effect of lateral particle-particle interactions. Understanding the link between the isotropic and anisotropic components of the surface stress and the particle microstructure, and how these components change when structured interfaces deform, is a crucial problem in the field of particle-laden interfaces. In this paper, we analyse static and transient three-dimensional simulations of a pendant drop whose surface is covered by colloidal particles displaying purely repulsive particle-particle interactions. We compute the isotropic and anisotropic surface stress from the inter-particle forces using a version of the Kirkwood-Irving formula suitable for interfacial suspensions; we validate the approach by comparing against surface tension values obtained using Fordham's method (Proc. R. Soc. London, Ser. A, 1948, 194). In the parameter range simulated, the combination of parameters for which the drop does not pinch off (stable drop) gives rise to a homogeneous and isotropic surface stress; we argue that in the absence of attractive interactions the drop becomes unstable before anisotropic effects can manifest themselves. For unstable drops, stress non-uniformity and anisotropy are significant when the drop deformation and the solid area fraction are sufficiently large. Our results have implications for the dynamic deformation of structured interfaces with geometrically complex and time dependent morphologies.

Modeling the Assembly of Polymer-Grafted Nanoparticles at Oil-Water Interfaces

Using dissipative particle dynamics (DPD), I model the interfacial adsorption and self-assembly of polymer-grafted nanoparticles at a planar oil-water interface. The amphiphilic core-shell nanoparticles irreversibly adsorb to the interface and create a monolayer covering the interface. The polymer chains of the adsorbed nanoparticles are significantly deformed by surface tension to conform to the interface. I quantitatively characterize the properties of the particle-laden interface and the structure of the monolayer in detail at different surface coverages. I observe that the monolayer of particles grafted with long polymer chains undergoes an intriguing liquid-crystalline-amorphous phase transition in which the relationship between the monolayer structure and the surface tension/pressure of the interface is elucidated. Moreover, my results indicate that the amorphous state at high surface coverage is induced by the anisotropic distribution of the randomly grafted chains on each particle core, which leads to noncircular in-plane morphology formed under excluded volume effects. These studies provide a fundamental understanding of the interfacial behavior of polymer-grafted nanoparticles for achieving complete control of the adsorption and subsequent self-assembly.