Interparticle Electrostatic Repulsion Research Articles

Oversaturating Liquid Interfaces with Nanoparticle‐Surfactants

AbstractAssemblies of nanoparticles at liquid interfaces hold promise as dynamic “active” systems when there are convenient methods to drive the system out of equilibrium via crowding. To this end, we show that oversaturated assemblies of charged nanoparticles can be realized and held in that state with an external electric field. Upon removal of the field, strong interparticle repulsive forces cause a high in‐plane electrostatic pressure that is released in an explosive emulsification. We quantify the packing of the assembly as it is driven into the oversaturated state under an applied electric field. Physiochemical conditions substantially affect the intensity of the induced explosive emulsification, underscoring the crucial role of interparticle electrostatic repulsion.

Oversaturating Liquid Interfaces with Nanoparticle-Surfactants.

Assemblies of nanoparticles at liquid interfaces hold promise as dynamic "active" systems when there are convenient methods to drive the system out of equilibrium via crowding. To this end, we show that oversaturated assemblies of charged nanoparticles can be realized and held in that state with an external electric field. Upon removal of the field, strong interparticle repulsive forces cause a high in-plane electrostatic pressure that is released in an explosive emulsification. We quantify the packing of the assembly as it is driven into the oversaturated state under an applied electric field. Physiochemical conditions substantially affect the intensity of the induced explosive emulsification, underscoring the crucial role of interparticle electrostatic repulsion.

Comparison of atomic force microscopy and dynamic light scattering on the size characterization of surfactant-modified WC/Co nanoparticles dispersed in aqueous media

Tungsten carbide cobalt nanoparticles (WC/Co NPs) are extensively employed to fabricate wear-resistant parts, cutting tools, and rock drills. The colloidal stability of the WC/Co NP suspension plays a vital role in determining the mechanical properties of WC/Co-based products. In this work, dispersion of WC/Co NPs in aqueous media was conducted using surfactants (cationic, anionic, nonionic) with different concentration conditions. The z-average hydrodynamic diameters measured using dynamic light scattering (DLS) revealed the optimal surfactant for WC/Co NP dispersion: cationic hexadecyltrimethylammonium bromide (CTAB) which induces interparticle electrostatic repulsion forces at a concentration of 2 g/L. The capping behavior of CTAB was demonstrated using Zeta-potential analysis, Fourier transform infrared spectroscopy, and X-ray diffraction measurements. The mean size measurement of immobilized WC/Co NPs was also conducted using atomic force microscopy (AFM). We demonstrate a comparative protocol between number-weighted mean sizes measured using AFM (dAFM) and DLS (dDLS) in relation to the relative standard deviation (RSD) from AFM measurements. A highly linear correlation was demonstrated between dDLS/dAFM and RSD. We indicate that for polydisperse (RSD > 0.1) and non-spherical NPs, the highest correlation between dDLS and dAFM exists when the RSD is 0.37, where dDLS/dAFM is close to 1.

PH-controllable stability of iron oxide@chitosan nanocomposite-stabilized magnetic Pickering emulsions

The pH-responsiveness of iron oxide@chitosan nanocomposite (IOCS)-stabilized Pickering emulsion (PE) (denoted as IOCS-PE) was investigated. Preparation of the IOCS-PE at pH 4.0 has resulted in PE with the smallest size (1.4 ± 0.2 μm) and highest stability against deformation as compared to those prepared at alkaline conditions (pH ≥ 6.3), due to the change in IOCS surface wettability at increasing pH. To further verify the pH-dependent stability, the IOCS-PE at pH 4.0 was subjected to a simple pH variation to pH 1.5 and pH 7.4, respectively. Superior stability was observed for IOCS-PE tuned to pH 1.5 as compared to pH 7.4 over 120 min with or without magnetic field. This difference in stability could be attributed to the reduction in interparticle electrostatic repulsion stemming from the deprotonation of chitosan’s amino group and IONP’s hydroxyl group at pH 7.4. The magnetic responsiveness of the IOCS-PE was also evaluated by subjecting it to an external magnetic field, demonstrating the feasibility of retaining the IOCS-PE in a specific location of interest via magnetic attraction. In conclusion, the stability of IOCS-PE was found to be pH-dependent, and its retention time could be increased by applying an external magnetic field. The outcomes displayed the potential of the IOCS-PE as stimuli-responsive emulsions for food or pharmaceutical applications.

Amine-Functionalized Quantum Dots as a Universal Fluorescent Nanoprobe for a One-Step Loop-Mediated Isothermal Amplification Assay with Single-Copy Sensitivity.

Loop-mediated isothermal amplification (LAMP) has received considerable attention for decentralized (point-of-care and on-site) nucleic acid testing in view of its simple temperature control (60-65 °C) and short assay time (15-60 min). There remains a challenge in its wide adoption and acceptance due to the limitations of the existing amplification result reporter probes, e.g., photobleaching of organic fluorophore and reduced sensitivity of the pH-sensitive colorimetric dye. Herein, we demonstrate CdSeS/ZnS quantum dots (semiconductor fluorescent nanocrystals with superior photostability than organic fluorophore) with surface modification of cysteamine (amine-QDs) as a new reporter probe for LAMP that enabled single-copy sensitivity (limit of detection of 83 zM; 20 μL reaction volume). For a negative LAMP sample (absence of target sequence), positively charged amine-QDs remained dispersed due to interparticle electrostatic repulsion. While for a positive LAMP sample (presence of target sequence), amine-QDs became precipitated. The characterization data showed that amine-QDs were embedded in magnesium pyrophosphate crystals (generated during positive LAMP), thus leading to their coprecipitation. This amine-QD-based one-step LAMP assay advances the field of QD-based nucleic acid amplification assays in two aspects: (1) compatibility─one-step amplification and detection (versus separation of amplification and detection steps); and (2) universality─the same amine-QDs for different target sequences (versus different oligonucleotide-modified QDs for different target sequences).

Size-encoded hierarchical self-assembly of nanoparticles into chains and tubules

Hierarchical structures with sophisticated patterns allow the emergence of challenging properties. However, the highly cooperative and specific interactions needed for assembly spanning different length scales are typically lacking in inorganic nanoparticles (NPs). Here we show that size can be a common structural driving force for controlling hierarchical assembly of inorganic NPs into anisotropic superstructures. It involves first the self-limiting assembly of small CdS NPs into large supraparticles and their subsequent spontaneous organization into chains and tubules hundreds of nanometers long. Our quantitative calculations based on DLVO theory reveals an intrinsic size effect relating to the dimension change of assembly units in accordance with a negative cooperativity. It is shown that the size increase in building blocks creates an effective kinetic barrier contrast at different attachment sites due to the increase of interparticle electrostatic repulsion, switching the assembly from thermodynamically preferred 3D to kinetically favored 1D pathway. The size-encoded hierarchical assembly is accompanied by the ligand-controlled Oswald ripening process, which is responsible for the variation of hierarchical patterns from chains to tubules. The general principle in governing multistage inorganic NP ordering represents an important guideline toward the complex mesoscale structures that may surpass traditional materials in both design and functionality.

Preparation and characterization of waterborne ceramic ink with submicron-sized praseodymium-doped zirconium silicate pigment by water-based diblock polymer dispersants

Two diblock polymer dispersants (i.e., PMAA-b-AMPS and PSSS-b-GMA) were synthesized and used as water-based dispersants for dispersion and stabilization of waterborne ceramic ink with submicron-sized praseodymium-doped zirconium silicate (Pr-ZrSiO4) pigment. The color property, dispersion, and stability of the pigment particles in aqueous suspension were determined by colorimetry, laser particle size analysis, and sedimentation test. The adsorption mechanism of carbon chains of dispersants on the particle surface was analyzed by Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and electrokinetic potential measurement. Also, the viscosity, surface tension and stability of waterborne ceramic ink with the pigment particles were measured by rheometry, surface tension analysis and standing settlement. The results show that the submicron-sized pigment particles in waterborne ceramic ink show optimum dispersibility and stability at 5 wt% addition of diblock polymer dispersants PMAA-b-AMPS and PSSS-b-GMA mixed in a mass ratio of 9:1. This could be since the mixed dispersants provide the more interparticle electrostatic repulsion and steric hindrance energies due to the proper adsorption on the particle surfaces. It is indicated that adding 20 wt% of diethylene glycol monobutyl ether into the waterborne ceramic ink can effectively enhance the viscosity and reduce the surface tension, thus satisfying the applied requirements in inkjet printing. In addition, the jettability of waterborne ceramic ink on a simulated ceramic green body was also evaluated based on semi-empirical analysis.

Inversion of thermodiffusive properties of ionic colloidal dispersions in water-DMSO mixtures probed by forced Rayleigh scattering.

Thermodiffusion properties at room temperature of colloidal dispersions of hydroxyl-coated nanoparticles (NPs) are probed in water, in dimethyl sulfoxide (DMSO) and in mixtures of water and DMSO at various proportions of water, [Formula: see text]. In these polar solvents, the positive NPs superficial charge imparts the systems with a strong electrostatic interparticle repulsion, slightly decreasing from water to DMSO, which is here probed by Small Angle Neutron Scattering and Dynamic Light Scattering. However if submitted to a gradient of temperature, the NPs dispersed in water with ClO4- counterions present a thermophilic behavior, the same NPs dispersed in DMSO with the same counterions present a thermophobic behavior. Mass diffusion coefficient [Formula: see text] and Ludwig-Soret coefficient [Formula: see text] are measured as a function of NP volume fraction [Formula: see text] at various [Formula: see text]. The [Formula: see text]-dependence of [Formula: see text] is analyzed in terms of thermoelectric and thermophoretic contributions as a function of [Formula: see text]. Using two different models for evaluating the Eastman entropy of transfer of the co- and counterions in the mixtures, the single-particle thermophoretic contribution (the NP's Eastman entropy of transfer) is deduced. It is found to evolve from negative in water to positive in DMSO. It is close to zero on a large range of [Formula: see text] values, meaning that in this [Formula: see text]-range [Formula: see text] largely depends on the thermoelectric effect of free co- and counterions.

Charge Melting of Liposome Colloidal Crystals

Recently we measured the charge dependence of electrostatic interparticle repulsions in liposome colloidal crystals (Cohen, Wei & Ou-Yang, ms. in preparation). The liposomes were made from binary mixtures of charged and neutral phospholipids at various ratios. The shear modulus (rigidity) of each liposome suspension in deionized water was measured by laser-tweezer methods. As a function of increasing liposome charge, suspension rigidity first increased, peaked at ∼20% charged-lipid content, then decreased, becoming watery near 100% charged lipids. By analogy to colloidal-crystal “shear melting,” we call this phenomenon “charge melting.” The effect was calculated quantitatively by use of the Wigner-Seitz (WS) model, which requires solving the electrostatics problem of a radially symmetric charged sphere and its counterions in an electroneutral spherical WS shell whose radius R is determined by the liposome concentration. Electroneutrality means the entire H+ screening cloud surrounding each liposome is compressed into its WS cell. The compression creates an osmotic pressure related to the electrostatic potential ψ(R) at the WS-cell surface. The osmotic pressure is a measure of the interparticle interaction strength, which is what stiffens the sample. “Melting” as a function of particle charge Z arises because electroneutrality requires (a) as Z increases, so does the number of counterions, which increases osmotic pressure and ψ(R). This effect is linear in Z. (b) More counterions also cause more screening of Z, which decreases ψ(R) and osmotic pressure. This effect is exponential in Z, so it ultimately dominates. We solve the WS electrostatics problem analytically in the Debye-Hückel approximation and numerically with the full Poisson-Boltzmann equation. The problem must be solved self-consistently for a closed system with no reservoir, a counterion-only electrolyte, surface H+ binding with charge regulation, and zero electric fields at the WS-cell surface and liposome center.

Binding of Lignin Nanoparticles at Oil-Water Interfaces: An Ecofriendly Alternative to Oil Spill Recovery.

Synthetic amphiphiles used for managing large-scale oil spills have a toxic impact on the environment and marine life. Developing new oil spill recovery technologies is critical to minimize the environmental and ecological impact of such disasters. Here, we show that a mixture of lignin nanoparticles and 1-pentanol forms a biocompatible alternative to nondegradable, synthetic amphiphiles used for oil spill recovery. The pentanol in the mixture generates initial Marangoni flow and confines the spilled oil into a thick slick on the surface of water. While the alcohol solubilizes, lignin nanoparticles irreversibly adsorb onto the oil-water interface. We find that the lignin nanoparticle adsorption to the oil-water interface is governed by a combination of electrostatic, van der Waals, and hydrophobic interactions between the particles and the interface. These interactions, combined with interparticle electrostatic repulsion between nanoparticles adsorbed at the oil-water interface, drive the formation of a submonolayer. The submonolayer transforms into a film of jammed nanoparticles due to compressive stress acting on the interface upon the solubilization of pentanol. This interfacial layer of lignin nanoparticles restricts oil from respreading and locks the oil in its confined state. The herded state of the oil with the interfacial layer of nanoparticles facilitates safe removal of the spilled oil using mechanical methods. The study presents a new principle of using a mixture of heavy alcohol and biocompatible nanoparticles for oil herding applications, thus providing an ecofriendly alternative to oil spill recovery.

Temperature-Induced, Selective Assembly of Supramolecular Colloids in Water.

In this article, we report the synthesis and physical characterization of colloidal polystyrene particles that carry water-soluble supramolecular N,N′,N″,-trialkyl-benzene-1,3,5-tricarboxamides (BTAs) on their surface. These molecules are known to assemble into one-dimensional supramolecular polymers via noncovalent interactions. By tethering the BTAs to charge-stabilized particles, the clustering behavior of the resulting colloids was dictated by a balance between interparticle electrostatic repulsion and the BTA-mediated attractions. Through careful tuning of the dispersing medium’s ionic strength, a regime was found in which particle aggregation could be reversibly induced upon heating the dispersion. These findings clearly indicate that hydrophobic interactions, which become stronger upon heating, play an important role during the clustering process. Besides the thermoreversible nature of the generated hydrophobic interparticle attractions, we found the clustering to be selective, that is, the BTA-functionalized colloids do not interact with nonfunctionalized hydrophobic polystyrene particles. This selectivity in the association process can be rationalized by the preferred stacking of the surface-tethered BTAs. These selective intermolecular/particle bonds are likely stabilized by the formation of hydrogen bonds, as previously observed for analogous molecular BTA assemblies. The resulting driving force responsible for particle clustering is therefore dual in nature and depends on both hydrophobic attractions and hydrogen bonding.

Lyotropic liquid crystal behavior of carboxylated cellulose nanocrystals

In this work, the lyotropic liquid crystal (LC) behavior of carboxylated cellulose nanocrystals (CNs) has been studied for cotton cellulose prepared by TEMPO-mediated oxidation of cellulose nanocrystals (TOCNs). It is found that suspensions of TOCNs at a critical concentration of 4.1wt% form anisotropic phase with clear optical birefringence. Furthermore, fingerprint texture is clearly observed when the concentration is 9.0wt% under a polarizing optical microscope. Fingerprint texture is characteristic of chiral nematic liquid crystal (N*-LC) phase. The band-spacing corresponds to half the helical pitch, and thus, the helical pitch of N*-LCs (9.0wt%) is about 6.0μm. The degree of oxidation (DO) and zeta potential of TOCNs in suspension are 0.15 and −59.5mV, respectively. Relatively larger aspect ratio and higher surface charges density of TOCNs make the critical concentration of LC formation decreased and the helical pitch increased compared to that of the LC prepared by H2SO4 hydrolysis system. The interparticle electrostatic repulsion between CNs with different surface charges influence the helical pitch of N*-LCs.

Effect of particle/polymer number ratio on the structure and dynamics of complex between large polymer and nanoparticle

The effect of particle/polymer number ratio on the structure and dynamics of particle-polymer complexes at a fixed polymer concentration has been investigated. Mixtures of poly(ethylene oxide) (PEO, Mv=1,000,000 gmol−1, Rh ∼39nm) aqueous solution and colloidal silica dispersion (Rh ∼10nm) were used as models. By adding silica particles, both zero-shear viscosity and Rh of the silica-PEO complexes increased with particle/polymer number ratio. The maximum zero-shear viscosity was reached when the PEO coils were saturated by silica particles. After that, a decrease in zero-shear viscosity and Rh was observed. SAXS results showed that PEO had a great effect on the inter-particle correlations before the PEO chains were saturated with silica particles, however, this effect diminished when even more silica particles were introduced. The reversible bridging interaction and inter-particle electrostatic repulsion were identified as the major factors accounting for the change in zero-shear viscosity and hydrodynamic size, and a tentative mechanism was proposed based on the spatial distribution of silica particles in the mixtures.

Stable Casein‐Hydroxypropyl Cellulose Complexes at Low pH

AbstractThe effect of pH at acidic pH range on the stability of hydroxypropyl cellulose (HPC) and casein complexes was studied. Results suggested the occurrence of clear and transparent appearance of HPC‐casein complexes at pH 2.5 and 3. After storage for 3 weeks, the complexes maintained the excellent stability, as HPC molecules attached onto the surfaces of casein micelles through electrostatic interactions with strong positive charges resulted in an interparticle electrostatic repulsion, thereby preventing aggregation. The particle size, zeta potential analysis, turbidity, the protein concentration and viscosity confirmed the hypothesis. In addition, microscopic images revealed that the polysaccharide (HPC) particles adsorbed onto the surfaces of casein micelles, and casein‐HPC complexes were uniformly dispersed in the system. Moreover, the effects of the ionic strength on the stability of the complex were investigated. It was indicated that the particle size of casein‐HPC complexes at low concentration (2 mM) of NaCl remained the identical level (P > 0.05), while the particle sizes significantly were increasing at the relatively higher concentrations (50 mM and 100 mM) of NaCl (P < 0.05).Practical ApplicationsIn this study, the casein‐HPC complexes at different pH values in aqueous systems were investigated. The results illustrate that the stable casein‐HPC complexes could be formed at low pH and the stability could be maintained over the duration of 3 weeks. The findings in this study would contribute to the production of acidified milk drinks in food industry.

Dynamics of Electrically Modulated Colloidal Droplet Transport.

Electrically actuated transport dynamics of colloidal droplets, on a hydrophobic dielectric film covering an array of electrodes, is studied here. Specifically, the effects of the size and electrical properties (zeta-potential) of the colloidal particles on such transport characteristics are investigated. For the colloidal droplets, the application of an electrical voltage leads to additional attenuation of the local dielectric-droplet interfacial tension. This is due to the electrically triggered enhanced colloidal particle adsorption at the dielectric-droplet interface, in the immediate vicinity of the droplet three-phase contact line (TPCL). The extent of such interfacial particle adsorption, and hence, the extent of the consequential reduction in the interfacial tension, is dictated by the combined effects of the three-phase contact line spreading, particle size, the interfacial electrostatic interaction between the colloidal particles (if charged) and the charged dielectric surface above the activated electrode, and the interparticle electrostatic repulsion. The electrical driving force of varying magnitude, stemming from this altered solid-liquid interfacial tension gradient in the presence of the colloidal particles, culminates in different droplet transport velocity and droplet transfer frequency for different colloidal droplets. We substantiate the inferences from our experimental results by a quasi-steady state force balance model for colloidal droplet transport. We believe that the present work will provide an accurate framework for determining the optimal design and operational parameters for digital microfluidic chips handling colloidal droplets, as encountered in a plethora of applications.

Centrifugation-assisted Assembly of Colloidal Silica into Crack-Free and Transferrable Films with Tunable Crystalline Structures.

Self-assembly of colloidal particles into colloidal films has many actual and potential applications. While various strategies have been developed to direct the assembly of colloidal particles, fabrication of crack-free and transferrable colloidal film with controllable crystal structures still remains a major challenge. Here we show a centrifugation-assisted assembly of colloidal silica spheres into free-standing colloidal film by using the liquid/liquid interfaces of three immiscible phases. Through independent control of centrifugal force and interparticle electrostatic repulsion, polycrystalline, single-crystalline and quasi-amorphous structures can be readily obtained. More importantly, by dehydration of silica particles during centrifugation, the spontaneous formation of capillary water bridges between particles enables the binding and pre-shrinkage of the assembled array at the fluid interface. Thus the assembled colloidal films are not only crack-free, but also robust and flexible enough to be easily transferred on various planar and curved substrates.

Spontaneous Formation of Gold Nanoparticles with Octa(3-aminopropyl) Polyhedral Oligomeric Silsesquioxane

Abstract Mixture of octa(3-aminopropyl) polyhedral oligomeric silsesquioxane (POSS-NH2) and tetrachloroauric(III) acid under basic condition spontaneously produced gold nanoparticles (AuNPs). The amino groups reduced Au(III) cation to Au(0) and interacted with the surface of AuNPs; POSS-NH2 worked as a reductant and a stabilizer at once. Sodium hydroxide partially hydrolyzed Si–O bonds of polyhedoral oligomeric silsesquioxane (POSS) to generate SiO− species, which stabilized AuNPs via electrostatic interparticle repulsion to avoid aggregation. Addition of hydrochloric acid neutralized the amino groups of POSS-NH2, and generated NH3+ interacted with SiO− to form aggregates. That is, pH-responsive AuNPs were prepared through the facile method.

Surface Pressure Isotherm for a Monolayer of Charged Colloidal Particles at a Water/Nonpolar-Fluid Interface: Experiment and Theoretical Model

Monolayers from electrically charged micrometer-sized silica particles, spread on the air/water interface, are investigated. Because of the electrostatic repulsion, the distances between the particles are considerably greater than their diameters, i.e., we are dealing with nondensely packed interfacial layers. The electrostatic repulsion between the particles occurs through the air phase. Surface pressure vs area isotherms were measured by Langmuir trough, and the monolayers' morphology was monitored by microscope. The mean area per particle is determined by Delaunay triangulation and Voronoi diagrams. In terms of mean area, the surface pressure for monolayers from polydisperse and monodisperse particles obeys the same law. The experiments show that Π ∝ L(-3) at large L, where Π is the surface pressure and L is the mean interparticle distance. A theoretical cell model is developed, which predicts not only the aforementioned asymptotic law but also the whole Π(L) dependence. The model presumes a periodic distribution of the surface charge density, which induces a corresponding electric field in the air phase. Then, the Maxwell pressure tensor of the electric field in the air phase is calculated and integrated according to the Bakker's formula to determine the surface pressure. Thus, all collective effects from the electrostatic interparticle interactions are taken into account as well as the effects from the particle finite size. By evaporation of water, the particle monolayers are deposited on a solid substrate placed on the bottom of the trough. The electrostatic interparticle repulsion is strong enough to withstand the attractive lateral capillary immersion forces that are operative during the drying of the monolayer on the substrate. The obtained experimental results and the developed theoretical model can be useful for prediction and control of the properties of nondensely packed interfacial monolayers from charged particles that find applications for producing micropatterned surfaces.

Morphological Control in Aggregates of Amphiphilic Cylindrical Metal–Polymer “Brushes”

Controlled self-assembly of gold nanorods (AuNRs) into nanostructures of various morphologies has attracted considerable interest because it provides a high degree of freedom in tailoring the properties of the nanostructures by the coupling of the optical and electronic properties of the individual AuNRs. This paper presents a new strategy for making AuNR aggregates of tunable morphologies. In this approach, the surface of AuNRs is chemically coated with an amphiphilic diblock copolymer. The coating gives the AuNRs a cylindrical brush structure. By varying the nature of the common solvent or the interparticle electrostatic repulsion, the self-assembly of the amphiphilic cylindrical AuNR–polymer “brushes” can produce water-soluble aggregates of controllable morphologies, including single-rod ellipsoidal micelles, curved circular lamellae, and nanospheres. The AuNRs in the various aggregates generate different surface plasmon resonance (SPR) absorption patterns, with the longitudinal SPR band in the near-in...

Size‐Dependent Electrostatic Chain Growth of pH‐Sensitive Hairy Nanoparticles

Big and small, short and tall: Nanoparticles (NPs) coated with pH-sensitive polymers underwent reversible chain growth as a result of varying electrostatic interparticle repulsion as determined by the pH value. Tuning of the pH value also enabled the coupling of differently sized NPs to form linear composite chains reminiscent of block copolymers (see picture).