Molecular simulation study on the colloidal suspension within dilute fibrous media: The effect of particle concentration on partitioning

We investigated the molecular simulations based on the Gibbs ensemble Monte Carlo method, which was previously applied to the colloidal suspensions confined with a narrow pore. The long-range electrostatic interactions between the particle and the adjacent fiber and between pairs of particles are evaluated by adopting the earlier methods. For uncharged cases, the partition coefficient with non-dilute particle concentration decreases with increases in the ratio of particle radius to fiber radius. For charged systems with constant surface charges of both the particle and the fiber, the electrostatic interactions result in a different behavior according to the variations of particle concentration. Note that the particle concentration is associated with the effects of steric exclusion as well as particle-particle interaction. By developing an effective adjusting trial, the particle concentration in the bulk region has been kept as a constant during the simulation. The predicted density profiles of colloids around a fiber indicate that, whether the particles are uncharged or charged, the particle-particle interaction plays a significant role.     

Molecular simulations of charged complex fluids: A review

Molecular simulation plays an increasingly important role in studying the properties of complex fluid systems containing charges, such as ions, piezoelectric materials, ionic liquids, ionic surfactants, polyelectrolytes, zwitterionic materials, nucleic acids, proteins, biomembranes and etc., where the electrostatic interactions are of special significance. Several methods have been available for treating the electrostatic interactions in explicit and implicit solvent models. Accurate and efficient treatment of such interactions has therefore always been one of the most challenging issues in classical molecular dynamics simulations due to their inhomogeneity and long-range characteristics. Currently, two major challenges remain in the application field of electrostatic interactions in molecular simulations; (i) improving the representation of electrostatic interactions while reducing the computational costs in molecular simulations; (ii) revealing the role of electrostatic interactions in regulating the specific properties of complex fluids. In this review, the calculation methods of electrostatic interactions, including basic principles, applicable conditions, advantages and disadvantages are summarized and compared. Subsequently, the specific role of electrostatic interactions in governing the properties and behaviors of different complex fluids is emphasized and explained. Finally, challenges and perspective on the computational study of charged systems are given.     

Experimental and MD simulation study on the physical and mechanical properties of organically modified montmorillonite clay and compatibilized linear low density polyethylene nanocomposites

Understanding the interfacial interactions plays a key role in controlling mechanical and physical properties of polymer/clay nanocomposites (PCNs). In this work, the surface interactions between constituents of experimentally prepared PCNs which are the pristine linear low density polyethylene (PE) chains, PE compatibilizers, montmorillonite clay surface layer, and surfactants were studied quantitatively by employing molecular dynamics simulation technique. The interaction energy between the polymer and the clay was found to be inversely proportional with the volume of the surfactant which decreases the electrostatic interactions between the compatibilizer and the hydrophilic clay surface. However, the van der Waals (vdW) interactions between alkyl tails of surfactants and the PE chains increase with the tail length of the surfactants. The most attractive interaction was between the surfactant's head group and the clay surface. We showed that there existed fine balance between the electrostatic and vdW type forces on the stability and the enhanced properties of the PE–organoclay nanocomposites. Calculated interaction energies were then correlated to the experimentally measured mechanical properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45817.     

Electrostatic effects on inertial particle transport in bifurcated tubes

Most aerosols found naturally in the ambient environment or those dispersed from artificial devices such as dry powder inhalers, are electrically charged. It is known that a strong electrostatic charge on aerosols can result in transport behavior dramatically different from that of uncharged aerosols, even in the absence of an external electric field. In the present work, we study pneumatic transport of corona‐charged particles in bifurcated tubes. This is accomplished by tracking the motion of discrete particles numerically under the influence of drag, gravitational, and electrostatic forces. The model aerosol is fly ash powder, whose size and charge distributions have been determined experimentally. The electrical mobility of the charged particle cloud is modeled through coulombic interactions between discrete point charges. For the case of polydispersed particles electrically charged across a distribution, the deposition efficiency was found to be greater than what is indicated by the mean charge and size. In particular, use of negatively charged fly ash powder of mean size of 2 μm and mean charge of ?1.5 C/kg led to significant increase in deposition efficiency (～29%) compared with uncharged fly ash powder of the same size distribution (～8%). Analysis of particle residence times suggests significant interaction between electrical and drag forces. These findings could have implications for pneumatic powder conveying or pulmonary drug delivery applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Zeta Potential Measurement in Catalyst Preparations

Oxide surfaces are covered with hydroxyl groups. In contact with water, positive or negative surface charges can be developed. The surface charge of oxide particles can be fine‐tuned by changing the calcination temperature of the oxides before dispersion in water or by variation of the suspension pH. Strong negative or positive surface charges stabilize a suspension and avoid particle aggregation. Nano‐structured catalysts suspended in water show surface charges different from those of compact TiO₂. For spray drying, the cationic or anionic additives used have to be strongly attached via electrostatic forces to the surface of the suspended oxide particles. When noble metal complexes have to be brought to the support surface, the positively or negatively charged complexes must have an opposite charge relative to the surface charge. Zeta potential measurements can solve these problems.     

Molecular dynamics study of TiO2/poly(acrylic acid‐co‐methyl methacrylate) and Fe3O4/polystyrene composite latex particles prepared by heterocoagulation

All‐atom molecular dynamics simulations were used to study the morphology of polymer/inorganic composite particles prepared by heterocoagulation. The results were also compared to those of our previous study of the preparation of TiO₂/poly(acrylic acid‐co‐methyl methacrylate) and Fe₃O₄/polystyrene composite particles. In the simulation system, polymer or inorganic particles were simulated by surface‐charge‐modified C₆₀ or Na atoms. Through a combination of analysis of the radial distribution functions of charged atoms and snapshots of the equilibrated structure, three kinds of particle distributions were observed under different conditions. When the polymer and inorganic particles had opposite surface charges and their sizes were very different, the composite morphology showed a core–shell structure with small particles adsorbed onto the surfaces of large particles. Furthermore, when the polymer and inorganic particles had opposite surface charges but comparable sizes, the polymer and inorganic particles aggregated domain by domain. Finally, when the polymer and inorganic particles were endowed with the same surface charge, the distribution of these two types of particles was homogeneous, regardless of their size difference. The simulation results were in agreement with the experimental results. The electrostatic interaction and the size of the particles dominated the final morphology of the composite particles when the heterocoagulation method was used. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

One-dimensional self-assembly of planar pi-conjugated molecules: adaptable building blocks for organic nanodevices

In general, fabrication of well-defined organic nanowires or nanobelts with controllable size and morphology is not as advanced as for their inorganic counterparts. Whereas inorganic nanowires are widely exploited in optoelectronic nanodevices, there remains considerable untapped potential in the one-dimensional (1D) organic materials. This Account describes our recent progress and discoveries in the field of 1D self-assembly of planar pi-conjugated molecules and their application in various nanodevices including the optical and electrical sensors. The Account is aimed at providing new insights into how to combine elements of molecular design and engineering with materials fabrication to achieve properties and functions that are desirable for nanoscale optoelectronic applications. The goal of our research program is to advance the knowledge and develop a deeper understanding in the frontier area of 1D organic nanomaterials, for which several basic questions will be addressed: (1) How can one control and optimize the molecular arrangement by modifying the molecular structure? (2) What processing factors affect self-assembly and the final morphology of the fabricated nanomaterials; how can these factors be controlled to achieve the desired 1D nanomaterials, for example, nanowires or nanobelts? (3) How do the optoelectronic properties (e.g., emission, exciton migration, and charge transport) of the assembled materials depend on the molecular arrangement and the intermolecular interactions? (4) How can the inherent optoelectronic properties of the nanomaterials be correlated with applications in sensing, switching, and other types of optoelectronic devices? The results presented demonstrate the feasibility of controlling the morphology and molecular organization of 1D organic nanomaterials. Two types of molecules have been employed to explore the 1D self-assembly and the application in optoelectronic sensing: one is perylene tetracarboxylic diimide (PTCDI, n-type) and the other is arylene ethynylene macrocycle (AEM, p-type). The materials described in this project are uniquely multifunctional, combining the properties of nanoporosity, efficient exciton migration and charge transport, and strong interfacial interaction with the guest (target) molecules. We see this combination as enabling a range of important technological applications that demand tightly coupled interaction between matter, photons, and charge. Such applications may include optical sensing, electrical sensing, and polarized emission. Particularly, the well-defined nanowires fabricated in this study represent unique systems for investigating the dimensional confinement of the optoelectronic properties of organic semiconductors, such as linearly polarized emission, dimensionally confined exciton migration, and optimal pi-electronic coupling (favorable for charge transport). Combination of these properties will make the 1D self-assembly ideal for many orientation-sensitive applications, such as polarized light-emitting diodes and flat panel displays.     

One‐Pot Route for Fe@Poly(styrene‐co‐divinylbenzene) Foam with Robust Physical/Chemical Stability and Remote Magnetic Driven Capacity for Oil Removal

Magnetic and superhydrophobic materials with robust physical/chemical stability for controllable and remote magnetic driven capacity for oil removal under harsh environments are meaningful for oil–water separation but still a challenge. Herein, an alternative strategy to address this challenge is demonstrated by decorating poly(styrene‐co‐divinylbenzene) (PSDVB) on Fe foam via one‐pot solvothermal method. Different from previous magnetic and superhydrophobic materials, Fe foam is chosen to replace Fe₃O₄ nanomaterials. Thus, complicated preparation procedures and the high cost for Fe₃O₄ nanomaterials can be avoided. Additionally, PSDVB coating provides the whole foam with robust physical/chemical stabilities: i) the surface wettability can be maintained after 50 abrasion cycles or exposed in humid air (relative humidity: 90%) for 14 days, and ii) the surface wettability does not change under different pH solutions (3 < pH < 12) or highly salty solution (NaCl 10 wt%) for 6 h. Besides, outstanding separation efficiency (>99.9%), high durability (>70 times), and excellent oil flux (16 963–75 156 L m^?2 h^?1) can be realized under gravity. Most importantly, the foam continuously removes oil from confine place (on water surface or under water) under magnetic driven force.     

Interactions between anionically modified hydroxyethyl cellulose and cationic surfactants

Macroscopic properties of aqueous solutions of several modified hydroxyethyl cellulose (HEC) samples and their interactions with cationic surfactants are studied by solubility, light scattering, electric birefringence, rheology, and surface tension measurements. Modified HEC samples carry anionic groups (an-HEC D0) and anionic and hydrophobic groups in random distribution (HM-an-HEC D1–D4). The molar substitution of anionic (an) groups is about 0.07 in all samples while that of the hydrophobic (HM) groups ranges from 0 in an-HEC D0 to 0.012 in HM-an-HEC D4. In a 1 wt% solution this corresponds to 2.7 mM anionic and 0 to 0.46 mM hydrophobic groups. In the dilute concentration range the polymers behave like typical polyelectrolytes whereas in the semi-dilute range they resemble uncharged polymers. On addition of oppositely charged surfactants the phase behavior of all polyelectrolytes is similar. With increasing surfactant concentrations the transparent solutions become turbid and the phases separate. Finally, resolubilization takes place with excess surfactant concentrations. With the HM-an-HEC compounds viscoelastic solutions are formed with cationic surfactants. The intermolecular interaction between hydrophobic parts of the polymers and the surfactants and interactions of oppositely charged ionic groups of the two components lead to formation of a temporary network with gel-like properties. With an-HEC the interaction can only take place via charges. Viscosity enhancement with increasing surfactant concentration is therefore lower with an-HEC than with HM-an-HEC compounds.     

Particle adhesion and removal in electrophotography

Particle adhesion and removal is often controlled by the interplay of electrostatic forces, related to electrical charges on the particles, and electrodynamic forces, such as those arising from van der Waals interactions. In addition, when electrostatically detaching a charged particle from a substrate, the manner in which the electric field is applied can alter the charge on the particle, thus changing both the attractive and detachment forces. The effects are clearly illustrated in the transfer of a toned image from the photoconductor in an electrophotographic engine. This paper reviews present day understanding of the interplay between electrostatic and electrodynamic interactions, as they occur within the electrophotographic process, and presents the results of previous studies in a unified manner.     

Molecular solutes in ionic liquids: a structural perspective

Understanding physicochemical properties of ionic liquids is important for their rational use in extractions, reactions, and other applications. Ionic liquids are not simple fluids: their ions are generally asymetric, flexible, with delocalized electrostatic charges, and available in a wide variety. It is difficult to capture their subtle properties with models that are too simplistic. Molecular simulation using atomistic force fields, which describe structures and interactions in detail, is an excellent tool to gain insights into their liquid-state organization, how they solvate different compounds, and what molecular factors determine their properties. The identification of certain ionic liquids as self-organized phases, with aggregated nonpolar and charged domains, provides a new way to interpret the solvation and structure of their mixtures. Many advances are the result of a successful interplay between experiment and modeling, possible in this field where none of the two methodologies had a previous advance.     

Facile synthesis of two-dimensional graphene/SnO₂ /Pt ternary hybrid nanomaterials and their catalytic properties

In this paper, we reported a simple, aqueous-phase route to the synthesis of two-dimensional graphene/SnO(2) composite nanosheets (GSCN) hybrid nanostructures consisting of 5 nm Pt nanoparticles supported on the both sides of GSCN. Functional two-dimensional GSCN were obtained through the reduction of graphene oxide (GO) using SnCl(2) in the presence of polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA). The main advantages of this preparation are that the reduction of GO, the formation of SnO(2) and the functionalization of GSCN were achieved simultaneously through one-pot reaction. GSCN/Pt ternary hybrid nanomaterials were generated by in situ reduction of negatively charged PtCl(6)(2-) precursors adsorbed on the positively charged surface of GSCN through electrostatic attraction. The as-synthesized GSCN/Pt ternary hybrid nanomaterials exhibited high cycle stabilization during the catalytic reduction of p-nitrophenol into p-aminophenol by NaBH(4). Additionally, our approach is expected to extend to other hybrid nanomaterials. We believe that the obtained GSCN/Pt ternary hybrid nanomaterials have great potential for applications in other field, such as electrochemical energy storage, sensors, and so on.     

Improving the antifouling properties of ceramic membranes via chemical grafting of organosilanes

For enhanced antifouling surface properties, the alumina membranes were modified through a simple silanization process. Three organosilanes presenting neutral, positive, and negative charges were allowed to graft onto alumina membranes. A small decrease in the pore size and the successful chemical binding of organosilanes were confirmed, respectively. The membrane filtration test using humic acid (HA) was conducted to evaluate the effect of surface charges on fouling resistance. The neutral and negatively charged membranes achieved remarkable flux behaviour due to no charge interaction and electrostatic repulsion force, respectively. Especially, the negatively charged membranes presented the lowest flux decline, the highest flux recovery, and the lowest membrane fouling.     

Oil–Water Separation Using Superhydrophobic PET Membranes Fabricated Via Simple Dip‐Coating Of PDMS–SiO2 Nanoparticles

The surfaces of commercially available polyester (PET) and polypropylene (PP) are superhydrophobically modified via the deposition of polydimethylsiloxane (PDMS)‐coated SiO₂ nanoparticles (P‐SiO₂) and PDMS binder. The adhesion of P‐SiO₂ is stronger on PET than on PP due to a stronger chemical interaction between PET and PDMS, which is attributed to the higher surface energy of PET than PP. The waterproof ability and oil separation rate of the P‐SiO₂‐coated PET (dip‐PET) membranes are studied as a function of membrane thickness, and the influence of oil viscosity on the oil separation efficiency is investigated. Optimal membrane thickness should be selected in a given environment for the facile oil–water separation and the dip‐PET membrane is chemically stable and can be used repetitively for oil–water separation. Finally, an automated prototype instrument is introduced for the dip‐coating process. It is suggested that our dip‐PET is a promising solution for oil–water separation in real‐world oil‐spill applications.     

3D Conformal Surface Engineering of Continuous Fibers with Porous Microstructures for 1D Advanced Functional Materials

One-dimensional (1D) continuous advanced functional materials and devices with inherent flexibility for complex deformations facilitate a broad range of applications in wearable technology. This communication presents a new electrostatic self-assembly strategy for controllable assembly of nanomaterials to fabricate 1D continuous materials with customizable functions based on a kind of continuous fiber fully surface-engineered with 3D conformal porous microstructures (F@3CPMs) by a unique self-assembly approach of breath figure using water microdroplet arrays. Through gently rubbing the modified fibers with suitable triboelectric materials, either positively or negatively charged F@3CPMs can be rationally prepared with adjustable triboelectric charge intensity. Besides showing superiority in incorporating desired components, such kind of F@3CPMs are demonstrated to have general applicability and enhanced performance in controllable self-assembly of polymeric, metal, and carbon nanomaterials for customizable functionalizations. Moreover, taking advantages of continuous fibers that can deform largely, functional F@3CPMs are further applied for development of 1D flexible motion sensing devices by twisting directly, which can be either used as 1D freestanding devices for straightforward integration with conventional fabrics or woven as a fabric structure integrity for a kind of self-powered interactive textiles without additional battery as power resources to detect and monitor the body motions of human beings.     

Electrostatics at the nanoscale

Electrostatic forces are amongst the most versatile interactions to mediate the assembly of nanostructured materials. Depending on experimental conditions, these forces can be long- or short-ranged, can be either attractive or repulsive, and their directionality can be controlled by the shapes of the charged nano-objects. This Review is intended to serve as a primer for experimentalists curious about the fundamentals of nanoscale electrostatics and for theorists wishing to learn about recent experimental advances in the field. Accordingly, the first portion introduces the theoretical models of electrostatic double layers and derives electrostatic interaction potentials applicable to particles of different sizes and/or shapes and under different experimental conditions. This discussion is followed by the review of the key experimental systems in which electrostatic interactions are operative. Examples include electroactive and "switchable" nanoparticles, mixtures of charged nanoparticles, nanoparticle chains, sheets, coatings, crystals, and crystals-within-crystals. Applications of these and other structures in chemical sensing and amplification are also illustrated.     

Self-Assembly of Proteinaceous Shells around Positively Charged Gold Nanomaterials Enhances Colloidal Stability in High-Ionic-Strength Buffers

The enzyme lumazine synthase (LS) has been engineered to self-assemble into hollow-shell structures that encapsulate unnatural cargo proteins through complementary electrostatic interactions. Herein, we show that a negatively supercharged LS variant can also form organic–inorganic hybrids with gold nanomaterials. Simple mixing of LS pentamers with positively charged gold nanocrystals in aqueous buffer spontaneously affords protein-shelled gold cores. The procedure works well with differently sized and shaped gold nanocrystals, and the resulting shelled complexes exhibit dramatically enhanced colloidal stability over a wide range of pH (4.0–10.0) and at high ionic strength (up to 1 m NaCl). They are even stable over days upon dilution in buffer. Self-assembly of engineered LS shells in this way offers an easy and attractive alternative to commonly used ligand-exchange methods for stabilizing inorganic nanomaterials.     

Spatially resolved electronic inhomogeneities of graphene due to subsurface charges

We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO₂ substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long-range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.     

Rational Design of Tryptophan‐Rich Antimicrobial Peptides with Enhanced Antimicrobial Activities and Specificities

Trp‐rich antimicrobial peptides play important roles in the host innate defense mechanism of many plants and animals. A series of short Trp‐rich peptides derived from the C‐terminal region of Bothrops asper myothoxin II, a Lys49 phospholipase A₂ (PLA₂), were found to reproduce the antimicrobial activities of their parent molecule. Of these peptides, KKWRWWLKALAKK—designated PEM‐2—was found to display improved activity against both Gram‐positive and Gram‐negative bacteria. To improve the antimicrobial activity of PEM‐2 for potential clinical applications further, we determined the solution structure of PEM‐2 bound to membrane‐mimetic dodecylphosphocholine (DPC) micelles by two‐dimensional NMR methods. The DPC micelle‐bound structure of PEM‐2 adopts an α‐helical conformation and the positively charged residues are clustered together to form a hydrophilic patch. The surface electrostatic potential map indicates that two of the three tryptophan residues are packed against the peptide backbone and form a hydrophobic face with Leu7, Ala9, and Leu10. A variety of biophysical and biochemical experiments, including circular dichroism, fluorescence spectroscopy, and microcalorimetry, were used to show that PEM‐2 interacted with negatively charged phospholipid vesicles and efficiently induced dye release from these vesicles, suggesting that the antimicrobial activity of PEM‐2 could be due to interactions with bacterial membranes. Potent analogues of PEM‐2 with enhanced antimicrobial and less pronounced hemolytic activities were designed with the aid of these structural studies.