An analytical expression for the van der Waals interaction in oriented-attachment growth: a spherical nanoparticle and a growing cylindrical nanorod

A mathematical derivation of an analytical expression is presented to evaluate the van der Waals interaction between a sphere and a cylindrical rod. This expression then is applied to study the growth of one-dimensional nanostructures, such as nanorods, using a common growth mechanism in colloidal chemistry, the oriented attachment growth mechanism. Parameters associated with the dimensions and the separation of nanoparticles and nanorods are varied in calculations to assess their influence on the magnitude of the van der Waals interaction.     

Mechanism and modeling of nanorod formation from nanodots

A population balance model based on Smoluchowski aggregation kinetics is developed to explain the formation of nanorods from a colloidal suspension of spherical nanoparticles (nanodots). Our model shows that linear pearl-chain aggregates form by the oriented attachment (OA) of nanodots during the early stages of synthesis, since it occurs with a time scale smaller than the coalescence time scale of nanodots present within an aggregate. The slower coalescence step leads to the transformation of the linear pearl-chain aggregate into a smooth nanorod over a longer time scale of many hours, as observed in experiments. The attachment kinetics is modeled by a modified Brownian collision frequency, with the latter decreasing with nanorod length, leading to the experimentally observed slower growth in nanorod length at longer times. The collision frequency also includes the effects of attractive dipole-dipole and van der Waals interactions between nanodots, which are primarily responsible for OA. Our model predictions are general, and they compare favorably with available experimental data in the literature on the distribution of the aspect ratio (length to diameter) of ZnO and ZnS nanorods over different time scales.     

End‐to‐End Self‐Assembly of Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design

One‐dimensional (1D) self‐assemblies of nanocrystals are of interest because of their vectorial and polymer‐like dynamic properties. Herein, we report a simple method to prepare elongated assemblies of semiconductor nanorods (NRs) through end‐to‐end self‐assembly. Short‐chained water‐soluble thiols were employed as surface ligands for CdSe NRs having a wurtzite crystal structure. The site‐specific capping of NRs with these ligands rendered the surface of the NRs amphiphilic. The amphiphilic CdSe NRs self‐assembled to form elongated wires by end‐to‐end attachment driven by the hydrophobic effect operating between uncapped NR ends. The end‐to‐end assembly technique was further applied to CdS NRs and CdSe tetrapods (TPs) with a wurtzite structure.     

A novel one‐dimensional silver cylinder stabilized by mixed 2‐mercaptobenzoic acid and ethylenediamine ligands

A novel infinite one‐dimensional silver cylinder, namely poly[μ‐ethylenediamine‐μ₅‐(2‐sulfanidylbenzoato)‐μ₄‐(2‐sulfanidylbenzoato)‐tetrasilver(I)], [Ag₄(C₇H₄O₂S)₂(C₂H₈N₂)]_n, has been synthesized by one‐pot reaction of equivalent molar silver nitrate and 2‐mercaptobenzoic acid (H₂mba) in the presence of ethylenediamine (eda). One Ag atom is located in an AgS₂NO four‐coordinated tetrahedral geometry, two other Ag atoms are in an AgS₂O three‐coordinated T‐shaped geometry and the fourth Ag atom is in an AgSNO coordination environment. The two mba ligands show two different binding modes. The μ₂‐N:N′‐eda ligand, acting as a bridge, combines with mba ligands to extend the Ag^I ions into a one‐dimensional silver cylinder incorporating abundant Ag...Ag interactions ranging from 2.9298 (11) to 3.2165 (13) Å. Interchain N—H...O hydrogen bonds extend the one‐dimensional cylinder into an undulating two‐dimensional sheet, which is further packed into a three‐dimensional supramolecular framework by van der Waals interactions; no π–π interactions were observed in the crystal structure.     

Ion‐Free and Ion‐Pairing Assemblies of Anion‐Responsive π‐Electronic Systems Possessing Directly Linked Alkyl Chains

Alkyl‐substituted pyrrole‐based anion‐responsive π‐electronic systems formed supramolecular gels and liquid crystals through effective π–π stacking and van der Waals interactions. The addition of chloride as a planar cation salt afforded ion‐pairing assemblies as soft materials comprising planar receptor‐Cl^? complexes and the cation.     

Self‐Sorting of Two Hydrocarbon Receptors with One Carbonaceous Ligand

Non‐directional van der Waals forces in biological and synthetic supramolecular systems play important roles in molecular assembly, particularly in determining the distances of the interacting species. The van der Waals forces are normally used in combination with other directional forces and are considered to play a secondary role in achieving specificity and fidelity in molecular recognition. Using an ideal supramolecular system consisting solely of hydrogen and carbon atoms, we found that the van der Waals interactions enable the high‐fidelity sorting of two homomeric receptors during ligand‐induced assembly. The self‐sorting occurred in a narcissistic manner by repulsion of a competing diastereoisomeric receptor from the assembly. The structure–sorting relationship study with enantiomers further revealed the dominant role of the van der Waals forces in shape recognition for high‐fidelity self‐sorting.     

Wide‐Range Light‐Harvesting Donor–Acceptor Assemblies through Specific Intergelator Interactions via Self‐Assembly

We have synthesized two new low‐molecular‐mass organogelators based on tri‐p‐phenylene vinylene derivatives, one of which could be designated as the donor whereas the other one is an acceptor. These were prepared specifically to show the intergelator interactions at the molecular level by using donor–acceptor self‐assembly to achieve appropriate control over their macroscopic properties. Intermolecular hydrogen‐bonding, π‐stacking, and van der Waals interactions operate for both the individual components and the mixtures, leading to the formation of gels in the chosen organic solvents. Evidence for intergelator interactions was acquired from various spectroscopic, microscopic, thermal, and mechanical investigations. Due to the photochromic nature of these molecules, interesting photophysical properties, such as solvatochromism and J‐type aggregation, were clearly observed. An efficient energy transfer was exhibited by the mixture of donor–acceptor assemblies. An array of four chromophores was built up by inclusion of two known dyes (anthracene and rhodamine 6G) for the energy‐transfer studies. Interestingly, an energy‐transfer cascade was observed in the assembly of four chromophores in a particular order (anthracene‐donor‐acceptor‐rhodamine 6G), and if one of the components was removed from the assembly the energy transfer process was discontinued. This allowed the build up of a light‐harvesting process with a wide range. Excitation at one end produces an emission at the other end of the assembly.     

Nonadditivity in van der Waals interactions within multilayers

Working at the macroscopic continuum level, we investigate effective van der Waals interactions between two layers within a multilayer assembly. By comparing the pair interactions between two layers with effective pair interactions within an assembly we assess the significant consequences of nonadditivity of van der Waals interactions. This allows us to evaluate the best numerical estimate to date for the Hamaker coefficient of van der Waals interactions in lipid-water multilamellar systems.     

A novel one‐dimensional metal–organic framework with a μ‐cyanido‐argentate group: catena‐poly[[(5,5′‐dimethyl‐2,2′‐bipyridyl‐κ2N,N′)silver(I)]‐μ‐cyanido‐κ2N:C]

The design and synthesis of metal coordination and supramolecular frameworks containing N‐donor ligands and dicyanidoargentate units is of interest due to their potential applications in the fields of molecular magnetism, catalysis, nonlinear optics and luminescence. In the design and synthesis of extended frameworks, supramolecular interactions, such as hydrogen bonding, π–π stacking and van der Waals interactions, have been exploited for molecular recognition associated with biological activity and for the engineering of molecular solids.The title compound, [Ag(CN)(C₁₂H₁₂N₂)]_n, crystallizes with the Ag^I cation on a twofold axis, half a cyanide ligand disordered about a centre of inversion and half a twofold‐symmetric 5,5′‐dimethyl‐2,2′‐bipyridine (5,5′‐dmbpy) ligand in the asymmetric unit. Each Ag^I cation exhibits a distorted tetrahedral geometry; the coordination environment comprises one C(N) atom and one N(C) atom from substitutionally disordered cyanide bridging ligands, and two N atoms from a bidentate chelating 5,5′‐dmbpy ligand. The cyanide ligand links adjacent Ag^I cations to generate a one‐dimensional zigzag chain. These chains are linked together via weak nonclassical intermolecular interactions, generating a two‐dimensional supramolecular network.     

Van der Waals Emulsions: Emulsions Stabilized by Surface‐Inactive,Hydrophilic Particles via van der Waals Attraction

Surface‐inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil‐in‐water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil–water interfaces when repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by van der Waals attraction between particle monolayer shells and oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by van der Waals attraction between single particles and oil droplets, which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion‐based technologies in practice.     

Van der Waals Emulsions: Emulsions Stabilized by Surface‐Inactive,Hydrophilic Particles via van der Waals Attraction

Surface‐inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil‐in‐water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil–water interfaces when repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by van der Waals attraction between particle monolayer shells and oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by van der Waals attraction between single particles and oil droplets, which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion‐based technologies in practice.     

Determination of van der Waals forces in monolayer films of lipids & biopolymers. Equation of state for two-dimensional films

Amphiphile molecules are characterized by the dual property arising from the interactions between the apolar [alkyl] and the polar part and the surrounding solvent, i.e., water. In assemblies which amphiphiles form in diverse systems, e.g., micelles, soap bubbles, monolayers or bilayers at interfaces, the attractive forces are attributed to the van der Waals forces. It is not easy to estimate the magnitude of van der Waals forces in some of these systems by any direct method.The magnitude of van der Waals forces in spread monolayers of lipids and biopolymers has been reported to be estimated from experimental data. The magnitude of these forces has been estimated by using an equation of state of a very general form, as delineated herein. In the current literature no such attempt has been reported in the analyses of these monolayers spread on aqueous surfaces. These analyses suggest that the predominant surface forces arise from van der Waals interactions, if the magnitude of electrostatic charge repulsions is weak. The equation-of-state as derived indicates that it is useful in providing information about the molecular interaction in monolayers, for both lipids and biopolymers.     

Formation of orientation-ordered superlattices of magnetite magnetic nanocrystals from shape-segregated self-assemblies

Magnetic magnetite (Fe3O4) nanocrystals have been synthesized by combining nonhydrolytic reaction with seed-mediated growth. The shape of these magnetite nanocrystals can be controlled either as pure spheres or a mixture of mainly faceted nanocrystals. Faceted magnetite nanocrystals consist of truncated tetrahedral platelets (TTPs), truncated octahedrons (TOs), and octahedrons (OTs). Transmission electron microscopy analysis indicates that the faceted nanocrystal mixture tends to self-segregate based upon the shape in a self-assembly process, and each shape forms its own distinct crystallographic orientation-ordered superlattice assemblies. Self-assemblies of the Fe3O4 nanocrystals in the shapes of TTP, TO, and OT show hexagonal, primitive cubic, and distorted body-centered cubic (bcc) superlattice structures, respectively. The possible mechanism for the formation of different superstructures is attributed to van der Waals interactions. Nanocrystals with different shapes provide diverse building blocks for bottom-up approaches in building nano- and mesosystems. Furthermore, the self-segregation phenomenon of different shaped nanocrystals in self-assembly processes could be very important in envisioning efficient assembly strategies for nanoscience- and nanotechnology-based devices.     

catena‐Poly[[[μ‐1,3‐bis(diphenylphosphanyl)propane‐κ2P:P′][O‐ethyl (4‐methoxyphenyl)phosphonodithioato‐κ2S,S′]silver(I)] chloroform monosolvate]

Reaction of a mixture of AgOAc, Lawesson's reagent [2,4‐bis(4‐methoxyphenyl)‐1,3‐dithiadiphosphetane‐2,4‐disulfide] and 1,3‐bis(diphenylphosphanyl)propane (dppp) under ultrasonic treatment gave the title compound, {[Ag(C₉H₁₂O₂PS₂)(C₂₇H₂₆P₂)]·CHCl₃}_n, a novel one‐dimensional chain based on the in situ‐generated bipodal ligand [ArP(OEt)S₂]⁻ (Ar = 4‐methoxyphenyl). The compound consists of bidentate bridging 1,3‐bis(diphenylphosphanyl)propane (dppp) and in situ‐generated bidentate chelating [ArP(OEt)S₂]⁻ ligands. The dppp ligand links the [Ag{ArP(OEt)S₂}] subunit to form an achiral one‐dimensional infinite chain. These achiral chains are packed into chiral crystals by virtue of van der Waals interactions. No π–π interactions are observed in the crystal structure.     

Concentration induced the interfacial self‐assembly polymorphism of 4, 4′‐dihexadecyloxy‐benzophenon by scanning tunneling microscopy

The concentration effect on a two‐dimensional (2D) self‐assembly of 4, 4′‐dihexadecyloxy‐benzophenon (DHB) has been investigated by scanning tunneling microscopy. The self‐assembly of DHB at the phenyloctane/graphite interface was concentration dependent. Under low concentration, the DHB molecules were adsorbed intactly on the graphite surface. With the increasing of concentration, one of side chains connecting the conjugated moiety stretched into the liquid phase. The coexistence of two self‐assembled structures was observed in a moderate concentration. The result indicated that the van der Waals interactions between the molecules and the graphite lattice were decreasing with the increasing concentration. After the samples were placed in ambient conditions over 24 h, a different self‐assembled structure was obtained on the gas/solid interface, in which the DHB molecules were adsorbed on the surface with only one of the side chains. Both the benzophenon core and the other side chain were extended to the gas phase. The results demonstrated that concentration played an important role in forming the 2D molecular self‐assembly and provided an efficient approach for the control of the DHB molecular nanostructure. Copyright © 2013 John Wiley & Sons, Ltd.     

Van der Waals surface graphs and molecular shape

A van der Waals surface graph is the graph defined on a van der Waals surface by the intersections of the atomic van der Waals spheres. A van der Waals shape graph has a vertex for each atom with a visible face on the van der Waals surface, and edges between vertices representing atoms with adjacent faces on the van der Waals surface. These are discrete invariants of three‐dimensional molecular shape. Some basic properties of van der Waals surface graphs are studied, including their relationship with the Voronoi diagram of the atom centres, and a class of molecular embeddings is identified for which the dual of the van der Waals surface graph coincides with the van der Waals shape graph. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trajectory Analysis and Collision Efficiency during Microbubble Flotation

The hydrodynamic interaction between a rising bubble and a sedimenting particle during microbubble flotation is considered. The effects of attractive van der Waals forces and attractive or repulsive electrostatic forces are included. A mathematical model is presented which is used to perform a trajectory analysis and to calculate collision efficiencies between the bubble and particle. It is shown that collision efficiencies and the nature of the bubble-particle interactions are strongly dependent on the relative strengths of the van der Waals and electrostatic forces and on the lengthscales over which these forces act. It is demonstrated that optimal operating conditions can be suggested to achieve efficient microbubble flotation by correctly accounting for the interaction of van der Waals, electrostatic, and hydrodynamic forces. Copyright 1999 Academic Press.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

取代烷基侧链长度对二维纳米结构的影响

合成了一系列烷基取代的间苯三酚衍生物,并在大气条件下用扫描隧道显微镜研究了它们在高定向裂解石墨表面的吸附和组装行为.实验结果表明,这些自组装分子具有条状结构特征.在链长较短的分子图像中,两条平行的烷氧基链肩并肩地排列在苯环的一侧,另一条烷氧基链则排列在苯环的另一侧,链与链之间彼此相互交错排列形成均一的烷基条带.当链长增加时,这种高稳定性和密排结构遭到破坏,出现单个分子和分子对共存的组装结构.这是由于烷基链与烷基链之间以及烷基链与基底之间的作用力共同决定的.通过调控分子烷基链的长度可以得到不同的表面二维纳米结构.     

Directing the self-assembly of nanocrystals beyond colloidal crystallization

This article gives an overview of recent progress in the self-assembly of nanocrystals. Classic self-assembly of nanocrystals, so-called colloidal crystallization driven by van der Waals interactions, is highlighted first with an emphasis on the recent realization of binary colloidal crystals. Next, new developments in the integration of nanocrystals into clusters based on electrostatic interactions, hydrogen bonding and dipole-dipole interactions are summarized, shedding light on the defined control of the interactions between the nanocrystals. Finally, the fabrication of heterogenous nanocrystals, obtained via either phase selective modification at the water/oil interface or facet-selective crystal growth on non-spherical nanocrystals is discussed. These last materials may provide significant building blocks for mimicking molecular self-assembly.