Anisotropic adsorption of molecular assemblies on crystalline surfaces

Orientational order of surfactant micelles and proteins on crystalline templates has been observed but, given that the template unit cell is significantly smaller than the characteristic size of the adsorbate, this order cannot be attributed to lattice epitaxy. We interpret the template-directed orientation of rodlike molecular assemblies as arising from anisotropic van der Waals interactions between the assembly and crystalline surfaces where the anisotropic van der Waals interaction is calculated using the Lifshitz methodology. Provided the assembly is sufficiently large, substrate anisotropy provides a torque that overcomes rotational Brownian motion near the surface. The probability of a particular orientation is computed by solving a Smoluchowski equation that describes the balance between van der Waals and Brownian torques. Torque aligns both micelles and protein fibrils; the interaction energy is minimized when the assembly lies perpendicular to a symmetry axis of a crystalline substrate. Theoretical predictions agree with experiments for both hemi-cylindrical micelles and protein fibrils adsorbed on graphite.     

A theoretical approach for estimation of ultimate size of bimetallic nanocomposites synthesized in microemulsion systems

In this research a new idea for prediction of ultimate sizes of bimetallic nanocomposites synthesized in water-in-oil microemulsion system is proposed. In this method, by modifying Tabor Winterton approximation equation, an effective Hamaker constant was introduced. This effective Hamaker constant was applied in the van der Waals attractive interaction energy. The obtained effective van der Waals interaction energy was used as attractive contribution in the total interaction energy. The modified interaction energy was applied successfully to predict some bimetallic nanoparticles, at different mass fraction, synthesized in microemulsion system of dioctyl sodium sulfosuccinate (AOT)/isooctane.     

On the decisive role of finite chain extensibility and global interactions in networks

The stress-strain dependence of dry networks at unidirectional extension and compression is studied. The phenomenological van der Waals equation of state is compared with different molecular models in order to provide an interpretation of the van der Waals corrections. It is shown that the stress-strain behavior predicted by the phantom, Langevin, and constrained junction fluctuation models are altogether covered by the van der Waals approach. The relationship between the suppression of junction fluctuation parameter introduced by Dossin and Graessley and the van der Waals corrections has been worked out. The effect of junction functionality on the small strain modulus as well as on the second Mooney-Rivlin coefficient is also presented.     

Effect of Rough Nanoparticle Orientation on DLVO Energy Interaction

This study investigated the effect of the orientation of rough nanoparticles on Derjaguin and Landau, Verwey and Overbeek (DLVO) energy interaction. Rippled sphere model was used to survey van der Waals attraction energy and repulsive double-layer interaction energy between nanoparticles. The effect of particle size, asperity size, number of asperities, and concentration was studied for different orientations. Spherical coordinates were used to evaluate the effect of changes in orientation under controlled conditions. Surface element integral method was used to calculate DLVO energy interactions between rough particles at specific orientations. The DLVO energy results show that a change in orientation significantly affected the stability of the nanoparticles. The stability of dispersion varied as the contact surface between nanoparticles changed.     

Using the van der Waals broadening of the spectral atomic lines to measure the gas temperature of an argon microwave plasma at atmospheric pressure

The ro-vibrational emission spectra of the molecular species are usually used to measure the gas temperature of a discharge at atmospheric pressure. However, under some experimental conditions, it is difficult to detect them. In order to overcome this difficulty and obtain the temperature, there are methods based on the relation between the gas temperature and the van der Waals broadening of argon atomic spectral lines with a Stark contribution negligible. In this work, we propose a method based on this relation but for lines with a Stark broadening comparable with the van der Waals one.     

Communication: Identical temperature dependence of the time scales of several linear-response functions of two glass-forming liquids

The frequency-dependent dielectric constant, shear and adiabatic bulk moduli, longitudinal thermal expansion coefficient, and longitudinal specific heat have been measured for two van der Waals glass-forming liquids, tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl-4-ether. Within the experimental uncertainties the loss-peak frequencies of the measured response functions have identical temperature dependence over a range of temperatures, for which the Maxwell relaxation time varies more than nine orders of magnitude. The time scales are ordered from fastest to slowest as follows: Shear modulus, adiabatic bulk modulus, dielectric constant, longitudinal thermal expansion coefficient, and longitudinal specific heat. The ordering is discussed in light of the recent conjecture that van der Waals liquids are strongly correlating, i.e., approximate single-parameter liquids.     

Characterization and stability of TiO2 nanoparticles in industrial dye stuff effluent

Stability studies were conducted in different solutions (deionized water (DI), NaCl, CaCl₂, and MgCl₂) at different pH. Agglomeration and zeta potential were influenced by ionic strength, type of electrolyte, and the presence of dye stuff. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to analyze the stability and/or agglomeration of the nanoparticles in the different solutions. Repulsive or attractive forces stipulated by the DLVO theory were used to quantitatively discuss the results. The increase in ionic strength increased agglomeration which was linked to pH_pzc, as there were minimal electrostatic repulsions at the pzc, yet the attractive van der Waals forces were dominant. Addition of the dye stuff significantly decreased the agglomeration as the dye stuff changed the overall zeta potential of TiO₂ nanoparticles to negative across the entire pH which improved stability as there were particle–particle repulsions. Monovalent and divalent cations were compared and Ca²⁺ increased the mean diameter of nanoparticles as it effectively decreased the EDL of the nanoparticles, thus enhancing agglomeration. The DLVO theory was successful at explaining, in terms of the interaction energies between nanoparticles, the phenomena that caused either agglomeration or stability of the as-synthesized TiO₂ nanoparticles in the different solutions.     

Temperature-induced gelation in dilute nanofluids

We report a temperature-induced gelation in dilute nanofluids containing surfactant capped iron oxide and alumina particles of average diameter ~10 nm. We observe a dramatic enhancement in the elastic modulus, viscous modulus, and viscosity, by 3-6 orders of magnitude for a volume fraction (φ) less than 0.035, above a critical shear rate ( ?γ(c)) and temperature (T(c)). The T(c) follows a weak power law scaling with φ as T(c) ~ φ(β), where the scaling exponent β is found to be -0.24. The observed gel-like transition at elevated temperature is attributed to strong van der Waals attractions on the kT energy scale due to poor solvent conditions, which is reminiscent of the phase behavior reported in polymer-coated colloids.     

Colloid stability of thymine-functionalized gold nanoparticles

Gold nanoparticles surface-coated with thyminethiol derivatives containing long hydrocarbon chains have been prepared. The diameter of the particles is 2.2 and 7.0 nm, respectively, with a relatively narrow size distribution. Thyminethiol derivatives are attached to the gold particle surfaces with thymine moieties as the end groups. The colloid stability of the gold nanoparticles as a function of the type and concentration of monovalent salt, pH, and particle size was investigated in alkaline, aqueous solutions. The gold particles are stable in concentrated NaCl and KCl solutions, but are unstable in concentrated LiCl and CsCl solutions. The larger gold particles are more sensitive to salt concentration and aggregate at lower salt concentrations. The reversible aggregation and dispersion of the gold particles can be controlled by changing the solution pH. The larger gold particles can be dispersed at higher pH and aggregate faster than the smaller particles, due to stronger van der Waals forces between the larger particles. Hydration forces play an important role in stabilizing the particles under conditions where electrostatic forces are negligible. The coagulation of the gold nanoparticles is attributed to van der Waals attraction and reduced hydration repulsion in the presence of LiCl and CsCl.     

Ionic Liquids: Dissecting the Enthalpies of Vaporization

We calculate the heats of vaporisation for imidazolium‐based ionic liquids [C_nmim][NTf₂] with n=1, 2, 4, 6, 8 by means of molecular dynamics (MD) simulations and discuss their behavior with respect to temperature and the alkyl chain length. We use a force field developed recently. The different cohesive energies contributing to the overall heats of vaporisations are discussed in detail. With increasing alkyl chain length, the Coulomb contribution to the heat of vaporisation remains constant at around 80 kJ mol^?1, whereas the van der Waals interaction increases continuously. The calculated increase of about 4.7 kJ mol^?1 per CH₂‐group of the van der Waals contribution in the ionic liquid exactly coincides with the increase in the heats of vaporisation for n‐alcohols and n‐alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions.     

Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity

The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading.     

Wetting behavior of pentane on water. The analysis of temperature dependence

The temperature dependence of wetting behavior for pentane on water is analyzed from the standpoint of the Derjaguin-Frumkin theory. The joint action of two mechanisms of surface forces, the van der Waals and the image charge interactions, are considered to calculate the isotherms of the disjoining pressure. To analyze the temperature influence on the magnitude of van der Waals forces, we have used the exact Dzyaloshinsky, Lifshitz, and Pitaevsky equation. It is shown that image forces, arising due to the restricted solubility of water in pentane, decay much faster with increasing the film thickness and can be considered as short ranged in comparison to the van der Waals forces. The competitive action of the image charge and the van der Waals forces provides the plausible explanation of the temperature dependence of wetting in the system under consideration.     

长程范德华力导向作用下胶体凝聚的计算机模拟

采用计算机模拟方法研究了长程范德华力在胶体凝聚过程中的作用, 发现由于胶粒间的范德华力是长程力, 它对胶粒或团簇运动将产生导向作用. 与不考虑导向作用的扩散控制团簇凝聚(DLCA)模型比较, 这种导向作用不仅加速了胶体的凝聚过程, 而且形成了更致密、分形维数更大的结构体. 研究还发现, 长程范德华力导向作用对胶粒的初始浓度非常敏感, 不论是在凝聚物的结构还是凝聚速率方面, 只有在胶粒初始浓度较低时, 该导向作用效应才明显. 其可能的原因是,在胶粒初始浓度较高时, 由于胶粒布朗运动的平均自由程很短而且位阻效应大, 从而使导向作用效应未能反映出来.     

The Effect of van der Waals Forces on the Deposition of Highly Dispersed Aerosol Particles on Ultrafine Fibers

The effect of van der Waals forces on the collection of highly dispersed aerosol particles with ultrafine fiber filters was studied theoretically. The capture coefficient was found from the numerical solution of the equation of convective diffusion with the account of the particle size, the effect of van der Waals forces acting between a particle and a fiber, and the gas slip effect at the surface of ultrafine fibers. It was shown that allowance being made for van der Waals forces markedly affects the capture coefficient within the maximal particle penetration range and that the radius of the most penetrating particles decreases with the rising effect of these forces.     

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.     

MOLECULAR DYNAMICS SIMULATION OF THE RELAXATION OF A FULLY EXTENDED POLYETHYLENE CHAIN

Molecular dynamics simulation of the relaxation at 300 K of a fully extended polyethylene chain of800 CH_2 units has been carried out by following the changes in morphology, van der Waals energy, radius ofgyration in the sense of mechanics and gyration radius in the sense of Flory, population of trans-conformation and orientation factor. The relaxation went through three stages: (1) relaxation from themorphology of a straight rod of 100 nm length to the molphology close to a random coil of gyration radius5.9 nm in 110 ps; (2) collapse of the morphology of a coil to a highly compact globule close to a sphere ofgyration radius 1.3 nm after 178 ps as the result of intersegmental van der Waals attractive interactions; (3)lateral ordering of the folded chain segments in the globule without appreciable changes in the chaindimension up to 1600 ps, the time limit of present simulation. Nearly complete relaxation of local segmentalorientation was performed much faster than the relaxation of globule chain orientation even for a single chainof low degree of polymerization and at a temperature some 155℃above its T_g. The lateral ordering of thechain segments during the period 178 to 680 ps of the simulation time was found to obey the Avramiequation with an Avrami index of 1 .44.     

Effect of surface treatment on the Hamaker constant of intermediate modulus carbon fibers

The Hamaker constant of unsized intermediate modulus carbon fibers was evaluated by contact angle measurement following the Fowkes theory of interfacial energetics. The effects of a surface treatment (performed by the manufacturer) and an oxygen plasma treatment (performed by us), both of oxidizing nature, proved to slightly increase the value of the Hamaker constant to a level that enhanced by about 9% the Lifshitz-van der Waals contribution to the theoretical adhesion force between the fiber and an epoxy matrix. A possible explanation of the observed increase of the Hamaker constant is the higher contribution to the overall Lifshitz-van der Waals interaction of Keesom (orientation) and Debye (induction) forces.     

A van der Waals density functional study of ice Ih

Density functional theory with the van der Waals density functional (vdW-DF) is used to calculate equilibrium crystal structure, binding energy, and bulk modulus of ice Ih. It is found that although it overestimates the equilibrium volume, vdW-DF predicts accurate binding energy of ice Ih, as compared with high level quantum chemistry calculations and experiment. Inclusion of the nonlocal correlation, i.e., van der Waals interaction, leads to an overall improvement over the standard generalized gradient approximation in describing water ice.     

溶致液晶中金属纳米粒子的掺杂及其作用机制研究

在用琥珀酸二异辛酯磺酸钠(AOT)构建的具有长程有序结构的层状溶致液晶内, 用不同方式导入预制的亲油或亲水贵金属纳米粒子, 可得到纳米粒子分布在不同介观空间内的无机/有机杂合体. 依据小角X射线散射和偏光显微镜结果, 通过分析掺杂纳米粒子与液晶模板的相互作用, 对掺杂前后体系结构的变化及制得杂合体的稳定性进行了表征. 结果表明, 除考虑掺杂粒子与层状模板空间的匹配外, 体系中静电斥力、范德华引力和Helfrich涨落力之间的平衡是维持液晶结构稳定的基本条件.