Concentration-Dependent Sedimentation of Stable Magnetic Dispersions

The concentration dependence of the sedimentation coefficient has been studied theoretically for a stable magnetic colloidal system under the influence of an external homogeneous magnetic field. The physical model of the magnetic colloid given by Donselaar et al. (1997, Langmuir 13, 6018) was adopted. It was found that the magnetic interaction accelerated the sedimentation of the particles. The stronger the external magnetic field is, the quicker the sedimentation of the particles. For sterically stabilized particles frequently used in experiments to simulate hard spheres, the combined influence of the magnetic attraction and van der Waals attraction on the sedimentation is also analyzed. Copyright 1999 Academic Press.     

EFFECT OF WEAK INTERACTIONS ON PHENOL ADSORPTION FROM AQUEOUS SOLUTIONS BY AMINATED POLYMERIC ADSORBENTS

Adsorption behaviors of phenol from aqueous solutions have been investigated in batch systems at 303 K and 318 K respectively, using hypercrosslinked polymeric adsorbent （CHA111）, aminated hypercrosslinked polymeric adsorbents （NDA101, NDA103, NDA105） and weakly basic polymeric adsorbent （D301） with a view to studying the effect of hydrogen bonding and Van der Waals interactions between adsorbate and the adsorbent. All adsorption isotherms can be well fitted by Langmuir and Freundlich equations. Compared with D301 driven by hydrogen bonding interaction only and CHA111 driven by Van der Waals interaction only, phenol adsorption on aminated adsorbents driven by both hydrogen bonding and Van der Waals interactions were apparently different, i.e., negative effect for NDA105, positive effect for NDA101 and synergistic effect for NDA103. In this synergistic action, some weak interactions would contribute more or less to the adsorption than they work individually.     

Theoretical studies on the ionization potential of interacting atoms at large separations

The theory of the one-particle Green's function is applied to calculations of the ionization potential of interacting atoms which are at large separations. Equations for the ionization potential involve terms which relate to Van der Waals interactions between separated atoms and long-range interactions between an atom and an ion. Numerical calculations of the ionization potential of two hydrogen atoms and two helium atoms at large separations are performed. Applications to the ionization potentials of weakly-interacting Van der Waals molecules (NeAr, NeKr, NeXe) are also reported.     

Stacking faults in a layered cobalt tellurium phosphate oxochloride

The new compound Co₂Te₃(PO₄)O₆Cl was synthesized by chemical reactions in a sealed and evacuated silica tube. The crystal structure was solved from single crystal diffraction data and is made up by charge neutral layers. Within the layers two types of chains are made up by edge sharing [CoO₆] and [CoO₅Cl] polyhedra respectively. The chains are separated by tellurium oxide and phosphate building blocks. There are only weak Van der Waals interactions in between the layers and severe diffuse scattering is observed due to faulted stacking of the layers. Structure solutions in a P-1 triclinic cell and a larger monoclinic cell in P2₁/c are discussed and compared to a computer generated model. The reasons for the stacking faults may be due to that there are two positions available for each layer that results in similar connectivity to the next layer in addition to the relatively wide channels in between the layers that reduce the Van der Waals interactions in between them.     

两个氢、氮、氧或氯原子间的Van der Waals能

根据气体的VanderWaals常数和两个氢原子间的VanderWaals能之理论值,计算了两个氢、氮、氧或氯原子间的VanderWaals能。     

The electronic states of naphthalene molecules adsorbed on a zeolite

Physical principles are considered for this method of examining interactions. Changes in absorption and fluorescence spectra are examined in relation to Van der Waals, electrostatic, and exchange interactions. Major spectral criteria are presented for the types of interaction and are illustrated by reference to naphthalene adsorbed on zeolites. Spectra at 77 K show that the adsorbed molecules are in two different states: in one they are bound by Van der Waals forces, while in the other a charge-transfer complex is formed. Examination at intervals after preparation shows that the adsorbed naphthalene is gradually oxidized.     

Van der Waals interactions between surfaces of biological interest

Microscopic and macroscopic approaches to calculations of long-range Van der Waals interactions between bodies are reviewed. Expressions are presented for various geometries, including planar-layered structures, sheathed spheres and rods. Retardation effects are shown to reduce dispersion interactions in a similar fashion in both approaches. Pair summation procedure gives 10–25% greater values of dispersion interactions than the macroscopic approach. Orientation effects, previously neglected in microscopic approaches are strongly dependent on many-body effects. When orientation effects are included in a pair summation procedure, its calculated values are close to those calculated with the macroscopic approach.Experimentally determined force values are in agreement with calculated ones for distances of separation above 15 Å in vacuum. In general, the theory is insufficient for yielding forces at distances of separation below 20 Å in water.Determination of Van der Waals parameters from refractive indices of pure liquids and solutions is described. Within 5%, dispersion coefficients are independent of concentration of solution, and isotropic electronic polarizabilities agree with those obtained by the addition of bond polarizabilities. Van der Waals parameters of several major components of cellular surfaces and intercellular media are arranged according to an ascending sequence: water<alkanes<phospholipids<proteins and cholesterol<sugars.Variations in compositions, distances of separation, and layer thicknesses are considered in the calculation of the interactions between cellular surfaces, both in planar and spherical systems, including phospholipid vesicles. In planar-cellular systems, Hamaker coefficients vary between 4 x 10^-15 and 6 x 10^-14 erg; at 50 Å distance of separation the free energies and forces are 210 to 1600 kT/μm², and 4 x 10^-5 to 3 x 10^-4 dynes/μm² respectively. The total potential curve, including electrostatic interactions, is calculated and the questions of cellular adhesion and fusion of phospholipid vesicles are discussed.     

取代芳烃水中溶解度的测定与估算

本文应用线性溶剂化能相关法回归分析了多种有机有机物的溶解度与范德华体积、Ｌｅｗｉｓ酸性及Ｌｅｗｉｓ碱性之间的相关性，结果表明，溶解度与范德华体积、Ｌｅｗｉｓ酸性及碱性之间呈良好的相关性，用这种方法估算的溶解度值与测定吻合得较好。     

Optimized Liquid-Phase Exfoliation of Magnetic van der Waals Heterostructures: Towards the Single Layer and Deterministic Fabrication of Devices

Van der Waals magnetic materials are promising candidates for spintronics and testbeds for exotic magnetic phenomena in low dimensions. The two-dimensional (2D) limit in these materials is typically reached by mechanically breaking the van der Waals interactions between layers. Alternative approaches to producing large amounts of flakes rely on wet methods such as liquid-phase exfoliation (LPE). Here, we report an optimized route for obtaining monolayers of magnetic cylindrite by LPE. We show that the selection of exfoliation times is the determining factor in producing a statistically significant amount of monolayers while keeping relatively big flake areas (~1 µm²). We show that the cylindrite lattice is preserved in the flakes after LPE. To study the electron transport properties, we have fabricated field-effect transistors based on LPE cylindrite. Flakes are deterministically positioned between nanoscale electrodes by dielectrophoresis. We show that dielectrophoresis can selectively move the larger flakes into the devices. Cylindrite nanoscale flakes present a p-doped semiconducting behaviour, in agreement with the mechanically exfoliated counterparts. Alternating current (AC) admittance spectroscopy sheds light on the role played by potential barriers between different flakes in terms of electron transport properties. The present large-scale exfoliation and device fabrication strategy can be extrapolated to other families of magnetic materials.     

Optical Trapping of Titania/Silica Core-Shell Colloidal Particles

Manipulation of colloidal systems via optical trapping techniques requires a refractive index mismatch between particles and solvent which leads to strong interparticle van der Waals interactions. Investigation of the behavior of systems without such strong attractive interactions, however, requires the uncoupling of particle refractive index and particle-particle interactions. To accomplish this, the synthesis of core-shell titania/silica particles has been performed. By index matching a silica shell on a titania core using a mixture of toluene and propanol, the van der Waals interactions between particles can be minimized. Due to the mismatch of the refractive index between the solvent and titania core, however, a strong trapping force can be generated, making optical manipulation feasible. In order to confirm that the silica shell was indeed matched, pure silica particles were synthesized by the method of St?ber (1968) and added to the core-shell system. In these mixed systems of core-shell and pure silica particles in silica-index-matching solvents, only the core-shell particles were trappable. Copyright 2000 Academic Press.     

Phase behavior of magnetic nanoparticles dispersions in bulk and confined geometries

Dispersions of magnetic nanoparticles (ferrofluids) have been for a long time taken as examples of dipolar fluids. Theoretical papers conclude now that an important parameter is the ratio of the anisotropic attractions (dipolar ones) to the isotropic attractions (Van der Waals ones). It is confirmed by the recent experimental results concerning the behavior of small magnetic particles in bulk or confined 2D geometries.     

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.     

Thermodynamic characteristics of adsorption of some 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from water-acetonitrile solutions on phenyl-bonded silica gel

Standard differential molar changes of the Gibbs energy, enthalpy, and entropy during adsorption in the Henry region of 17 derivatives of 1,3,4-oxadiazole and 1,2,4,5-tetrazine from water-acetonitrile solutions with acetonitrile concentrations of 20 to 80 vol % and a variation step of 5 vol % on silica gel with grafted phenyl groups at column temperatures of 313.15 to 333.15 K with at a step of 5 K are determined under dynamic conditions by high-performance liquid chromatography. The dependence of the thermodynamic characteristics of adsorption on the molecular structure of adsorbates is discussed. It is established that the absolute values of the changes in the Gibbs energy and enthalpy during adsorption as a whole increase with increasing Van der Waals surface area and volume of the adsorbate molecules, while the adsorption of most 1,3,4-oxadiazoles is characterized by a dependence close to isentropic. It is assumed that the determining role in the change of entropy during adsorption from solutions is played by the polarity of the adsorbate molecule and the conformational flexibility of its substituents.     

Simulating van der Waals interactions in water/hydrocarbon-based complex fluids

In systems composed of water and hydrocarbons, Van der Waals interactions are dominated by the nonretarded, classical (Keesom) part of the Lifshitz interaction; the interaction is screened by salt and extends over mesoscopic distances of the order of the size of the (micellar) constituents of complex fluids. We show that these interactions are included intrinsically in a recently introduced local Monte Carlo algorithm for simulating electrostatic interactions between charges in the presence of nonhomogeneous dielectric media.     

Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.     

Online stacking of carboxylated magnetite core–shell nanoparticles in capillary electrophoresis

The stacking effect on carboxylated magnetite core–shell nanoparticles using sodium borate buffer pH 9.5 as the background electrolyte is presented. The ionic strength of the background electrolyte ranged from 5 to 100 mM, and the ionic strength of a sample zone ranged from 5 to 100 mM. Moreover, water was used as the sample dispersant. Both stacking and de‐stacking effects were observed when conductivities of the sample zone and the background electrolyte differed. An explanation of carboxylated magnetic core‐shell nanoparticles behavior was suggested based on the Derjaguin–Landau–Verwey–Overbeek theory supposing that the aggregation point is defined by the energetic barrier as the sum of energies given by electrostatic interactions and Van der Waals interactions. Moreover, the stacking conditions were applied for the evaluation of the lowest detectable dilution of magnetic nanoparticles. The carboxylated magnetic nanoparticles were dispersed in 10 mM borate/NaOH pH 9.5 and injected for 60 s to the background electrolyte composed of 100 mM borate/NaOH pH 9.5 that allowed the detection of 100‐fold diluted nanoparticles.     

Force interactions of porous silica glass microspheres against mirror-polished stainless steel in nonaqueous solvents

Force interactions of porous silica particles against mirror-polished stainless steel surfaces were quantified in the presence of various solvents to facilitate processing of ceramics with less reliance on organic aids which subsequently need to be burned off. The results were compared to and found to be in good agreement to idealized models of van der Waals force interactions. Significantly, van der Waals attractive forces between steel surfaces and silica surfaces were minimized through the use of tetrahydrofuran and enhanced using methanol. The solvent selections were further extended to settling behavior and were found to follow the general trends determined by Stokes law. The methods presented herein can be extended to other real-world systems.     

Biomimetic particles: optimization of phospholipid bilayer coverage on silica and colloid stabilization

The effect of ionic strength and pH on phosphatidylcholine (PC) adsorption from vesicles on silica nanoparticles was investigated over a range of NaCl concentrations (0.1-150 mM) at pH 6.3 and 7.4 from determination of adsorption isotherms, colloid stability, particle sizing, and zeta-potentials. At and above 10 mM ionic strength, pH 6.3, high-affinity adsorption isotherms with limiting adsorption indicative of one-bilayer deposition on each silica particle were obtained. At 10 mM ionic strength, adsorption isotherms indicated lower affinity between PC and silica at pH 7.4 than at pH 6.3, suggesting a role of hydrogen bonding between silanol on silica and phosphate on PC in promoting bilayer deposition at low pH. Under conditions where high affinity and bilayer deposition were achieved, silica sedimentation documented from photographs was absent, suggesting particle stabilization induced by bilayer coverage. However, at physiological (150 mM NaCl) or close to physiological ionic strength (140 mM NaCl), the large colloid stability similarly achieved at pH 6.3 or 7.4 suggested the major role of van der Waals attraction between the PC bilayer vesicle and silica particle in determining bilayer deposition. The effect of increasing ionic strength was increasing van der Waals attraction, which caused PC vesicle disruption with bilayer deposition and bilayer-induced silica stabilization.     

Chromatographic separation and molecular modelling of triazines with respect to their inhibition of the growth of L1210/R71 cells.

The potential anti-cancer activity of triazines was characterized by the inhibition of the growth of L1210/R71 cells. The retention times for fifteen triazine derivatives were measured by high-performance liquid chromatography on octyl silica and silica gel columns. The slope and intercept values of the plot of the logarithmic capacity factor versus acetonitrile concentration were calculated from the reversed-phase retention measurements. The adsorption properties of the compounds were characterized by the retention data obtained on silica gel columns using high and low concentrations of ammonium salts in the hydro-organic mobile phase. The non-polar, non-polar unsaturated and polar surface areas, the surface energies, the dipole moments and the Van der Waals radii of the molecules were calculated from their chemical structures after energy minimization on the basis of molecular mechanics. Correlation analysis of these parameters showed that the inhibitory effect is dependent on the polar and non-polar surface areas of the molecules. The reversed-phase slope showed a significant correlation with the difference between the accessible and the total non-polar surface areas of the compounds, whereas the intercept values correlated with the non-polar accessible surface area. The adsorption properties of the triazines on silica gel cannot be described by the molecular parameters investigated here.     

Van der Waals Radii of Hydrogen in Gas-Phase and Condensed Molecules

Van der Waalsa radii of hydrogen in the different gas-phase and condensed molecules are determined and shown that a value of the van der Waals radius depends on the effective charge of the H atom. Is described also the van der Waals anisotropy of H in some molecules.