Structure of simple liquids as a problem of percolation in the Voronoi grid

Institute for Chemical Cybernetics and Combustion, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 2, pp. 98–105, March–April, 1989.     

Adiabatic polarization energy in a simple dense fluid

In this note we prey300505 sent the results of a calculation of the adiabatic electrostatic polarization energy, P⁺, of Xe⁺ in fluid Ar over the density range 0.1–1.4 g cm^?3. P₊ was expressed in terms of Lekner's screening function which, within the framework of the Kirkwood approximation, is determined by the (neutral) solute-solvent and the solvent-solvent pair correlation functions. The density dependence of P₊ can be quite well approximated (within ≈10%) by the Born charging energy with the effective ionic radius being identified with the effective hard-core diameter for the (neutral) solute-solvent separation. The P₊ data, together with experimental spectroscope results for the density dependence of the ionization potential of Xe and of CH₃I, result in preliminary information concerning the ground state energy of the quasi-free electron in fluid Ar over a wide density range.     

Odd-even staggering in simple models of metal clusters

The odd-even staggering of free-electron metal clusters is studied using several simple models: Noninter-acting electrons in a rectangular box, triaxial harmonic oscillator, and Hückel model. Finite temperature effects are studied using the Monte Carlo method. All the models show qualitatively similar odd-even staggering. In the ground state the HOMO-LUMO gap is larger than the neighbouring energy gaps. The reduction of the odd-even staggering due to exchange and correlation is studied using the local-spin-density approximation.     

Clustering versus percolation in the assembly of colloids coated with long DNA

We report an experimental study in which we compare the self-assembly of 1 mum colloids bridged through hybridization of complementary single-stranded DNA (ssDNA) strands (12 bp) attached to variable-length double-stranded DNA spacers that are grafted to the colloids. We considered three different spacer lengths: long spacers (48 500 bp), intermediate length spacers (7500 bp), and no spacers (in which case the ssDNA strands were directly grafted to the colloids). In all three cases, the same ssDNA pairs were used. However, confocal microscopy revealed that the aggregation behavior is very different. Upon cooling, the colloids coated with short and intermediate length DNAs undergo a phase transition to a dense amorphous phase that undergoes structural arrest shortly after percolation. In contrast, the colloids coated with the longest DNA systematically form finite-sized clusters. We speculate that the difference is due to the fact that very long DNA can easily be stretched by the amount needed to make only intracluster bonds, and in contrast, colloids coated with shorter DNA always contain free binding sites on the outside of a cluster. The grafting density of the DNA decreases strongly with increasing spacer length. This is reflected in a difference in the temperature dependence of the aggregates: for the two systems coated with long DNA, the resulting aggregates were stable against heating, whereas the colloids coated with ssDNA alone would dissociate upon heating.     

Metal clusters on an inert surface: a simple model

The shapes of metal clusters (with 2 to 14 valence electrons) on an inert surface are studied with a simple model based on the ultimate jellium model. It is shown that within certain approximations the surface-cluster interaction can be described with an external potential in the Kohn-Sham method. No restrictions for the cluster geometry are imposed. The results show that depending on the strength of the interaction and on the size of the cluster, the ground state is either planar or three-dimensional, but in many cases both geometries are stable and there is a marked energy barrier between them. The results agree qualitatively with ab initio calculations of Na clusters on a NaCl(100) surface.     

Analysis of the odd-even alternation in simple metal clusters

A set of MO-LCAO calculations within the LSDA formalism has been performed for the analysis of the odd-even alternation in simple metal clusters. Electronic properties, including ionization potentials and partial density of states analyses were evaluated for clusters of Na, K, Cu and Ag ranging from two to nine atoms. The present study focus on the differences in magnitude of the odd-even alternation, which is attributed to the electronic level separation close to the Fermi level of the clusters. For the coinage metals, the hybridization between s, d and p states is shown to strongly influence the alternation, reducing the magnitude for copper to about the same value as for silver. The small reduction of the alternation magnitude due to a finite spin density for the odd clusters is also investigated by means of comparative LDA calculations.     

Local pressure tensor of a simple fluid in a conelike cage of a solid

Asymptotic formulas are derived for distribution functions and components of the pressure tensor for a Van der Waals fluid in a conelike cage of a solid. The formulas describe the local behavior of the fluid far from the vertex of the cone and at rather large distances from solid walls. It is shown that, due to the symmetry of the system, three diagonal components and one off-diagonal component of the pressure tensor differ from zero. The effect of the deviation of conelike pore walls from a cylindrical shape on the appearance of a noticeable contribution to the local characteristics of the fluid in the pore is estimated.     

Static polarizability of small simple metal clusters in dielectric media

Using the local-density-functional method and spherical jellium model, the electronic structure and static polarizability α(0) of small simple metal clusters (Al, Li, Na, K, Rb, and Cs) surrounding by media with dielectric constants ε = 1 ÷ 25 have been calculated. It has been obtained that α(0) of the smallest clusters with a low mean valence electron density is a decreasing function of ε, whereas for the clusters with a high valence electron density it rises with ε.     

The solvophobic effect in simple fluid mixtures

The “solvophobic” effect is the tendency of solute particles to cluster as the attractive interaction between solvent particles is strengthened. The potentials of mean force in the hypernetted chain approximation have been obtained for a simple fluid mixture consisting of two hard spheres immersed in a hard core/Yukawa tail solvent. The results clearly exhibit features attributable to the “solvophobic” effect.     

Extending the simple weighted density approximation for a hard-sphere fluid to a Lennard-Jones fluid II. Application

A simple weighted density approximation (SWDA) was extended to nonuniform Lennard-Jones fluids by following the spirit of a partitioned density function theory [S. Zhou, Phys. Rev. E 68 (2003) 061201] and mapping the hard-core part onto an effective hard-sphere fluid whose higher order terms beyond the second order of the functional perturbation expansion are treated by the SWDA. The resultant DFT formalism performs well for Lennard-Jones fluids under the influence of diverse external fields. With the present DFT formalism, we investigate in detail the structure and adsorption properties of a low-density LJ gas in a spherical cavity with a wall consisting of hard-sphere or LJ particles. It was found that when the cavity wall exerts an attractive external potential on the LJ particles in the cavity, the excess adsorption decreases as the temperature increases, while when the cavity wall exerts a hard repulsive external potential on the LJ particles in the cavity, the excess adsorption increases as the temperature increases.     

Supercritical fluid chromatography/mass spectrometry using a simple capillary-direct interface

Summary A simple method for interfacing capillary supercritical fluid chromatography with a commercial quadrupole mass spectrometer (SFC/MS) has been developed that yields good chromatographic peak shapes and good sensitivity. No modification of the mass spectrometer is required, and a single instrument can be used for both SFC/MS and GC/MS with conversion between modes requiring less than 20 min. SFC/MS separations and chemical ionization mass spectra of wax components, a triazine pesticide metabolite, abietic acid, and high molecular weight polycyclic aromatic hydrocarbons are reported.

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Kombination von Chromatographie mit überkritischen fluiden Phasen und Massenspektrometrie unter Verwendung eines einfachen direkten Capillar-Interface

     

A simple theory of the interaction between polymer brushes immersed in a supercritical fluid

We use a simple two-order parameter model to describe the interaction between the brushes of polymers terminally attached to flat surfaces immersed in a supercritical solvent. Our approach makes it possible to take into account the high compressibility of the supercritical solvent, which proves to give a significant contribution to the disjoining force acting between polymer brushes. Our theory explains why the interaction between brushes can change from repulsive to attractive with decreasing solvent density. This theoretical finding is verified by making a comparison with recent computer simulations. A reasonably good agreement between the results of the present theory and the simulations is found.     

Phase behavior of a simple dipolar fluid under shear flow in an electric field

Nonequilibrium molecular dynamics simulations are performed on a dense simple dipolar fluid under a planar Couette shear flow. Shear generates heat, which is removed by thermostatting terms added to the equations of motion of the fluid particles. The spatial structure of simple fluids at high shear rates is known to depend strongly on the thermostatting mechanism chosen. Kinetic thermostats are either biased or unbiased: biased thermostats neglect the existence of secondary flows that appear at high shear rates superimposed upon the linear velocity profile of the fluid. Simulations that employ a biased thermostat produce a string phase where particles align in strings with hexagonal symmetry along the direction of the flow. This phase is known to be a simulation artifact of biased thermostatting, and has not been observed by experiments on colloidal suspensions under shear flow. In this paper, we investigate the possibility of using a suitably directed electric field, which is coupled to the dipole moments of the fluid particles, to stabilize the string phase. We explore several thermostatting mechanisms where either the kinetic or configurational fluid degrees of freedom are thermostated. Some of these mechanisms do not yield a string phase, but rather a shear-thickening phase; in this case, we find the influence of the dipolar interactions and external field on the packing structure, and in turn their influence on the shear viscosity at the onset of this shear-thickening regime.     

A simple semiclassical model for ionic structure effects in large metal clusters

A semiclassical version of the density functional approach is used to investigate the structure of metal clusters. The effect of the ionic structure is included in a schematical way, assuming that the ions are distributed on concentric shells. The method, which allows a simultaneous investigation of geometrical and electronic effects, is computationally very simple and can be extended up to very large cluster sizes. Predictions of this model in the medium size range are compared with the results of available microscopic calculations, yielding a very good agreement.     

Ionic structure and global deformation of axially symmetric simple metal clusters

The ground-state structure and shape parameters of neutral and singly charged simple metal clusters with up to 40 valence electrons have been calculated within the cylindrically averaged pesudopotential scheme (CAPS). Na and Ba have been chosen as typical monovalent and divalent metals. The obtained structures agree very well with the results of more elaborate methods. A simple growth pattern for the most stable ionic geometries is deduced. The ionic and electronic multipole moments from CAPS are very similar to those of the structure averaged jellium model (SAJM) and of the plasma model. The surface energies derived from the CAPS results agree with experimental values.     

A methodology to define the Cubic Equation of State of a simple fluid

A method to define the Cubic Equation of State (CES) of a simple substance is presented in this work. CES is constructed with only three parameters of the fluid, namely, the critical compressibility Z_c≡P_cv_c/RT_c, the acentric factor ω ≡ − log  (P^(sat)/P_c) − 1 (where P^(sat) is the saturated vapor pressure), and the saturated vapor volume v^(sat) at the temperature T^(sat)/T_c = 0.7 (where T_c is the critical temperature, v_c is the critical volume, and P_c is the critical pressure). The resulting CES is unique for each substance and, in general, it is different from other known CES in the literature.     

Computation of the pair correlation function of a repulsive finite-intercept hard-core simple fluid

We consider a range of integral equation theories for a simple fluid with a pair potential of the type that arises for fused-hard-sphere molecular fluids in the RAM perturbation theory. The potential is of finite range with a finite intercept at contact, and is everywhere repulsive. Results based on a corrected form of the hypernetted chain theory are excellent and are superior to all others.     

A simple large-scale synthesis of very long aligned silica nanowires

A simple method based on the thermal oxidation of Si wafers has been discovered to provide a large-scale synthesis of very long, aligned silica nanowires. The as-grown product was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence. The obtained SiO₂ nanowires had no metal contaminations, ultralong lengths of millimeters, and most diameters of ∼50 nm. The PL spectra of the SiO₂ nanowires showed a strong and stable green emission at 540 nm. The nucleation and growth of the SiO₂ nanowires were investigated.