A. Yu. Vlasov

The equation of state of isotropic fluids of hard convex bodies from a high-level virial expansion

We have calculated virial coefficients up to seventh order for the isotropic phases of a variety of fluids composed of hard aspherical particles. The models studied were hard spheroids, hard spherocylinders, and truncated hard spheres, and results are obtained for a variety of length-to-width ratios. We compare the predicted virial equations of state with those determined by simulation. We also use our data to calculate the coefficients of the y expansion [B. Barboy and W. M. Gelbart, J. Chem. Phys. 71, 3053 (1979)] and to study its convergence properties. Finally, we use our data to estimate the radius of convergence of the virial series for these aspherical particles. For fairly spherical particles, we estimate the radius of convergence to be similar to that of the density of closest packing. For more anisotropic particles, however, the radius of convergence decreases with increased anisotropy and is considerably less than the close-packed density.

The eighth virial coefficient of four- and five-dimensional hard hyperspheres

The eighth virial coefficient for hard hyperspheres is calculated by Monte Carlo techniques. It is found that B8/B(7)2=0.000 274+/-0.000 014 and -0.000 115+/-0.000 012 in four and five dimensions, respectively. The results are in good agreement with the findings of Clisby and McCoy (e-print arXiv:cond-mat0410511), and confirm that B8 is negative in five dimensions.

A quasichemical lattice model for a binary mixture of hard rectangular parallelepipeds Application to systems composed of nematic and non-mesogenic molecules

Abstract The hole lattice model of rectangular parallelepipeds is presented to describe the structural and excess thermodynamic properties of nematic-non-mesogenic mixtures. The molecular attractions are taken into account within the quasi-chemical approximation. A procedure for evaluating model parameters from data on the thermodynamic characteristics of pure components and the activity coefficients of the non-mesogen at infinite dilution at the nematic-isotropic transition temperature of the mesomorphic component is proposed. The mixing functions (enthalpy and volume), activity coefficient of the non-mesogen and the order parameters of the components are calculated at a molecular level for systems composed of 4-methoxybenzylidene-4′-propylaniline and a non-mesogen (tetrachlormethane, benzene and n-heptane). The calculated results are in quite good agreement with experiment in the temperature rang from 319·2 to 335·4 K.

Compositional surface-layered sorbents for pre-concentration of organic substances in the air analysis.

We juxtapose methods of synthesis of non-polar surface-layered sorbents on the platform of a carrier, which combines macro-porous coarse-dispersed polytetrafluoroethylene with micro-dispersed activated carbon. Further, we present data about the sorption properties of the said materials alongside perspectives of their analytical applications. Our study established that with respect to efficacy of dynamic sorption from gaseous phases these sorbents outperform bulk-porous analogues in their normal, granular form.

Zwanzig model of multi-component mixtures of biaxial particles: y 3 theory re-visited

The paper considers the thermodynamic and phase ordering properties of a multi-component Zwanzig mixture of hard rectangular biaxial parallelepipeds. An equation of state (EOS) is derived based on an estimate of the number of arrangements of the particles on a three- dimensional cubic lattice. The methodology is a generalization of the Flory–DiMarzio counting scheme, but, unlike previous work, this treatment is thermodynamically consistent. The results are independent of the order in which particles are placed on the lattice. By taking the limit of zero lattice spacing, a translationally continuous variant of the model (the off-lattice variant) is obtained. The EOS is identical to that obtained previously by a wide variety of different approaches. In the off-lattice limit, it corresponds to a third-level y-expansion and, in the case of a binary mixture of square platelets, it also corresponds to the EOS obtained from fundamental measure theory. On the lattice it is identical to the EOS obtained by retaining only complete stars in the virial expansion. The off-lattice theory is used to study binary mixtures of rods (R 1 − R 2) and binary mixtures of platelets (P 1 − P 2). The particles were uniaxial, of length (thickness) L and width D. The aspect ratios Γ i  = Li /Di of the components were kept constant (Γ1 R  = 15, Γ1 P  = 1/15 and Γ2 R  = 150, Γ2 P  = 1/150), so the second virial coefficient of R 1 was identical to P 1 and similarly for R 2 and P 2. The volume ratio of particles 1 and 2, v 1/v 2, was then varied, with the constraints that viR  = viP and Results on nematic–isotropic (N − I) phase coexistence at an infinite dilution of component 2, are qualitatively similar for rods and platelets. At small values of the ratio v 1/v 2, the addition of component 2 (i.e. a thin rod (e.g. a polymer) or a thin plate) results in the stabilization of the nematic phase. For larger values of v 1/v 2, however, this effect is reversed and the addition of component 2 destabilizes the nematic. For similar molecular volumes of the two components strong fractionation is observed: shorter rods and thicker platelets congregate in the isotropic phase. In general, the stabilization of the ordered phase and the fractionation between the phases are both weaker in the platelet mixtures. The calculated spinodal curves for isotropic-isotropic demixing are noticeably different between the R 1 − R 2 and the P 1 − P 2 systems. The platelet mixtures turn out to be stable with respect to de-mixing up to extremely high densities. The values of the consolute points for the R 1 − R 2 blends are remarkably similar to those obtained using the Parsons–Lee approximation for bi-disperse mixtures of freely rotating cylinders with similar aspect ratios [S. Varga. A. Galindo, G. Jackson, Mol. Phys., 101, 817 (2003)]. In a number of R 1 − R 2 mixtures, phase diagrams exhibiting both N − I equilibrium and I − I de-mixing were calculated. The latter is pre-empted by nematic ordering in all the cases studied. Calculations show the possible appearance of azeotropes in the N − I coexistence domain.

Aqueous suspensions of single-wall carbon nanotubes: Degree of aggregation into bundles and optical properties

Aqueous suspensions of nanotubes, as well as the structure and optical properties of their aggregates (bundles), are studied by spectroscopy and high-resolution electron microscopy. The structure of nanoparticles is controlled by varying the ultrasonication time during preparation of suspensions. It is found that the defectiveness of nanotubes increases with decreasing bundle size. A correlation is shown to take place between the suspension preparation regime, the structure of nanoparticles, and the relaxation of the photoexcitation energy of their electronic shells. It is found that the efficiency of photoexcitation energy conversion into heat increases with increasing degree of aggregation of nanotubes into bundles.

An analysis of approximations for calculating the electrostatic contribution to the Gibbs energy of micelle formation

Approximations used to calculate the electrostatic contribution to the Gibbs energy of micelle formation are discussed as applied to the problem of describing the influence of salt admixtures on the aggregation of ionic surface active substances in aqueous solutions. The known approximate solutions to the Poisson-Boltzmann equation for curved charged surfaces (spheres and cylinders) in the presence of electrolytes are analyzed. It is proposed that the electrostatic Gibbs energy be estimated from the Mitchell-Ninham equation for surface electric potentials. The dependences of Gibbs energy estimates on the radius of the surface and surface charge density are compared with the results of the exact numerical solution to the Poisson-Boltzmann equation for spheres and cylinders over a wide range of background salt concentrations. Recommendations are given for selecting the approximation for estimating the electrostatic contribution taking into account the size and shape of micelles, solution ionic strength, and charge density on the surface of micelles. Two approximations that gave the best description of the exact solution over the whole range of the conditions under consideration were used within the framework of the molecular thermodynamic aggregation model for calculating the critical micelle concentration and the size of aggregates depending on the salt content in aqueous solutions of sodium dodecyl sulfate.

ROLE OF STERIC FACTORS IN THE MECHANISM OF SEPARATION OF STRUCTURAL ISOMERS WITH NEMATIC SORBENTS

The model of a nematic binary mixture of rigid biaxial molecules is used to calculate the coefficients of selectivity enhancement Sm/p (the ratio between the activity coefficients ofmeta- andpara-substituted benzenes as sorbates at infinite dilution) in the nematogenic matrix of a nematic liquid-crystal sorbent and the parameters of orientation order of the components. It was found that in systems of particles in which interaction between the particles consists in steric repulsion, the Sm/p coefficient is less than 1 and is practically constant in the isotropic phase, whereas in the nematic phase, Sm/p is greater than 1 and increases in parallel with the order parameter.

Elimination of bleaching in a material based on aqueous fluid with carbon nanoparticles for optical limiters

The possibilities of correcting the functional properties of nonlinear optical limiters based on carbon nanoparticles using polymer dopants have been considered. The results of analyzing the characteristics of nonlinear optical limiting of a suspension of carbon black with addition of sodium dodecylbenzene sulfonate and polyvinyl alcohol are given. The forms in which particles exist in the suspension have been investigated by different physicochemical methods. Optical measurements have confirmed the efficiency of these additives.

Nonlinear and electro-optical properties of fluid organo/inorganic matrices with supra-molecular organization, containing nano-sized and molecular anisometric moieties

The design of materials for light harvesting applications or information display technique (switching or limiting laser radiation, electrically administered light scattering, enhancement of pixel response time, etc.) poses issues of employing matrices with lowered phase symmetry (gels, aggregated polymer coils in solutions, biaxial nematics, polymer network liquid crystals (PNLC)) as the milieu for photo- and electro-active components. We tackled this scenario on a number of examples.