E. P. Sokolova

Stability and optical limiting properties of a single wall carbon nanotubes dispersion in a binary water-glycerol solvent

We report the temporal stability of the dispersion of single-wall carbon nanotubes in a binary solvent “water + glycerol” having eutectic composition (ca. 67% wt.) with sodium dodecylbenzene sulfonate as a dispersant. The system procures good temporal and thermal stability: its absorption spectra demonstrate no changes during one-year storage with temperature spanning −40 to +40 °C. The system provides non-linear optical power limiting of the incident laser radiation (532 nm) in a one-shot and pulse-periodic regimes of its applying.

A differential scanning calorimetry study of a binary system: II. Phase diagram and transition enthalpies for the lyoptropic liquid crystal dimethyldecylphosphine oxide—water system

Abstract A system containing water and a nonionic surfactant dimethyldecylphosphine oxide, in which lyotropic liquid crystalline hexagonal and lamellar phases are known to exist, is studied by means of differential scanning calorimetry (DSC). Invariant points and boundaries of phases and of two-phase regions are determined and a phase diagram is obtained. Lyomesophase/liquid transition enthalpies are shown to grow with the increasing surfactant content. Enthalpy of the eutectic lamellar + crystals ⇄ liquid phase reaction with the system is compared to the heat effects of the three-phase transitions in the DPPC—water system. Limitations of DSC method in the study of phase diagrams are discussed.

Orientational and thermodynamic properties of rod-plate mixtures

The molecular-statistical model of a fluid composed of rectangular parallelepipeds, that is continuous in translations and discrete in orientation distribution, is employed in calculations relating to regions corresponding to the stability of uniaxial and biaxial nematic phases in model mixtures of uniaxial rod-like and plate-like particles, interacting via steric repulsion. The calculations are performed taking into account three-particle correlations. It is shown that for fixed overall mole fraction, the value of the configurational Gibbs free energy of a biaxial phase is equal to that of a system with a miscibility gap containing two uniaxial nematic phases, one rich in rod-like molecules and the other in plate-like ones. The results of the present study are compared with those of studies of related model systems.

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.

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.

Phase Diagrams of Lyotropics

Abstract Principles of the interpretation of DSC data for binary systems are considered. The phase diagrams obtained must satisfy the requirements of the phase rule. DSC curves and the phase diagram for the dimethyloctylphosphine oxide - water system are given. The effect of the alkyl chain length upon the enthalpies of the hexagonal-isotropic and lamellar-isotropic transitions is discussed.

A Van Der Waals Model for the Nematic Liquuid Crystal Mixtures

Abstract Phase diagrams and excess molar volumes of binary nematogenic mixtures are investigated in the frame of a van der Waals - type theory for systems made up of hard rectangular parallelepipeds. It is shown that the excess volumes can be positive or negative or can change sign depending on temperature and composition. In agreement with experiment the model predicts a coexistence of two nematic phases for mixture of rod-like and plate-like molecules.

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.