Ultrasound Absorption Anisotropy in the Vicinity of Smectic A-Nematic Transition

Abstract

The results of a low frequency ultrasonic study of 4-n-butyloxybenzylidene-4'-n-butylaniline (BBBA) in the vicinity of the smectic A-nematic transition are presented. The frequency and temperature dependence of the parameters describing the anisotropic ultrasound absorption are determined experimentally and compared with the predictions of fluctuation and relaxation theories of the smectic A-nematic transition. It is shown that no single theory could explain all of the experimental data.     

三乙酸甘油酯玻璃化转变的焓弛豫与介电弛豫研究

脆性较高的玻璃形成分子液体通常在焓弛豫和介电弛豫动力学上表现出明显的差异性, 为了深入理解这一问题, 本文针对具有较高液体脆性的三乙酸甘油酯对比研究了焓弛豫与介电弛豫行为. 利用这两个技术分别研究了结构弛豫动力学过程的非Arrhenius 与非指数特征, 液体脆性因子与非指数性因子的对比显示很好的一致性. 分析表明分子的柔性有可能对这两种弛豫过程中分子运动的相关性产生明显的影响. 讨论了玻璃形成液体的分子结构与动力学参数之间的关联.     

Mutual Interdependence of Partitions Functions in Vicinity T g of Transition

The solid—liquid transition is described by use of the model incorporating the non-linear interactive oscillators. The individual non-linear oscillators are formed from the mechanical units of about the monomer size and produce the vibrations on the lower amplitude level, for the solid phase, either in amorphous or in crystal-like form. As the temperature starts to overpass the Vogel's temperature, the vibrations of individual units are big enough to cause the permanent displacements of the individual vibrating particles; the material starts to flow and the process of diffusion begins as well. As the temperature is passing through T _g vicinity, the large mechanical heterogeneity's start to appear as the small percentage of oscillators enlarge their amplitude of vibrations enormously and act as the local stress perturbations centers. These centers are responsible for the destruction of original matrixes and the sharp onset of fluidity and diffusion takes place. The upper amplitude of vibration motion is the basic property of a liquid state. The whole system of vibrations in matrix is described by use of techniques of deterministic chaos theories. It is shown as well, how the mutual interplay of the partition functions (vibration and cohesive), plays the important role in transition from liquid to solid states.This revised version was published online in November 2005 with corrections to the Cover Date.     

Computer Simulation Studies of Anisotropic Systems XVI. The Smectic E-Smectic B Transition

We have investigated the smectic E-smectic B transition with the aid of a model smectogen whose properties have been calculated using the Monte Carlo technique of computer simulation. The lath-like mesogenic molecules are defined to lie in a plane with their centres on a triangular lattice and with their long axes orthogonal to the smectic layer. The quadrupolar interaction, restricted to nearest neighbours, is assumed to be responsible for the herring-bone arrangement of the molecular short axes, characteristic of the smectic E phase. The computer simulations have been employed to evaluate both thermodynamic and structural parameters as a function of temperature. The model smectogen is found to exhibit a continuous transition at which the long range herring-bone structure of the smectic E phase is destroyed only to be replaced by the analogous short range structure of the smectic B. Where possible the results simulated for the model smectogen are compared with the behaviour of real mesogens and the predictions of molecular field theories for the transition. In particular, contact is made with X-ray diffraction studies of the two phases by using optical techniques to generate the diffraction patterns associated with configurations produced by the simulation. The model is found to be in good accord with experiment but the molecular field prediction of the smectic E-smectic B transition temperature is shown to be poor.     

HCN→HNC异构化反应过渡态附近的拓扑特性

电荷密度拓扑分析方法(topological analysis of charge distribution)已经被广泛应用于研究化合物的各种静态性质及反应特性.对IRC反应途径上过渡态附近构型的拓扑性质研究报导较少.本工作是用电荷密度分布的拓扑分析方法对HCN→HNC异构化反应IRC过渡态附近的性质进行了研究.对电荷密度分布拓扑分析和IRC途径确定选用相同的基组,得到了一些新的结论。     

Thermodynamic Parameters of Cholesteric/Smectic A Transition in Cholesteric Myristate and Its Binary Mixture CM/PCPB

Thermodynamic properties of the cholesteryl myristate (CM) and its binary mixture CM/PCPB (p-pentylphenyl-2-chloro-4(p-pentylbenzoyl)-benzoate) are studied at the concentrations of x_PCPB = 0.052 and 0.219 as a function of temperature near the cholosteric/smectic A transition. By analyzing the observed molar volume from the literature, the temperature dependences of the thermal expansion, isothermal compressibility and the difference in the specific heat are calculated and, the Pippard relations are established for those compounds close to the cholesteric/smectic A transition. Predictions of the thermodynamic quantities and the Pippard relations can be examined by the experimental measurements of the CM and its binary mixture of CM/PCPB close to the cholesteric/smectic A transition.     

Determination of Fluorescence Polarization and Absorption Anisotropy in Molecular Complexes Having Threefold Rotational Symmetry

Abstract— The current work concerns investigation of the polarization properties of complex molecular ensembles exhibiting threefold (C₃) rotational symmetry, particularly with regard to the interplay between their structure and dynamics of internal energy transfer. We assume that the molecules or chromophores in such complexes possess strongly overlapped spectra both for absorption and fluorescence. Such trimeric structures are widely found in biological preparations, as for example the trimer of C-phycocyanin (C-PC). Higher order aggregates, e.g. hex-amers and three-hexamer rods, are also investigated and compared with the trimer case. The theory addresses both steady-state and 8-pulse excitation and establishes some links between them. Monochromophoric, bichro-mophoric and trichromophoric molecular complexes are individually examined. For steady-state excitation, analytical formulas are reported for the degree of fluorescence polarization and absorption anisotropy. It is shown that the polarization is dependent on the chromophore inclination relative to the symmetry axis, the relative efficiencies of absorption and fluorescence by chromophores of different spectral types, and the rates of energy equilibration. To assess the validity of the theory, it has been applied to C-PC aggregates. Here it was found that different C-PC aggregates provide practically identical polarization response. For S-pulse excitation we give analytical formulas for determination of the fluorescence depolarization, and also the depolarization associated with absorption recovery, both for a monochromophoric trimer and some particular cases of bichromophoric trimer. More complicated systems are analyzed by computer modeling. Thus it transpires that the initial polarization anisotropy r(t = 0) takes the value 0.4 for all considered aggregates; the long-time limit r(t →∞) has about the same value as is associated with steady-state excitation. We also show that with steady-state excitation the degree of fluorescence polarization is practically equal for various C₃ aggregates of C-PC, and that the major factor determining the polarization is the chromophore orientation relative to the symmetry axis.     

Systematic Variation of Length Ratio and the Formation of Smectic A and Smectic C Phases

The phase diagrams of four binary mixtures of chemically similar smectogenic mesogens differing only in molecular length are investigated. In these bidisperse systems the length ratio varies systematically. The phase diagrams show the stabilization of the smectic A and the destabilization of the smectic C phase with increasing length ratio as a general trend. Detailed small‐angle X‐ray diffraction and electro‐optic measurements revealed a decrease in smectic translational order and a continuous reduction of the tilt angle with increasing length difference. These surprising results are of general interest for the understanding of the structure and dynamics of smectic phases. The remarkably strong impact of the length difference on the smectic layer structure and the phase behavior is discussed from a mechanistic point of view taking into account sterical interactions. For the observed structural changes in these bidisperse smectics we propose pronounced out‐of‐layer fluctuations with increasing length difference as driving force, causing neighboring molecules within nearest layer into a smectic A‐like packing.     

Non-Steady-State Temperature Field of the Vapor-Gas Mixture in the Vicinity of a Growing Droplet: Balance of the Heat of Phase Transition

The non-steady-state temperature field of the vapor-gas medium in the vicinity of a droplet growing in supersaturated vapor is constructed. In the conduction problem, a time-dependent boundary condition is used which ensures the fulfillment of the balance condition of the heat of phase transition. The resultant temperature field is compared with the one obtained in the heat conduction problem with the equilibrium boundary condition on the surface of a droplet of a fixed radius. Although the solution with the equilibrium boundary condition does not ensure the balance between the heat released on the growing droplet and the heat distributed due to heat conduction in the vapor-gas medium, the difference between the two solutions is not very large. This difference is important for describing the homogeneous nucleation of supersaturated vapor in the vicinity of a growing droplet, as is indicated by comparison of the vapor supersaturation fields constructed with and without allowance for thermal effects, as well as with the use of solutions to the diffusion and heat conduction problems with various boundary conditions.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 333–341.Original Russian Text Copyright © 2005 by Grinin, Zhuvikina, Gor.     

金属有机化合物中过渡金属的原子吸收分光光度法测定

本文报导应用火焰原子吸收分光光度法对金属有机化合物中过渡金属铜、镍、钴、铁、钼及铬的定量测定。我们将金属有机化合物溶解在适当的有机溶剂或水中,不必分离基体,即可用标准加入法直接进行测定。操作简单,速度快,相对误差<0.4％。     

Atomic Absorption Assay of Noble Metals in Ores with the Use of Sample Decomposition by Ultrasound

An atomic absorption assay of gold, silver, and palladium in various types of ores is performed with the use of ultrasound at the fire assay stage. It is shown that ultrasound can reduce the temperature and duration of fusion and increase the recovery of noble metals into the lead button.     

Use of Ultrasound in Sample Preparation for the Determination of Mercury Species by Cold-Vapor Atomic Absorption Spectrometry

Ultrasound was used for the intensification of sample preparation in the determination of different mercury species in brines and solutions of sodium chloride and common salt. Procedures were proposed for the determination of total mercury and inorganic, organic, and methyl mercury species.     

Biologically Active Radiation in the Vicinity of a Single Tree

The horizontal photon flux density of photosynthetically active radiation (PAR) and flux density of ultraviolet A (UVA) and ultraviolet B (UVB) radiation were measured in the vicinity of isolated single trees during the summer of 1996. Measurements were made under shade and sunlit conditions along a transect aligned with the solar disk and the tree trunk. Flux density measurements were normalized by the flux density at a reference location away from the tree. Results showed (1) a more rapid decline in the flux density of UVA and UVB radiation than PAR with decreasing distance to the tree trunk on both the sunlit and shaded side of a tree and (2) more rapid changes in the flux density of UVB radiation UVA radiation, and PAR with distance from the tree on the sunlit side of the tree than the shaded side of the tree. The UVB/PAR ratio was found to increase in the shadow of a tree with increasing distance from the tree to between 4 and 6 for the conditions of the study. The potential for detrimental effects by UVB flux density under conditions of the high ratio may be mitigated by sunflecks at a given location over the course of a day.     

Smectic polymorphism in a series of three-ring enaminoketone compounds

Abstract

Liquid crystalline properties of 1 - (4′ - alkoxyphenylamino) - 3 - (4′ - hexyloxyphenyl)-prop-1-en-3-ones, from methoxy to heptadecyloxy, have been examined by optical, DSC, and X-ray methods. The phase diagram for the series exhibits a rich polymorphism of tilted smectic phases, for example, five mesophases were found for the hexyloxy derivative. A characteristic feature of the phase diagram is a gap in the crystal G phase area. For the heptyloxy homologue, a direct crystal H-smectic F phase transition was found; in the case of shorter as well as longer terminal substituents, the phase sequence crystal H–crystal G–smectic F is observed. Calorimetric and X-ray studies revealed the existence of a tricritical point on the crystal G–smectic F transition line.     

Smectic liquid crystalline character of n-alkylammonium pyroglutamates

A series of n-alkylammonium and n-dialkylammonium pyroglutamates were synthesized and characterized. Their thermotropic liquid crystal behaviour was investigated using differential scanning calorimetry and optical microscopy. The structures of their crystal and smectic A phases were studied by X-ray diffraction.     

Thermal Decomposition of Alcohols in the Vicinity of Critical Point

On the basis of experimental data, the thermodynamic properties of alcohols in different regions of their phase diagram, including the critical point, were analyzed as depending on composition changes caused by thermal decomposition. Original Russian Text ? V.I. Dvoryanchikov, M.M. Zaripilova, G.A. Rabadanov, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 4, pp. 782–785.     

Smectic ordering in polymer liquid crystal-silica aerogel nanocomposites

Summary Two series of side chain liquid crystal (SCLC) polyacrylate-silica aerogel nanocomposites have been investigated by small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). The first series (ex-situ nanocomposite) was obtained by infiltration of a smectic SCLC polyacrylate prepared by polymerisation in solution into monolithic aerogel slabs. The second one (in-situ nanocomposite) was prepared by photopolymerisation of the monomer infiltrated in the aerogel. The results are compared with those obtained for bulk polyacrylates. It is shown that the smectic ordering is not destroyed by confinement in the aerogel. Spacing of the smectic layers and smectic correlation lengths were deduced from the fit of the SAXS profiles to a Lorentzian function with a quadratic correction. The principal results suggest that in-situ polymerisation enhances the degree of order and the stability of the smectic phase in the nanocomposite.     

Smectic ordering in athermal systems of rodlike triblock copolymers

The phase behavior of the system of parallel rigid triblock copolymers is examined using the second virial density functional theory. The triblock particle consists of two identical infinitely thin hard rods of finite lengths on the opposite ends of one central hard cylinder with nonzero length and diameter. Stability analyses and free energy calculations show that the system of parallel particles can form not only uniform nematic and smectic A phases but also a smectic C phase. The stability and structure of the tilted structure are controlled by only the diameter and the length of the central cylinder segment. Interestingly, the diameter affects only the layer tilting and the periodicity, but not the packing fraction of the nematic to smectic-C transition. For all values of cylinder length the usual smectic A and smectic C transitions compete with each other and no nematic-columnar transition is observed. At low and high cylinder lengths the smectic A phase is stabilized first, while the smectic C is the most stable for intermediate length values.     

Anisotropy in the wetting of rough surfaces

Surface roughness amplifies the water-repellency of hydrophobic materials. If the roughness geometry is, on average, isotropic then the shape of a sessile drop is almost spherical and the apparent contact angle of the drop on the rough surface is nearly uniform along the contact line. If the roughness geometry is not isotropic, e.g., parallel grooves, then the apparent contact angle is no longer uniform along the contact line. The apparent contact angles observed perpendicular and parallel to the direction of the grooves are different. A better understanding of this problem is critical in designing rough superhydrophobic surfaces. The primary objective of this work is to determine the mechanism of anisotropic wetting and to propose a methodology to quantify the apparent contact angles and the drop shape. We report a theoretical and an experimental study of wetting of surfaces with parallel groove geometry.     

Anisotropy in the interaction of ultracold dysprosium

The nature of the interaction between ultracold atoms with a large orbital and spin angular momentum has attracted considerable attention. It was suggested that such interactions can lead to the realization of exotic states of highly correlated matter. Here, we report on a theoretical study of the competing anisotropic dispersion, magnetic dipole-dipole, and electric quadrupole-quadrupole forces between two dysprosium atoms. Each dysprosium atom has an orbital angular momentum of L = 6 and a magnetic moment of μ = 10 μ(B). We show that the dispersion coefficients of the ground state adiabatic potentials lie between 1865 a.u. and 1890 a.u., creating a non-negligible anisotropy with a spread of 25 a.u. and that the electric quadrupole-quadrupole interaction is weak compared to the other interactions. We also find that for interatomic separations R < 50a(0) both the anisotropic dispersion and magnetic dipole-dipole potential are larger than the atomic Zeeman splittings for external magnetic fields of order 10 G to 100 G. At these separations the atomic angular momentum can be reoriented. We finish by describing two scattering models for these inelastic m-changing collisions. A universal scattering theory is used to model loss due to the anisotropy in the dispersion and a Born approximation is used to model losses from the magnetic dipole-dipole interaction for the (164)Dy isotope. These models find loss rates that are of the same order of magnitude as the experimental value.