Volume 57, Issue 3-4

A statistical theory of cholesteric liquid crystals composed of short rigid biaxial molecules is presented. It is derived in the thermodynamic limit at a small density and a small twist. The uniaxial (biaxial) cholesteric phase is regarded as a distorted form of the uniaxial (biaxial) nematic phase. The chirality of the interactions and the implementation of the inversion to the rotation matrix elements are discussed in detail. General microscopic expressions for the elastic constants are derived. The expressions involve the one-particle distribution function and the potential energy of two-body short-range interactions. It is shown that the elastic constants determine the twist of the phase. The stability condition for the cholesteric and nematic phases is presented. The theory is used to study unary and binary systems. The temperature and concentration dependence of the order parameters, the elastic constants and the twist of the phase are obtained. The possibility of phase separation is not investigated.

An alternative approach to the dielectric relaxation and resonance absorption is proposed. Two assumptions were taken into consideration. One, that there exists a dielectric in which particular molecules have different freedom of motion. The other, that the value of the motion damping coefficient can be described by the distribution function. The obtained formula for the description of the experimental data of dielectric relaxation and resonance absorption measurements is presented.

The phase detection in cholesteryl oleyl carbonate was studied over a broad temperature range by dielectric spectroscopy. Tests were conducted in the static and the low frequency ionic regions. They point to the possibility of relatively simple detection of phase transition. Some manifestations of pretransitional behaviours were also found. - Pacs: 64.70Md, 77.22Ch, 64.30+t

The conductivities of the binary room-temperature molten salt (RTMS) systems ZnCl 2 -N-nbutylpyridinium chloride (BPC), ZnCl 2 -1-ethyl-3-methylimidazolium chloride (EMIC) and ZnCl 2 - benzyltriethylammonium chloride (BTEAC) have been measured at different temperatures and compositions by a d.c. four-probes method. The conductivities of the three RTMS are in the order ZnCl 2 -EMIC > ZnCl 2 -BPC > ZnCl 2 -BTEAC. In ZnCl 2 -BPC the conductivity at 70 to 150 °C, is maximal for 40 mol% ZnCl 2 . In ZnCl 2 - EMIC, the conductivity below 130 °C is almost constant for 30 to 50 mol% ZnCl 2 and has the lowest activation energy 25.21 kJ/mol. For these two systems, the conductivities decrease rapidly beyond 50 mol% ZnCl 2 owing to the rapid increase in cross-linking and resultant tightening of the polyelectrolyte structure. As to the ZnCl2-BTEAC system, the conductivities at 110 - 150 °C decrease slowly for 30 - 60 mol% ZnCl 2 . The conductivities of the ZnCl 2 -EMICmelt are compared with those of the AlCl 3 -EMIC melt previously studied. The stability of the ZnCl 2 -EMIC melt system is explored by the effect of the environment on the conductivity and the Far Transmission Infra Red (FTIR) spectrum. It reveals that the effect is slight, and that the ZnCl 2 -EMIC melt may be classified as stable.

The molar enthalpies of mixing, Δ mix H m , of the liquid alkali metal iodide - neodymium iodide binary systems have been measured over the whole composition range at 1068 K with an accuracy of about 6%. The apparatus used was a Calvet - type high - temperature microcalorimeter. In all the systems under investigation the enthalpies of mixing are negative, with minimum values close to −1.1, −4.8, −10.3, −16.2, and −20.0 kJ mol −1 for LiI-NdI 3 , NaI-NdI 3 , KI-NdI 3 , RbI-NdI 3 , and CsI-NdI 3 , respectively, at the mole fractions x NdI3 ⋍ 0.30 - 0.40 except the LiI-NdI 3 system, where it is at the NdI 3 -rich compositions. The molar enthalpies of formation Δ form H m at 1068 K for M = Li, Na, K, Rb, and Cs, arising from the reaction 3 MI (l) + NdI 3(l) , are found to be −4.5, −17.5, −39.1, −59.9, and −73.3 kJmol −1 , respectively They are compared with the formation enthalpies determined previously for the (3MCl, NdCl 3 ) and (3MBr,NdBr 3 ) liquid mixtures. These enthalpies become less negative with increase of the radius of the halide ion.

Two graph invariants were encountered that form the link between (molecular) walk counts and eigenvalues of graph adjacency matrices. In particular, the absolute value of the sum of coefficients of the first or principal (normalized) eigenvector, s 1 , and the analogous quantity s n , pertaining to the last eigenvector, appear in equations describing some limits (for infinitely long walks) of relative frequencies of several walk counts. Quantity s 1 is interpreted as a measure of mixedness of a graph, and s n , which plays a role for bipartite graphs only, is interpreted as a measure of the imbalance of a bipartite graph. Consequently, s n is maximal for star graphs, while the minimal value of s n is zero. Mixedness s 1 is maximal for regular graphs. Minimal values of s 1 were found by exhaustive computer search within the sample of all simple connected undirected n-vertex graphs, n≤10: They are encountered among graphs called kites. Within the special sample of tree graphs (searched for n≤20) so-called double snakes have maximal s 1 , while the trees with minimal s 1 are so-called comets. The behaviour of stars and double snakes can be described by exact equations, while approximate equations for s 1 of kites and comets could be derived that are fully compatible with and allow to predict some pecularities of the results of the computer search. Finally, the discriminating power of s 1 , determined within trees and 4-trees (alkanes), was found to be high.

A series of Fe 2 O 3 -SiO 2 nanocomposites (25 weight % of Fe 2 O 3 ) has been prepared using a sol-gel method. Samples showing different features were obtained by varying the evaporation conditions acting on the surface / volume ratio of the starting sol. The samples were investigated using Mössbauer spectroscopy at low temperature. The Fe 2 O 3 nanoparticles have been identified as maghemite ( γ-Fe 2 O 3 ) particles. The samples show a superparamagnetic behavior with a blocking temperature that depends on the average particle size.

The stability of a viscoelastic interface acted upon by an oscillating azimuthal magnetic field is studied. The interface separates two rigid magnetic fluid columns. Only azimuthal disturbrance modes are considered in a linear perturbation technique. Weak viscoelastic effects are taken into consideration, so that their contributions are demonstrated in the boundary conditions. The presence or absence of free surface currents resulted in a dispersion equation with complex coefficients of the Mathieu type. It is found that the surface currents disappear when the stratified magnetic field becomes unity. The phenomenon of coupled resonance is observed. Several special cases are reported. A set of graphs are drawn to illustrate the influence of the various parameters on the stability of the considered system.

1,1,1-Trichloroethane (CH 3 CCl 3 ), 2,2-Dibromopropane ((CH 3 ) 2 CBr 2 ), and Eicosane (nC 20 H 42 ), have been investigated with differential thermal analysis (DTA) under high pressure using compressed argon and helium as a pressure medium. In the case of the chemically reactive halogenated samples the measurements were performed in open DTA containers. It was found that compressed helium modified the phase boundaries much less than compressed argon. In the case of eicosane there was no reaction with the DTA containers, and the measurements performed in closed containers were well reproducible, independent of the pressurized gas used. Furthermore pVT measurements have been carried out in the neighbourhood of the melting region for CH 3 CCl 3 and (CH 3 ) 2 CBr 2 . The pVT measurements yield the volume change of melting, from which the enthalpy change is calculated with the aid of the Clausius-Clapeyron equation. The specific volumes of the solid phases below the melting point are compared with crystallographic data.

New liquid crystalline compounds involving intermolecular hydrogen bonding between mesogenic p-n-alkoxybenzoic acids (nABA) (where n denotes the alkoxy carbon number varying from propylto decyl- and dodecyl-) and p-aminobenzonitrile (ABN) are synthesized. The thermal and phase behaviour of these materials is studied by Thermal Microscopy and Differential Scanning Calorimetry. Adetailed IR spectral investigation in solid and solution states confirms the formation of H-bonding between cyano and −COOH groups of ABN and nABA, respectively. Comparative thermal analyses of both free p-n alkoxybenzoic acids and H-bonded complexes suggest the induction of smectic-G phase in all the complexes.

CNDO/2 A computational analysis based on quantum mechanics has been carried out to determine the association energy of seven homologues of the 4-n-alkylbenzoic acid series (n = 3, 4, 5, 6, 7, 8, and 9), using the Rayleigh-Schrödinger perturbation method for various nearest neighbour configurations of interacting pairs. The net atomic charges and dipoles have been computed using the method. An attempt is made to explain the odd-even effects at the molecular level on the basis of these results.

DADMBP A computational analysis of the molecular ordering of the non-liquid crystalline biphenyl derivative 4,4ʹ-diamino-3,3ʹ-dimethylbiphenyl () has been performed. The CNDO/2 method has been employed to compute the net atomic charge and atomic dipole components at each atomic centre. The modified Rayleigh-Schrödinger perturbation theory along with the multicentered-multipole expansion method has been employed to evaluate the long-range interactions, while a “6-exp” potential function has been assumed for short-range interactions. On the basis of stacking, in-plane and terminal interaction energy calculations, all possible arrangements of a molecular pair have been considered. It is found that, if a suitable functional group is attached to DADMBP , so that the length to breadth ratio is increased, the molecule will show a change in the long-range order, the phase transition temperature and other liquid crystalline properties. The results are compared with those for the liquid crystalline compounds 4-(4ʹ-ethoxyphenylazo)phenyl valerate ( EPPV ) and 4,4ʹ-di-n-propoxy-azoxybenzene ( DPAB ).

An antiferroelectric liquid crystal material, (S)-4-(2-chloro-3-phenyl-1-(2-methylbutyloxy)carbomato- 4ʹ-n-dodecyloxy biphenylcarboxylate (CPCDBD), has been synthesized by using (S)-2-amino- 3-(4-hydroxy)phenyl propionic acid (L-tyrosine) as one the optically active ingredients. Preliminary investigations on this material reveal high spontaneous polarization (~ 145 nC/cm 2 ) in the antiferroelectric Sm-C A * phase. Possible structural contributions towards the appearance of antiferroelectric ordering are discussed.

According to the Planck aether hypothesis, the vacuum of space is a superfluid made up of Planck mass particles, with the particles of the standard model explained as quasiparticle - excitations of this superfluid. Astrophysical data suggests that ≈70% of the vacuum energy, called quintessence, is a negative pressure medium, with ≈26% cold dark matter and the remaining ≈4% baryonic matter and radiation. This division in parts is about the same as for rotons in superfluid helium, in terms of the Debye energy with a ≈70% energy gap and ≈25% kinetic energy. Having the structure of small vortices, the rotons act like a caviton fluid with a negative pressure. Replacing the Debye energy with the Planck energy, it is conjectured that cold dark matter and quintessence are Planck mass rotons with an energy below the Planck energy.

Mean amplitudes of vibration of F 3 ClO have been calculated from vibrational spectroscopic data in the temperature range between 0 and 1000 K. Its bond properties are discussed on the basis of the obtained results and by comparison with known data for related species. Mean amplitudes of vibration for the related F 3 BrO molecule are also estimated and briefly discussed