Band 54, Heft 5

The static and complex permittivity of 4-n-octyl-4'-cyanobiphenyl (8CB) has been measured for the isotropic, nematic and smectic A phases as functions of temperature and pressure. The ranges of 297 - 361 K, 0.1 - 220 MPa, and 0.1 - 13 MHz, were covered. Only the parallel component of the complex permittivity, ɛ*( f ) = ɛ' ( f ) - iɛ"(f), was measured. The relaxation times T || (p, T ) as well as T is (p, T) were analysed at constant temperature, pressure and volumes yielding the activation volume, Δ ≠ V(T), activation enthalpy Δ ≠ H(p), and activation energy Δ ≠ U(V), respectively. All activation parameters calculated for the smectic A phase of 8CB are smaller than those obtained for the nematic phase. The activation energy constitutes approximately half of the activation enthalpy value in all three phases studied. The pressure study allowed to calculate the pressure dependence of the retardation factor g || ( p, T), from which the nematic potential q(p,T ) can be derived. Using the relationships between g\\ and q/ RT proposed by Kalmykov and Coffey, the order parameter (P 2 (p, T)) was calculated as a function of pressure.

The phase diagram of trans-4-n-hexyl-(4'-cyanophenyl)-cyclohexane, (6PCH) has been established by high-pressure differential thermal analysis. Specific volumes are presented for temperatures between 300 and 370 K up to 300 MPa. The p,V m ,T data have been determined for the nematic, isotropic, and (partly, in the neighbourhood to the melting curve) solid phases. Volume and enthalpy changes along the phase transitions have also been calculated. As previously, the p,V m ,T data were used to calculate the volume entropy for the nematic-isotropic transition. The molar volumes along the clearing line T NI (p) enabled us to calculate the molecular field parameter γ = ∂ln T NI /∂In V NI , being 4.1.

The phase behaviour of a new liquid crystal, belonging to the series l-[4-n-alkyl-biphenyl]-2-[4-isothio-cyanato-phenyl]ethane ( n TPEB), n = 10, has been investigated with differential scanning calorimetry at ambient and high pressure. The phase behaviour depends on the thermal treatment. Phase transition temperatures have been determined as a function of pressure up to 300 MPa. No pressure-induced or pressure-limited phases are observed in this pressure range. Enthalpy-and volume-changes accompanying the phase transitions have been calculated using the Clausius-Clapeyron equation.

Starting from two line solitons, the solution of an integrable (2 + 1)-dimensional sine-Gordon equation in bilinear form yields a dromion solution that is localized in all directions for a suitable potential. The interaction between two dromions is studied in detail through the method of figure analysis. For different selections of parameters, the interactions between two dromions may be elastic, not completely elastic, or completely inelastic.

The electron paramagnetic resonance spectra of 3/ß-doxyl-5α-cholestane dissolved in five liquid crystals have been recorded as a function of temperature in the isotropic and mesogenic phases. From these spectra the order parameter (P 2 ) has been determined. The results have been com-pared with the data obtained from the optical birefringence measurements and from the polarized absorption spectra of the dichroic dye dissolved in liquid crystal host.

This study reports on a theoretical calculation of Hahn's spin-echo experiment in case of a model A 2 B 2 spin system with a strongly coupling character and gives the experimental results of one-dimension 1 H high-resolution NMR spectra of taurine and aspartate. The calculated amplitudes of the spin-echoes for two different proton groups of taurine are given. Using results of our calculations for taurine, the computer simulations of J -modulation are implemented. It is shown that the agreement be-tween the experimental and simulated spectra is good.

Classical Monte Carlo calculations have been performed in order to investigate the ability of the TIP4P, SPC, and SPCE water models to reproduce the structural features of liquid water. The simulations were carried out in the NPT ensemble at 4 thermodynamic conditions. The results are compared with recent neutron diffraction data. Essentially, the three models capture equally well the thermodynamic and structural features of water. Although they were parametrized to reproduce the water properties at ambient conditions, the agreement with the experimental pair correlation functions was even better at super-critical conditions. This is because the effective pair potentials have some difficulty to re-produce cooperative interactions, like hydrogen bonds. These interactions are less effective at super-critical conditions, where the liquid behaves roughly like a gas

From the knowledge of the spin correlation functions derived from Mössbauer relaxation spectra of quasi-one-dimensional A 2 Mn 0.98 Fe ()()2 F 5 (H 2 O) we could fit the antiferromagnetic susceptibilities of A 2 MnF 5 (H 2 O) with A = Na + , (NH 4 ) + , K + , Rb + obtained for single crystal samples. The calculation yielded characteristic parameters such as the local anisotropy D, the intra-chain exchange energy J, the inter-chain exchange energy |Jʹ|, the Neel temperature T N , and the spin canting angle φ.

The ε values in the internal cation mobilities of (Li,K,Cs)NO 3 of the eutectic composition (35.2-39.8-25.0 mol%) in the temperature range from 453 to 673 K were measured by Klemm's countercurrent electromigration method, where e is defined as (b i -b j )/b a ; b a is the average internal cation mobility and the subscripts i and j refer to any two out of these three cations. The conductivity was measured by a direct current method. From these data and the molar volume calculated from those of the pure salts on the assumption of additivity, the internal cation mobilities, b Li , b K and b Cs , have been calculated. The b Li is well expressed by the empirical equation presented for binary alkali nitrates; the negative devia-tion for b K and b Cs may be accounted for on the assumption of the tranquilization effect of Li + . The orders of those internal cation mobilities are bCs<bK<bLi at 453-473 K, bCs<bLi<bK at 473 -583 K, and b Li <bCs<bK at 583-673 K. These orders can be interpreted in terms of the dynamic dissociation model previously presented.

The method of multiple scales is used to analyse the second harmonic resonance of weakly nonlinear progressive waves on the surface of a fluid column in the presence of a magnetic field. The dynamical equations governing the second harmonic resonance are obtained. Numerical results are given graphically.

The stability of the plane interface separating two viscoelastic (Walters B') superposed fluids in por-ous medium in the presence of suspended particles is considered. For the case of two uniform Walters B' fluids separated by a horizontal boundary, the system is found to be stable or unstable under certain conditions for the stable configuration. However, the system is found to be unstable for the unstable configuration. The case of an exponentially varying density is also considered. For the stable stratifica-tion, the system is found to be stable or unstable under certain conditions, whereas the system is found to be unstable for the unstable stratification. The behaviour of growth rates with respect to suspended particle, number density, and medium permeability is examined analytically.

The third-order optical nonlinearity of a nitrogen doped car-bon nanotube is studied. It is found that carbon nitride nano-tubes are potentially important in photonics owing to their large nonlinear optical response.