Field-induced uniaxial and biaxial nematic phases in the Maier–Saupe–Zwanzig (MSZ) lattice model

We report detailed statistical mechanics calculations to analyse the field-temperature phase diagram of a fully connected Maier–Saupe–Zwanzig (MSZ) lattice model for the nematic transitions in liquid-crystalline systems. If the field is applied along the uniaxial direction, with positive anisotropy, there is a first-order boundary between distinct uniaxial paranematic phases, which ends at a simple critical point. For negative values of the anisotropy, the first-order boundary becomes continuous at a tricritical point, and the transition is between field-induced uniaxial and biaxial structures. This elementary MSZ lattice model provides a way to systematically regain all of the main features of earlier phenomenological and model calculations. Due to the simplicity of the calculations, critical and tricritical points, as well as critical lines, can be located analytically, which leads to a possibility of comparisons with values for real experimental systems. Also, we formulate a generalised six-state MSZ model, which takes into account intrinsic molecular biaxiality, and perform calculations to determine the location of the critical point in the field-temperature phase diagram.     

Elektrostatische Kapazität pflanzlicher Gewebe und organischer Kolloide

Ohne Zusammenfassung     

Phase separation of the Potts model in the square lattice

When the two dimensional q-color Potts model in the square lattice is quenched at zero temperature with Glauber dynamics, the energy decreases in time following an Allen-Cahn power law, and the system converges to a phase with energy higher than the ground state energy after an arbitrary large time when q>4. At low but finite temperature, it cesses to obey the power-law regime and orders after a very long time, which increases with q, and before which it performs a domain growth process which tends to be slower as q increases. We briefly present and comment numerical results on the ordering at nonzero temperature.     

Carboxylate Catalyzed Isomerization of β,γ-Unsaturated N-Acetylcysteamine Thioesters**

We demonstrate herein the capacity of simple carboxylate salts – tetrametylammonium and tetramethylguanidinium pivalate – to act as catalysts in the isomerization of β,γ-unsaturated thioesters to α,β-unsaturated thioesters. The carboxylate catalysts gave reaction rates comparable to those obtained with DBU, but with fewer side reactions. The reaction exhibits a normal secondary kinetic isotope effect (k_1H/k_1D=1.065±0.026) with a β,γ-deuterated substrate. Computational analysis of the mechanism provides a similar value (k_1H/k_1D=1.05) with a mechanism where γ-reprotonation of the enolate intermediate is rate determining.     

Phase equilibria on binary systems containing diethyl sulfide

Isothermal vapor-liquid equilibrium (VLE) of the following systems was measured with a recirculation still: diethyl sulfide + ethanol at 343.15 K, diethyl sulfide + 1-propanol at 358.15 K, and diethyl sulfide + propyl acetate at 363.15 K. Diethyl sulfide + ethanol at 343.15 K and diethyl sulfide + 1-propanol at 358.15 K systems exhibit positive deviation from Raoult's law, whereas diethyl sulfide + propyl acetate at 363.15 K system exhibits only slight positive deviation from Raoult's law. A maximum pressure azeotrope was found in the systems diethyl sulfide + ethanol (x₁ = 0.372, P = 88.4 kPa, T = 343.15 K) and diethyl sulfide + 1-propanol (x₁ = 0.640, P = 96.8 kPa, T = 358.15 K). No azeotropic behavior was found in diethyl sulfide + propyl acetate system at 363.15 K. The experimental results were correlated with the Wilson model and compared to COSMO-SAC predictive model. Liquid and vapor phase compositions were determined with gas chromatography. All measured data sets passed the thermodynamic consistency tests. The activity coefficients at infinite dilution are also presented.     

Coherence for closed categories with biproducts

A coherence result for symmetric monoidal closed categories with biproducts is shown in this paper. It is also explained how to prove coherence for compact closed categories with biproducts and for dagger compact closed categories with dagger biproducts by using the same technique.     

Stoichiometric Determination of Carbon in Ceramic Materials by Low Energy Deuteron Activation Analysis

The ¹²C(d,n)¹³N reaction, at 2 MeV, was used for the stoichiometric determination of carbon in reference materials and sintered pellets of SiC ceramics. The preliminary results show higher percentage of carbon on the surface in comparison to the core of the sintered ceramics, indicating the existence of carbon contamination on the surface, possibly caused by the sintering process itself.