Towards Nematic Phases in Ionic Liquid Crystals - A Simulation Study.

Ionic liquid crystals (ILCs) are soft matter materials with broad liquid crystalline phases and intrinsic electric conductivity. They typically consist of a rod-shaped mesogenic ion and a smaller spherical counter-ion. Their mesomorphic properties can be easily tuned by exchanging the counter ion. ILCs show a strong tendency to form smectic A phases due to the segregation of ionic and the non-ionic molecular segments. Nematic phases are therefore extremely rare in ILCs and the question of why nematic phases are so exceptional in existing ILCs, and how nematic ILCs might be obtained in the future is of vital interest for both the fundamental understanding and the potential applications of ILCs. Here, we present the result of a simulation study, which highlights the crucial role of the location of the ionic charge on the rod-like mesogenic ions in the phase behaviour of ILCs. We find that shifting the charge from the ends towards the centre of the mesogenic ion destabilizes the liquid crystalline state and induces a change from smectic A to nematic phases.

Molecular Electron Density Distribution and X-Ray Diffraction Patterns of Smectic A Liquid Crystals - A Simulation Study.

X-ray diffraction (XRD) is one of the most important methods to assess the long-range translational order in smectic A (SmA) liquid crystals. Nevertheless, the knowledge about the influence of the molecular electron density distribution (MEDD) on the XRD pattern is rather limited because it is not possible to vary the orientational order, the translational order and the MEDD independently in an experiment. We here present a systematic simulation study in which we examine this effect and show that the MEDD indeed has a major impact on the general appearance of the XRD pattern. More specifically, we find that the smectic layer peaks and the intensity ratios thereof strongly depend on the width of the MEDD. The classic approach by Leadbetter et al. to determine the smectic translational order parameter ∑ from XRD intensities works if the MEDD is quite narrow. In all other cases the influence of the MEDD has to be taken into account.

Reliability of Orientational Order Parameters Determined from Two-dimensional X-ray Diffraction Patterns: A Simulation Study.

The orientational order parameter S2 is one of the most important quantities to describe the degree of long-range orientational ordering of liquid crystals. There are several approaches to experimentally measure this order parameter of liquid crystalline phases but every method includes substantial simplifications and assumptions. We present a simulation-based approach to elucidate the reliability of the method of Davidson, Petermann and Levelut to measure S2 via 2D X-ray experiments. We have found that this method slightly underestimates S2 by an absolute value of only 0.05 and thus provides reliable measures of S2 by X-ray diffraction.

Orientational distribution functions and order parameters in "de Vries"-type smectics: A simulation study.

Simple smectic A liquid crystal phases with different types of prescribed orientational distribution functions have been simulated and compared in order to study the possibility to distinguish between the Maier-Saupe type and cone-like orientational distributions using the popular method of Davidson et al. [J. Phys. II 5, 113 (1995)]. This method has been used to extract the orientational distribution functions from simulated diffraction patterns, and the results have been compared with actual distribution functions which have been prescribed during simulations. It has been shown that it is indeed possible to distinguish between these two qualitatively different types of orientational distribution already from the shape of the 2D diffraction pattern. Moreover, typical experimental diffraction patterns for "de Vries"-type smectic liquid crystals appear to be close to the ones which have been simulated using the prescribed Maier-Saupe orientational distribution function.

Orientational fluctuations near the smectic A to smectic C phase transition in two "de Vries"-type liquid crystals.

On the basis of thorough analysis of 2D X-ray diffraction patterns from smectic monodomains, we examine the influence of orientational fluctuations on the weakly first-order smectic A (SmA) to smectic C (SmC) transitions in two nonchiral organosiloxane "de Vries"-type liquid crystals. We find that these materials exhibit very large molecular tilt fluctuations with magnitudes of up to 35°--thus larger than the average tilt itself. This is essential to understand the underlying molecular mechanism behind the practical absence of smectic layer contraction in these materials: in the SmA phase, the nematic order parameter is very low (molecular fluctuations correspondingly high), and the expected layer shrinkage at the SmA to SmC transition is almost fully compensated by the increase in orientational order, as the fluctuations diminish with decreasing temperature. In addition to the general tilt fluctuations, we observe intrinsic soft-mode fluctuations. They have a λ-shaped temperature dependence that peaks at the SmA-SmC transition with a maximum amplitude of about 2°.

Thermal stability of carbonyl radicals. Part II. Reactions of methylglyoxyl and methylglyoxylperoxy radicals at 1 bar in the temperature range 275-311 K.

Reactions of methylglyoxyl and methylglyoxylperoxy radicals were investigated at a total pressure of 1 bar in oxygen. Methylglyoxyl radicals were generated by stationary photolysis of Br2-CH3C(O)C(O)H-NO2-O2-N2 mixtures at wavelengths > or =480 nm and of Cl2-CH3C(O)C(O)H-NO2-O2-N2 mixtures in the wavelength range 315-460 nm. In the bromine system, rate constant ratios for the reactions CH3C(O)CO --> CH3CO + CO (kdis) and CH3C(O)CO + O2 --> CH3C(O)C(O)O2 (kO2) were measured as a function of temperature in the range 275-311 K. Assuming the constant value kO2 = 5.1 x 10(-12) cm3 molecule(-1) s(-1) for our reaction conditions, kdis = 1.2 x 10(10.0+/-0.7) x exp(-11.7 +/- 3.8 kJ mol(-1)/RT) s(-1) (2sigma errors) was obtained for ptot = 1 bar (M = O2), in good agreement with the kinetic parameters calculated by Méreau et al. [R. Méreau, M.-T. Rayez, J.-C. Rayez, F. Caralp and R. Lesclaux, Phys. Chem. Chem. Phys., 2001, 3, 4712]. CH3C(O)C(O)O2 radicals oxidise NO2, forming NO3, CH3CO and CO2. This experimental result is supported by DFT and ab initio calculations. Possible mechanisms for the observed formation of several % of ketene and bromoacetyl peroxynitrate are discussed. Use of Cl rather than Br atoms to abstract the aldehydic H atom from methylglyoxal leads to chemically activated CH3C(O)CO radicals, thus substantially increasing the fraction of CH3C(O)CO radicals that decompose rather than add O2.

Reaction of phenylperoxy radicals with NO(2) at 298 K.

In the present work, phenylperoxy radicals were generated by stationary 254 nm photolysis of iodobenzene and nitrosobenzene in the presence of O(2) and NO(2) at 298 K and a total pressure of 1 bar (M = N(2)). Experiments were performed on time scales of seconds or minutes in a temperature controlled photoreactor made of quartz (v = 209 L). Major gas phase products identified and quantified in situ by long-path IR absorption include N(2)O(5), NO, HONO, HNO(3), CO, and o-nitrophenol. In addition, evidence is presented for the formation of an aerosol consisting of p-nitrophenol. The occurrence of N(2)O(5) as a major product in both reaction systems, the strong loss of NO(2) in the iodobenzene system and the comparison of measured product distributions with the results of numerical model calculations suggest that the reaction C(6)H(5)O(2) + NO(2) --> C(6)H(5)O + NO(3), k(5)occurs in both photolysis systems, a major part of the NO(3) being scavenged as N(2)O(5). The results of ab initio calculations imply that proceeds via a short-lived peroxynitrate intermediate. In the photolysis of nitrosobenzene-NO(2)-O(2)-N(2) mixtures, NO and NO(2) compete for C(6)H(5)O(2) radicals. Comparison of measured and modelled product distributions allows to set a lower limit of k(5) > 1 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K. This lower limit is consistent with the assumption that k(5) is equal to the high pressure recombination rate constant of RO(2) + NO(2) --> RO(2)NO(2) reactions, i.e. with k(5) approximately 7 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K, 1bar.