Molecular dynamic study of the odd-even effect in some 4- n -alkyl- 4' -cyanobiphenyls

Using a united atom model, we performed molecular dynamics (MD) simulations on the 4- n -alkyl- 4' -cyanobiphenyl (nCB) liquid crystal homologous series for n=5-9 at a constant reduced temperature in the nematic phase. To evaluate the rotational diffusion coefficient (RDC), the second- and fourth-rank order parameters (OP's) and correlation time were calculated from MD trajectories. We analyzed the rotational viscosity coefficient (RVC) by using the Nemtsov-Zakharov and Fialkowski methods based on statistical-mechanical approaches. The simulated RDC, RVC, and OP's were found to be reasonable when compared with previous theoretical and experimental data. These quantities were also calculated for the rigid core and alkyl chain segments separately for each mesogen, to take a closer look at the molecule tail-chain flexibility. The properties calculated for a rigid core were compared with those for an alkyl chain, and satisfactory results were obtained. Odd-even effects for RDF, RVC, and molecular ordering parameters were calculated for five mesogens. © 2006 The American Physical Society

Miscibility and diffusivity of water in organic acids: Molecular dynamics simulations

Molecular dynamics simulations were performed to study both the structural and dynamic properties of n-alkyl carboxylic acid/water/n-alkyl carboxylic acid system with different alkyl chain length, such as acetic (CH3COOH) (A/W/A system), butyric (CH3(CH2)2COOH) (B/W/B system) and hexanoic (CH3(CH2)4COOH) (H/W/H system) acids. The mass density profiles along the interface normal to the organic acid/water system, the difference in the local structure of H2O molecules in bulk and in the vicinity of interface for A/W/A, B/W/B and H/W/H systems, as well as the diffusion behavior of water molecules at the interface with abovementioned organic acids, in the wide range of the temperature changes, have been investigated. Because of the central importance of water to the biology and pharmacology a number of new calculated regimes of miscibility and diffusivity of water molecules in carboxylic acids help us to better analyze such organic systems. © 2019Ege Üniversitesi BIDEB-2221 Türkiye Bilimsel ve Teknolojik Araştirma KurumuThe Turkish group used the facilities of the TUBITAK ULAKBIM High Performance Computing Center. A. V. Z. acknowledges financial support from Scientific and Technological Research Council of Turkey (TUBITAK) BIDEB-2221 , which funded his stay at Ege University. Appendix A -

Molecular structure and elastic properties of thermotropic liquid crystals: Integrated molecular dynamics - Statistical mechanical theory vs molecular field approach

PubMed ID: 23534657The connection between the molecular structure of liquid crystals and their elastic properties, which control the director deformations relevant for electro-optic applications, remains a challenging objective for theories and computations. Here, we compare two methods that have been proposed to this purpose, both characterized by a detailed molecular level description. One is an integrated molecular dynamics-statistical mechanical approach, where the bulk elastic constants of nematics are calculated from the direct correlation function (DCFs) and the single molecule orientational distribution function [D. A. McQuarrie, Statistical Mechanics (Harper Row, New York, 1973)]. The latter is obtained from atomistic molecular dynamics trajectories, together with the radial distribution function, from which the DCF is then determined by solving the Ornstein-Zernike equation. The other approach is based on a molecular field theory, where the potential of mean torque experienced by a mesogen in the liquid crystal phase is parameterized according to its molecular surface. In this case, the calculation of elastic constants is combined with the Monte Carlo sampling of single molecule conformations. Using these different approaches, but the same description, at the level of molecular geometry and torsional potentials, we have investigated the elastic properties of the nematic phase of two typical mesogens, 4'-n-pentyloxy-4-cyanobiphenyl and 4'-n-heptyloxy-4-cyanobiphenyl. Both methods yield K3(bend) >K1 (splay) >K2 (twist), although there are some discrepancies in the average elastic constants and in their anisotropy. These are interpreted in terms of the different approximations and the different ways of accounting for the structural properties of molecules in the two approaches. In general, the results point to the role of the molecular shape, which is modulated by the conformational freedom and cannot be fully accounted for by a single descriptor such as the aspect ratio. © 2013 American Institute of Physics