Microscopic structure and dynamics of a partial bilayer smectic liquid crystal (original) (raw)
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ChemPhysChem, 2009
We have studied the important n-cyano biphenyl series of mesogens, n=4-8 using modelling and molecular dynamics simulations. We have been able to obtain spontaneously ordered nematics upon cooling isotropic samples of 250 molecules. We show that, using the united atom force field developed here, the experimental isotropic-nematic transition temperatures are reproduced within 4 K, allowing a molecular level interpretation of the odd-even effect along the series. Other properties, like densities, orientational order parameters and NMR residual dipolar couplings are also well reproduced, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure.
ChemPhysChem, 2014
We report an investigation of the alignment of biphenyl (2P) in the liquid crystal phases of 5CB and 8CB, combining predictive atomistic Molecular Dynamics (MD) simulations and 1 H-LXNMR residual dipolar couplings measurements. We provide first a detailed comparison and validation of the MD results with LXNMR, showing a good agreement between the simulated and experimental dipolar couplings at the same reduced temperature. We then use MD to examine the location of biphenyl in the smectic, by itself unavailable to LXNMR and find that, surprisingly, it is rather uniformly distributed. We show that the combination of MD and NMR provides very detailed information about the order, the interconnection between orientation and conformation, the local positional order and interactions with the liquid crystalline solvent.
The Journal of Physical Chemistry B, 2007
Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4′-cyanobiphenyls (nCB) with n ) 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336), successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here been extended to higher homologues. Nematic and isotropic phases were found for all members of the series, and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures reproduce the experimental values within (10 K. Also, structural properties as second and fourth rank orientational order parameters are in good agreement with the corresponding experimental quantities. This means that the well-known odd-even effect, observed for many properties along the nCB series, is well reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the correlation between molecular properties and odd-even effects is presented. *
Molecular organization in freely suspended nano- thick 8CB smectic films. An atomistic simulation
We present an atomistic molecular dynamics simulation of freely suspended films of the smectic liquid crystal 8CB formed by n l = 2, 3,. . .,10, 20 theoretical monolayers, determining their orientational and positional order as a function of the film thickness. We find that films are always composed by bilayers of antiparallel molecules, and that in the case of odd n l , the system prefers to self-assemble in (n l + 1)/2 bilayers, with an increase of surface tension with respect to even n l samples. We also show that external layers have higher positional and orientational order, and that upon heating the disordering of the system proceeds from the inside, with the central layers progressively losing their smectic character, while the external ones are more resistant to temperature changes and keep the film from breaking.
The European Physical Journal E, 2012
Solute molecules were dissolved in the liquid crystal 4-cyano-4-n-octyloxybiphenyl (8OCB), known to form a partial bilayer smectic-A phase. Through measurement of solutes' and solvent's orientational order parameters via nuclear magnetic resonance spectroscopy, and their analysis via a statistical thermodynamic density functional theory, values of the solvent's positional order parameters and solutes' positional-orientational distribution functions were obtained. Near to the transition to the nematic phase, the main positional order parameter of the smectic liquid crystal turned out to be comprised in the interval 0.4-0.6, though the quality of the fittings assuming the phase as nematic all across the temperature range investigated was only slightly worse. This may be ascribed to the looseness of the partial bilayer smectic structure. Solutes were found to preferentially lie in those regions where liquid crystal molecule terminal chains are located.
Physical Review E, 1999
Using Monte Carlo simulation methods, we explore the role of molecular shape in the phase behavior of liquid crystals and the electroclinic effect. We study a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres bonded rigidly together with no intra-molecular degrees of freedom. For strongly angled molecules, we find that steric repulsion alone provides the driving force for a smectic-C phase, even without intermolecular dipole-dipole interactions. For weakly angled (nearly rod-like) molecules, we find a stable smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt angle of about 19 •. In the SmA phase we find evidence of vortex-like point defects. We also observe a field-induced nematic-smectic phase transition.