Chiral induction in nematics A computer simulation study

Abstract

We have studied chiral induction in a nematic in contact with a chiral surface using computer simulations. Nematic and surface particles have been modelled using the GayÈBerne (GB) potential considering, additionally, a short-range chiral term for the inducing surface. We Ðnd that, close to the chiral surface, a twist of the local director with respect to the surface molecules is induced, even in the isotropic phase. In the nematic phase, the twist is maintained through orientational correlation and, even well inside, the sample molecules are e †ectively twisted with respect to the surface director. This process can highlight the basic mechanism of the chiral induction of a cholesteric, i.e. chiral nematic, phase. A detailed description of the molecular organization at various distances from the inducing surface is presented, using scalar and pseudoscalar orientational correlation functions.A well known but still fascinating e †ect is the induction of a chiral nematic phase upon dissolving a small quantity of a chiral solute in a nematic,1,2 which can exhibit a huge susceptivity to this chiral perturbation. Such induced chiral nematic phases have a helical structure with a repeat distance that can be of a length comparable to the wavelength of visible or IR light, depending on soluteÈsolvent characteristics and solute concentration. The induction of a macroscopic twist seems likely to be connected to the extent of orientational pair correlation. In fact, the induced optical activity, which is a measurable quantity related to chirality, is very weak in the isotropic phase compared to that measurable in nematic phases.3,4 Since the concentration of chiral dopants needed to induce a cholesteric phase can be very low, even mole fractions of the chiral inducer below 10~4 are sufficient,5 it is difficult to understand how the local director twist is generated and propagated. Indeed, every chiral molecule is expected to have an environment with a large predominance of achiral molecules and yet the chiral induction is very e †ective. This is further complicated by the fact that chiral interactions are expected to be, on one hand, weak and, on the other, fairly shortranged.6,7 The induction of a chiral nematic phase has been extensively studied, both experimentally and theoretically (see e.g. ref. 8 and references therein). However, we are aware of no detailed computer simulation study of systems of chiral molecules dissolved in a nematic phase. Such a study would, in any case, be very complicated, using computer simulations, particularly since the low concentrations involved would make it very difficult to calculate meaningful statistics.We have thus decided to study by computer simulation the chiral induction in a simpler and more controlled system, where a nematic is put in contact with a surface covered by chiral molecules which cannot di †use into the nematic bulk. Such systems could be prepared by coating the surface in contact with the nematic with a layer of chiral molecules or with a chiral polymer.In this work ...

Chiral doping of nematic phases and its application to the determination of absolute configuration

“…These approaches have played an important role to explain the mechanism of chiral induction, but they can be of limited usefulness if the link between dopant configuration and cholesteric handedness is concerned. Also Monte Carlo techniques have been used to evaluate the helical twisting power of solutes 57,68,69 ; atomistic force fields allow a detailed account of the structure of the dopant, but long trajectories are needed for an exhaustive sampling of chiral properties, and the high computational demand of simulations is certainly a limit for practical purposes. Phenomenological models, able to account for the molecular structure with a modest computational cost, seem to provide more suitable tools for the assessment of the absolute configuration.…”

Twisting with a twist: supramolecular helix fluctuations in chiral nematics

“…For weakly chiral interactions (U c k B T ) it is justified to approximate Φ ≈ −βU c . To mimic the effective potential between soft helical filaments [22] [44,45] but has since then been consistently used in simulation models to capture chiral interactions (whether caused by quantum-mechanical or steric factors) between nonspherical mesogens [46][47][48][49][50]. As for the remaining parameters, ε c is an amplitude parameter and γ c a cut-off value for the angle, such that U c (γ c ) = 0.…”

Studies of Nanocomposites of Carbon Nanotubes and a Negative Dielectric Anisotropy Liquid Crystal

“…A three parameter (amplitude, T AC , and susceptibility exponent) power-law fit of e c −1 versus temperature resulted in γ = 0.99 ± 0.06. Despite the localization of the chirality induced in the liquid crystal to within a few nanometers of the CNT surface [58], the susceptibility exponent clearly is mean-fieldlike and apparently unaffected by the presence of the nanotubes. This result suggests that the Carbon Nanotubes -Recent Progress 196 director rotation extends a length scale ξ, comparable to the SmC correlation length, into the bulk liquid crystal from the narrow chiral region immediately surrounding the CNTs, which is the region that responds directly to the applied field.…”

“…Note that the negative dielectric anisotropy is due to the O-C=O group in the benzoate group linking the two phenyl rings. Molecular modeling suggests that the effective dipole moment of about 2.5 is pointing approximately perpendicular to the twisted phenyl rings, essentially out of the page [58].Studies of Nanocomposites of Carbon Nanotubes and a Negative Dielectric Anisotropy Liquid… http://dx.doi.org/10.5772/intechopen.73391 191 achieve the desired final weight percent ϕ w of CNTs. The mixture was then sonicated for 3 hours to facilitate good dispersion.…”

“…Prior to all measurements, temperature calibration was done with a sapphire disc, under the same experimental conditions used for all 9OO4/CNT samples. The analysis method used to extract the complex specific heat is based on linear response theory [50,58]. In general, a temperature oscillation is described as T(t) = T 0 + T˙0 t + A T sin(ωt), where T 0 is the initial temperature at time t = 0, T indicates the absolute temperature at time t, T˙0 represents the baseline temperature scan rate, A T is the temperature amplitude, and ω (ω = 2πf) is the angular frequency of the temperature modulation.…”