1,721,115 research outputs found
Modeling of Macromolecular Alignment in Nematic Virus Suspensions. Application to the Prediction of NMR Residual Dipolar Couplings
No full textThe alignment of macromolecules in dilute suspensions of filamentous phages is described in terms of steric and electrostatic contributions; the former are modeled as excluded volume interactions between a viral particle and a macromolecule, and the latter are treated at the mean field level, through the Poisson-Boltzmann equation for the virus surrounded by an ion density. The virus is represented as a uniformly charged rod, whereas the relevant features of the macromolecule, i.e., shape and charge distribution, are explicitly taken into account. As an application, the residual dipolar couplings between N-15 and H-1 nuclear spins in the Ig-binding domain of streptococcal protein G are calculated, and their dependence on ionic strength and virus dimension or concentration is analyzed. The theoretical predictions enable us to explain the NMR observations reported for this protein
Dielectric permittivity of nematics with a molecular based continuum model
No full textA theoretical model for the dielectric permittivity of nematics has been recently proposed [1], based on the atomistic representation of a probe molecule interacting with a medium which is characterised by its macroscopic properties. Electrostatic interactions are described through the classical model of a charge distribution contained in a molecular- shaped cavity embedded in an anisotropic dielectric continuum. Short-range intermolecular interactions are parameterized in terms of the anisometry of the molecular surface, which is defined according to the "rolling sphere" representation. The results obtained for the isotropic and nematic phases of 4,4'-pentyl-cyanobiphenyl and 4,4'-pentyl-cyanobicyclohexyl are reported; a good agreement with experiment appears, with a significant improvement with respect to the Maier-Meier theory
Shape model for the molecular interpretation of the flexoelectric effect
No full textA mean-field model for the flexoelectric polarization in nematics is presented, based on a continuous description of director deformations coupled to the molecular degrees of freedom via surface interactions. In such a framework, a consistent picture of the flexoelectric effect is obtained, including both dipolar and quadrupolar contributions, with a realistic account of the molecular characteristics of shape and charge distribution. The method is aimed at establishing a quantitative link between chemical structure and flexoelectric response. It provides numerical estimates of the effect and its temperature dependence and allows the recognition of the relevant molecular features for its emergence. Application to some representative systems, comprising mesogenic molecules and photoisomerizable dopants, is considered; it is shown that simple interpretative schemes can be misleading and a comparison with experimental data is reported
The theory of elastic constants
No full textThe elastic theory of liquid crystals can be traced back to the early 1930s, but the origin of the molecular theory of elastic constants must be postponed to more than 30 years later, when Alfred Saupe wrote his famous papers on this subject. At approximately the same time, the seminal works by Priest and Straley also appeared. Since then, several theories have been developed to connect intermolecular interactions to curvature deformations, on a quite different length-scale, in liquid crystals. This field was particularly alive between the end of the 1970s and the beginning of the 1980s, in parallel with experimental investigations. In more recent times, a renewed interest was aroused by the controversy about the second-order splay-bend contribution, which appears in the Nehring-Saupe expression for the deformation energy density. In the first part of the present contribution the molecular theory of elastic constants is briefly reviewed. This paper focuses on the effects of molecular structure on the elastic constants of thermotropic nematics and the ability of different models to account for them. A few classical examples are discussed to illustrate these issues. The second part of this paper is dedicated to our recent 'Surface Interaction' model, a molecular field approach based on the Maier-Saupe theory, implemented into a framework allowing for atomistic molecular modelling. The theoretical background is outlined, then some new results are reported and the insights derived from a realistic molecular representation are discussed. We conclude that, after about 40 years of theoretical investigations, there is a general consensus on the importance of the molecular shape in determining the elastic constants of nematics: for fairly rigid compounds these can be simply related to the length-to-width ratio, but for the general case of non-rigid mesogens the molecular flexibility and shape curvature have to be taken into account
Curvature elasticity of nematic liquid crystals: simply a matter of molecular shape? Insights from atomistic modeling
No full textThe elastic moduli of low-molar-mass thermotropic liquid crystals (LCs) exhibit an intriguing dependence on the molecular structure of the constituents, which can be very important for applications. We have recently developed a molecular field theory, wherein the elastic constants of nematics are expressed in terms of integrals over the molecular surface. This theory, combined with molecular geometry optimization, allows us to connect mesoscale deformations in liquid crystals to atomic-scale details. Here we investigate typical mesogenic systems, i.e. para-azoxyanisole (PAA) and 4-n-alkyl,-4'-cyanobiphenyls (nCBs), whose elastic properties exhibit clear differences. We show that these can be traced back to differences in molecular shape. Our calculations also highlight the importance of the flexibility of mesogens, which was generally ignored by previous theories: in view of their different shape, conformers are shown to give different contributions to the elastic constants. The key role of deviations from a rod-like shape, which is generally assumed by models of mesogens, emerges from our calculations. The bend elastic constant is shown to be particularly sensitive to this feature; for a given compound, rod-like conformers give a high contribution to the bending stiffness, whereas the contribution of bent conformers is low or even negative. The possible implications of these findings are discussed, with special reference to the behavior of bent-core mesogens. Finally, we predict the temperature dependence of the surface-like elastic constants, whose experimental determination is still controversial; we find that these are generally smaller than the bulk moduli and even more sensitive to changes in the molecular shape
Electroclinic effect in nematic liquid crystals: The role of molecular and environmental chirality
No full textThe electroclinic (EC) effect is the tilt of the optical axis of a liquid crystal in the plane perpendicular to an
applied electric field. Chirality plays a key role for its emergence. Based on the molecular and phase symmetry
we derive a molecular expression for the EC coefficient, the material property that quantifies the linear coupling
between tilt and electric field, in nematic liquid crystals. Modeling the relevant molecular properties (shape, elec-
tricdipolemoment,andpolarizability)withatomicresolution,wecalculatetheECcoefficientforprototypemolec-
ular structures. We demonstrate that molecular chirality, needed for the occurrence of the EC effect in nematics
withauniformdirector,isnotanecessaryrequirementinthepresenceofatwisteddirector.Ourresultsshowthatin
the latter case conformational deracemization, invoked to explain recent experiments, is not the only mechanism
On the elusive saddle–splay and splay–bend elastic constants of nematic liquid crystals
Full text linkThe elastic behavior of nematics is commonly described in terms of the three so-called bulk deformation modes, i.e., splay, twist, and bend. However, the elastic free energy contains also other terms, often denoted as saddle–splay and splay–bend, which contribute, for instance, in confined systems. The role of such terms is controversial, partly because of the difficulty of their experimental determination. The saddle–splay (K24) and splay–bend (K13) elastic constants remain elusive also for theories; indeed, even the possibility of obtaining unambiguous micro- scopic expressions for these quantities has been questioned. Here, within the framework of Onsager theory with Parsons–Lee correction, we obtain microscopic estimates of the deformation free energy density of hard rod nematics in the presence of different director deformations. In the limit of a slowly changing director, these are directly compared with the macroscopic elastic free energy density. Within the same framework, we derive also closed microscopic expressions for all elastic coefficients of rodlike nematics. We find that the saddle–splay con- stant K24 is larger than both K11 and K22 over a wide range of particle lengths and densities. Moreover, the K13 contribution comes out to be crucial for the consistency of the results obtained from the analysis of the microscopic deformation free energy density calculated for variants of the splay deformation
Interplay of Particle Morphology and Director Distortions in Nematic Fluids
No full textThe existing microscopic theories for elasticity of nematics are challenged by recent findings on systems, whether bent molecules or semiflexible polymers, which do not comply with the model of rigid rodlike particles. Here, we propose an extension of Onsager-Straley second-virial theory, based on a model for the orientational distribution function that, through explicit account of the director profile along a particle, changes in the presence of deformations. The elastic constants reveal specific effects of particle morphology, which are not captured by the existing theories. This paves the way to microscopic modeling of the elastic properties of semiflexible liquid crystal polymers, which is a longstanding issue
Spontaneous Twisting of Achiral Hard Rod Nematics
No full textSince Onsager’s seminal work, hard rods have been taken as a prototype of nematic liquid crystals, characterized by uniaxial order and a uniform director field as a ground state. Here, using Onsager theory to calculate the free energy in the presence of arbitrary deformations, we find that hard rod nematics have an intrinsic tendency to twist around their ordering axis (double twist), driven by a mechanism in which the orientational fluctuations of particles play a key role. The anisotropic hard core potential used here is arguably the simplest form of interaction able to originate spontaneous breaking of mirror symmetry in a 3D fluid. Our results are discussed in relation to the recent discovery of a double twisted ground state in cylindrically confined lyotropic chromonic liquid crystals
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