Simple Molecular Model for the Smectic A Phase of Liquid Crystals
TL;DR: In this article, the Maier-Saupe model with an orientational order parameter is extended to the smectic $A$ phase by introducing a new order parameter, the amplitude of a density wave in the direction of the nematic preferred axis.
Abstract: The Maier-Saupe model of the nematic phase with an orientational order parameter is extended to the smectic $A$ phase by introducing a new order parameter, the amplitude of a density wave in the direction of the nematic preferred axis. Self-consistent equations for the two order parameters are derived from an anisotropic model interaction and are solved numerically. We calculate the order parameters, the entropy, and the specific heat as a function of temperature for several values of dimensionless interaction strength $\ensuremath{\alpha}$ for the smectic $A$ phase. The transition temperatures plotted versus $\ensuremath{\alpha}$ provide a theoretical phase diagram which resembles experimental plots of transition temperature versus alkyl chain length for homologous series of compounds. The model qualitatively reproduces chemical trends in transition entropies. Experiments are suggested to measure the order parameters in the smectic $A$ phase.
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TL;DR: In this article, the physical properties of nematic, cholesteric, and smectic liquid crystals are discussed and a wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties.
Abstract: This review discusses the physical properties of nematic, cholesteric, and smectic liquid crystals. Molecular theories of the liquid crystal phases are discussed and the molecular field theories of the phase transitions between the various liquid crystal phases are presented. The elastic theory and hydrodynamics of liquid crystals is developed. A wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties, is discussed.
978 citations
TL;DR: In this paper, a review focusing on nematic liquid crystals is presented, where three main kinds of effects can be distinguished: the perturbation of the liquid crystalline structure close to the surface, the bulk liquid crystal structure is recovered with an orientation which is fixed by the surface and critical adsorption or wetting can occur at surfaces.
Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.
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TL;DR: In this paper, the authors used a self-consistent molecular field approximation to calculate the nematic orientational ordering of a system where molecules consist of rigid parts and chains, and provided an explanation for the even-odd effect in isotropic-nematic transition temperatures.
Abstract: In the liquid crystalline phases of many organic materials molecular end chains take part in the ordering process. For a single chain, this ordering is calculated by exact summation over all conformations of a chain in the molecular field due to the neighboring molecules. The nematic orientational ordering of a system where molecules consist of rigid parts and chains is then obtained within a self‐consistent molecular field approximation. The results are in very good agreement with the experimental phase diagrams for homologous series of liquid crystals, and for the first time provide the explanation for the well known even‐odd effect in isotropic‐nematic transition temperatures. In agreement with observations, the corresponding transition entropies exhibit both the increase with increasing end‐chain length and the even‐odd variations. The decrease of order along the chain is also calculated and compared with the available data.
388 citations
TL;DR: Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment in gas-phase spectroscopy.
Abstract: Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.
359 citations
Cites background from "Simple Molecular Model for the Smec..."
...If both the density profile and the direct correlation function are expressed in terms of the lowest-order spherical harmonics and with some further approximations, Equation 29 reduces to the familiar Maier-SaupeMcMillan theory of liquid crystals (66)....
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TL;DR: In this paper, the authors describe a mittlere Innere Feld einer nematischen Molekülanordnung abgeleitet, and gezeigt, daß this Innere Field in einem gewissen Temperaturbereich einen nematicalhen Ordnungszustand bewirken muß and daß dieser ordnung sustand was with einer bestimmten Temperatur diskontinuierlich und unter Energieaufnahme in die isot
Abstract: Mit Beschränkung auf den Dipol-Dipol-Anteil der reinen Dispersionswechselwirkungen als dem für die Existenz einer kristallinflüssigen Phase vermutlich maßgebenden Teil der zwischenmolekularen Kräfte wird durch geeignete Mittelungen ein Ausdruck für das auf das Einzelmolekül wirkende mittlere Innere Feld einer nematischen Molekülanordnung abgeleitet. Es wird gezeigt, daß dieses Innere Feld in einem gewissen Temperaturbereich einen nematischen Ordnungszustand bewirken muß und daß dieser Ordnungszustand bei einer bestimmten Temperatur diskontinuierlich und unter Energieaufnahme in die isotrope Molekülanordnung übergehen muß. Die Theorie liefert also einen Umwandlungspunkt 1. Ordnung, wie er experimentell auch beobachtet wird. Die Umwandlungstemperatur wird im wesentlichen durch die Anisotropie der optischen Übergangsmomente der Moleküle bestimmt.
1,328 citations
932 citations
TL;DR: A theory of translational and orientational melting with application to liquid crystals is presented in this article in a manner analogous to that of Kirkwood and Monroe by introducing both translational or orientational order parameters.
Abstract: A theory of translational and orientational melting with application to liquid crystals is presented in a manner analogous to that of Kirkwood and Monroe by introducing both translational and orientational order parameters.
121 citations
TL;DR: The theory of translational and orientational melting with application to liquid crystals is presented in this article in a manner analogous to that of Kirkwood and Monroe, and the theory is applied to liquid crystal crystallography.
Abstract: The theory of translational and orientational melting with application to liquid crystals is presented in a manner analogous to that of Kirkwood and Monroe.
106 citations
TL;DR: In this article, an additional lower temperature liquid crystalline phase is observed with chloro-substituted compounds for n-alkoxy chain lengths of C8 and longer and with the methyl substituted compounds for chain length of C10 and longer.
Abstract: Bis-(4′-n-alkoxybenzal)-2-chloro-1, 4-phenylenediamines and bis- (4′-n-alkoxybenzal)-2-methyl-1, 4-phenylenediamines with different alkoxy groups have been synthesized. Similar compounds with fluoro, bromo and chloro groups replacing the alkoxy groups have also been prepared. All the compounds show a nematic liquid phase. An additional lower temperature liquid crystalline phase is observed with chloro-substituted compounds for n-alkoxy chain lengths of C8 and longer and with the methyl substituted compounds for chain lengths of C10 and longer. This lower temperature phase can show a threaded texture very similar to the nematic phase. Furthermore, both phases can be uniformly oriented by surface action and can assume a twisted structure. We suppose that the lower temperature phase has a layered structure in which the molecules are inclined to the layers, and that it corresponds to a smectic liquid crystal classified by Sackmann and Demus as smectic C.
68 citations