Orientational Ordering Of Molecules Research Articles

Effects of Surface Charge Distribution and Electrolyte Ions on the Nonlinear Spectra of Model Solid-Water Interfaces.

Molecular dynamics simulations of model charged solid/water interfaces were carried out to provide insight about the relationship between the second-order nonlinear susceptibility χ(2) and the structure of the interfacial water layer. The results of the calculations reveal that the density fluctuations of water extend to about 12 Å from the surface regardless of the system, while the orientational ordering of molecules is long-ranged and is sensitive to the presence of electrolytes. The charge localization on the surface was found to affect only the high-frequency part of the Im[χ(2)] spectrum, and the addition of salt has very little effect on the spectrum of the first water layer. For solid/neat water interfaces, the spectroscopically active part of the liquid phase has a thickness largely exceeding the region of density fluctuations, and this long-ranged nonlinear activity is mediated by the electric field of the molecules. The electrolyte ions and their hydration shells act in a destructive way on the molecular field. This effect, combined with the screening of the surface charge by ions, drastically reduces the thickness of the spectroscopic diffuse layer. There is an electrolyte concentration at which the nonlinear response of the diffuse layer is suppressed and the χ(2) spectrum of the interface essentially coincides with that of the first water layer.

Quantum Rotation and Orientational Ordering of Molecules in Crystals

Quantum Rotation and Orientational Ordering of Molecules in Crystals

Differential Capacitance of Electric Double Layer – Influence of Asymmetric Size of Ions, Thickness of Stern Layer and Orientational Ordering of Water Dipoles

The mean-field theoretical model of the electric double layer which takes into account the asymmetric finite size of anions and cations and the orientational ordering of water dipoles in the Stern and the diffuse layers is described together with a short description of the main concepts and a brief review of the literature in the theory of the electric double layer. As an example of the application of the described mean-field lattice model of the electric double layer, the influence of different sizes of anions and cations, the influence of the thickness of the Stern layer and the influence of the orientational ordering of water molecules on the asymmetric, bimodal camel-like dependence of differential capacitance on the surface potential is theoretically considered. The presented theoretical model of the electric double layer is flexible enough to be in the future extended to more complicated multicomponent systems with molecules of different sizes and the orientational ordering of molecules.

Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering.

Periodic structures of phases with orientational order of molecules but homogenous electron density distribution: a short pitch cholesteric phase, blue phase and twist-bend nematic phase, were probed by resonant soft X-ray scattering (RSoXS) at the carbon K-edge. The theoretical model shows that in the case of a simple heliconical nematic structure, two resonant signals corresponding to the full and half pitch band should be present, while only the full pitch band is observed experimentally. This suggests that the twist-bend nematic phase has a complex structure with a double-helix built of two interlocked, shifted helices. We confirm that the helical pitch in the twist-bend nematic phase is in a 10 nm range for both the chiral and achiral materials. We also show that the symmetry of the blue phase can be unambiguously determined through a resonant enhancement of the X-ray diffraction signals, by including polarization effects, which are found to be an important indicator in phase structure determination.

An analytical equation of state for describing isotropic-nematic phase equilibria of Lennard-Jones chain fluids with variable degree of molecular flexibility.

We develop an equation of state (EoS) for describing isotropic-nematic (IN) phase equilibria of Lennard-Jones (LJ) chain fluids. The EoS is developed by applying a second order Barker-Henderson perturbation theory to a reference fluid of hard chain molecules. The chain molecules consist of tangentially bonded spherical segments and are allowed to be fully flexible, partially flexible (rod-coil), or rigid linear. The hard-chain reference contribution to the EoS is obtained from a Vega-Lago rescaled Onsager theory. For the description of the (attractive) dispersion interactions between molecules, we adopt a segment-segment approach. We show that the perturbation contribution for describing these interactions can be divided into an "isotropic" part, which depends only implicitly on orientational ordering of molecules (through density), and an "anisotropic" part, for which an explicit dependence on orientational ordering is included (through an expansion in the nematic order parameter). The perturbation theory is used to study the effect of chain length, molecular flexibility, and attractive interactions on IN phase equilibria of pure LJ chain fluids. Theoretical results for the IN phase equilibrium of rigid linear LJ 10-mers are compared to results obtained from Monte Carlo simulations in the isobaric-isothermal (NPT) ensemble, and an expanded formulation of the Gibbs-ensemble. Our results show that the anisotropic contribution to the dispersion attractions is irrelevant for LJ chain fluids. Using the isotropic (density-dependent) contribution only (i.e., using a zeroth order expansion of the attractive Helmholtz energy contribution in the nematic order parameter), excellent agreement between theory and simulations is observed. These results suggest that an EoS contribution for describing the attractive part of the dispersion interactions in real LCs can be obtained from conventional theoretical approaches designed for isotropic fluids, such as a Perturbed-Chain Statistical Associating Fluid Theory approach.

Orientational order and polarizability of molecules in a nematic liquid crystal

The influence of the orientational order of molecules in a nematic liquid crystal on the mean value and anisotropy Δγ of the molecular polarizability has been studied in the framework of the molecular sta� tistical approach with allowance for the perturbation of the electronic structure of molecules due to the change in the conformation of their aromatic core and intermolecular interactions. Experimental depen� dences of and Δγ on the molecular orientational order parameter S have been derived, and their specific features for the known objects have been explained. The possibility of separating the contributions of opposite signs to the dependence Δγ(S) due to the change in the conformation of molecules and intermolecular inter� actions has been shown using nematic MBBA as an example.

Molecular reorientation of a nematic liquid crystal by thermal expansion

A unique feature of nematic liquid crystals is orientational order of molecules that can be controlled by electromagnetic fields, surface modifications and pressure gradients. Here we demonstrate a new effect in which the orientation of nematic liquid crystal molecules is altered by thermal expansion. Thermal expansion (or contraction) causes the nematic liquid crystal to flow; the flow imposes a realigning torque on the nematic liquid crystal molecules and the optic axis. The optical and mechanical responses activated by a simple temperature change can be used in sensing, photonics, microfluidic, optofluidic and lab-on-a-chip applications as they do not require externally imposed gradients of temperature, pressure, surface realignment, nor electromagnetic fields. The effect has important ramifications for the current search of the biaxial nematic phase as the optical features of thermally induced structural changes in the uniaxial nematic liquid crystal mimic the features expected of the biaxial nematic liquid crystal.

Local-field anisotropy of a light wave in quasi-two-dimensional soft-matter objects

Experimental values of the Lorentz tensor components Lj for uniaxial quasi-two dimensional “soft matter” objects on substrates (bilayer lipid membranes, multilayer Langmuir films, smectics A, hexatic smectics B, submicron films of discotics Colhd, micron anisotropic films of liquid-crystal comblike polymers and macromolecular polymers, submicron films of conjugated conductive polymers), freely suspended submicron films of smectics A, and uniaxially stretched micron films of conjugated conductive polymers have been determined using dispersion of refractive indices in the visible range. The dependences of the components Lj on the type of orientation (axial, planar) of uniaxial molecules (structural units of the film) with respect to the optical axis of the film, the film thickness, the substrate type, the chemical structure of molecules, and their long-range orientational order are established. It is revealed that the smectic A-hexatic B phase transition and two-dimensional crystallization of the smectic layer lead to changes in the components Lj due to the change in the orientational ordering of molecules as a result of the relation between the orientational and hexatic order parameters. All the above objects are characterized by isotropization of the Lorentz tensor L and the local-field tensor f with a simultaneous decrease in the birefringence of the sample and in the anisotropy of the molecular polarizability due to the change in the electronic structure of molecules. The correction for the anisotropy of the local-field tensor f to the orientational order parameter or the anisotropy of the molecular polarizability increases. The existing model approaches to calculating the components Lj for the objects under consideration are compared with the experimental data.

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L j and the mean molecular polarizability \( \bar \gamma \) for five nematic liquid crystals belonging to two homological series. The dependence of L j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence \( \bar \gamma \)(S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences \( \bar \gamma \)(S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.

Anisotropy of the local field of the light wave in cholesteric liquid crystals

The experimental values of the Lj components of the Lorentz tensor have been obtained for the first time for the quasinematic layer of the cholesteric phase and in the smectic phase A for homologs of cholesteric fatty ethers using the dispersion dependences of the refraction indices for the planar texture of cholesteric liquid crystals (CLCs). The dependence of the Lj components on the homolog number, mesophase temperature, the magnitude of birefringence, and the change in the orientational ordering of molecules in the cholesteric phase and at the cholesteric-smectic A phase transition was determined. Isotropization of the Lorentz tensor L and the local field tensor f was found for CLCs when the birefringence of LCs and the anisotropy of molecular polarizability decreased simultaneously. The anisotropy Δf was found to be negative for the quasinematic layer of CLCs and the smectic phase in the visible range of the spectrum. The values of Lj, obtained with known local field models for CLCs and smectics A, gave positive Δf irrespective of the chemical structure of molecules, optical anisotropy of LCs, and the spectral region, which contradicts to the experiment.

Long-range orientational order, local-field anisotropy, and mean molecular polarizability in liquid crystals

The problems on the relation of the mean effective molecular polarizability to the long-range ori- entational order of molecules (the optical anisotropy of the medium) in uniaxial and biaxial liquid crystals, the local anisotropy on mesoscopic scales, and the anisotropy of the Lorentz tensor L and the local-field tensor f are formulated and solved. It is demonstrated that the presence of the long-range orientational order of molecules in liquid crystals imposes limitations from below on the molecular polarizability , which differs for uniaxial and biaxial liquid crystals. The relation between the local anisotropy and the molecular polarizability is inves- tigated for calamitic and discotic uniaxial liquid crystals consisting of lath- and disk-shaped molecules. These liquid crystals with identical macroscopic symmetry differ in the local anisotropy and the relationships between the components L || L ⊥ , f || > f ⊥ (discotic) for an electric field oriented parallel and perpendicular to the director. The limitations from below and above on the molecular polarizability due to the anisotropy of the tensors L and f are established for liquid crystals of both types. These limitations indi- cate that the molecular polarizability depends on the phase state and the temperature. The factors responsible for the nonphysical consequences of the local-field models based on the approximation = const are revealed. The theoretical inferences are confirmed by the experimental data for a number of calamitic nematic liquid crys- tals with different values of birefringence and the discotic liquid crystal .

Structure of the Acetone Liquid−Vapor Interface as Seen from Monte Carlo Simulations

The orientational order of the molecules at the liquid-vapor interface of acetone has been investigated by computer simulation. To fully describe the orientational preferences of the acetone molecules, the bivariate joint distribution of two independent orientational parameters has been determined at different layers of the interface. The strength of the orientational ordering of the interfacial molecules has been found to be liquid-like rather than crystal-like. The obtained results have revealed that the interfacial acetone molecules have dual orientational preferences. The main symmetry axis of the molecules declines by about 50-70 degrees from the interface normal axis, pointing toward the liquid phase in both of the preferred orientations. However, the plane of the molecules in the orientation preferred on the liquid side of the interface is perpendicular to the interfacial plane, whereas the other preferred orientation, which is present on the vapor side of the interface, corresponds to the alignment obtained from this orientation by an almost 90 degrees rotation around the main symmetry axis. Because the population of the liquid side is higher than that of the vapor side of the interface, the first of the two preferred orientations is the dominant alignment over the entire interface, in good agreement with recent experimental findings (Chen, H.; Gan, W.; Wu, B. H.; Wu, D.; Zhang, Z.; Wang, H. F. Chem. Phys. Lett. 2005, 408, 284).

DYNAMIC CHARACTERISTICS OF LIQUID CRYSTAL FILM IN A SLIDE BEARING UNDER ELECTRIC FIELD

This paper describes the viscosity variation of a liquid crystal under electric field, and its application to a controllable sliding bearing. Liquid crystal is known as a homogeneous organic liquid characterized by the long-range order of its molecular orientation. When an electric field is applied to a liquid crystal film as lubricant, the orientational order of molecules become parallel to the applied field, whch causes apparent viscosity variation. In this paper, a controllable slinding bearing system was constructed and its dynamic characteristics were studied. When step load or sinusoidal load was applied to the bearing pad, the film thickness was successfully controlled by a conventional PID controller. The response frequency was also studied in the experiment.

Molecular crystal global phase diagrams. I. Method of construction

A method is described to produce global phase diagrams for single-component molecular crystals with separable internal and external modes. The phase diagrams present the equilibrium crystalline phase as a function of the coefficients of a general intermolecular potential based on rotational symmetry-adapted basis functions. It is assumed that phase transitions are driven by orientational ordering of molecules with a fixed time-averaged shape. The mean-field approximation is utilized and the process begins in a high-temperature disordered reference state, then spontaneous symmetry-breaking phase transitions and phase structure information at lower temperature are sought. The information is mapped onto phase diagrams using the intermolecular expansion coefficients as independent variables. This is illustrated by global phase diagrams for molecules having tetrahedral symmetry (e.g. carbon tetrachloride, adamantane and white phosphorus). Uses of global phase diagrams include crystal structure data mining, guidance for crystal design and enumeration of likely or missing polymorphic structures.

Spectral features of a nematic liquid crystal consisting of biaxial molecules with internal rotation

Expressions for the components of the optical permittivity tensor of a nematic liquid crystal consisting of biaxial π-conjugated molecules with a conformational degree of freedom (internal rotation) are obtained. A relationship between the intensity and dichroism of absorption bands in isotropic and nematic phases and the parameters of conformational, orientational, and mixed conformational and orientational orders of the molecules is established. A distribution function for the molecules is obtained that takes into account the mutual correlation of their conformational and orientational degrees of freedom. The effect of this correlation on the above-noted order parameters, the dichroism of the absorption bands, and the dependence of the measured oscillator strengths of molecular transitions on the character and degree of orientational order of biaxial molecules is studied. On the basis of comparison with experimental data, the relative role of different terms in the distribution function that are responsible for the correlation between the conformational and orientational degrees of freedom of the molecules is ascertained.

Structural Effects of Self-Organization of Biaxial Molecules with Internal Rotation in Nematic Liquid Crystals

The effect of correlation between the conformational and orientational degrees of freedom of biaxial molecules with internal rotation on the conformational, orientational, and mixed order parameters of the molecules and on the function of the conformational distribution of molecules in nematic liquid crystals was studied in terms of molecular statistical theory. The correlation has a strong effect on polarizability of molecules with π-conjugated fragments. An explanation is offered to the experimental dependences of the mean value and anisotropy of polarizability on the character and degree of orientational ordering of molecules in the nematic phase.

Dipole moment and mobility of molecules in nematic liquid crystals of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid in the absence of external orienting fields

Dielectric polarization was studied in a nematic liquid crystal of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid (BE[HOP]BA) in the absence of external orienting fields in the isotropic and mesophase states in a frequency range of 103–107 Hz. In the isotropic melt, three regions of dielectric absorption of a relaxation origin were revealed. It is shown that two of them are related to a reorientation motion of individual molecules about the longitudinal axes (process I) and about the short axes (process II). Processes I and II have relaxation times τ ∼ 10−9 and 10−8 s and activation energies ΔU ∼ 16 and 23 kcal/mol, respectively; the energy of activation of process II in the liquid-crystal phase increases to ΔU ∼ 38 kcal/mol. In the isotropic melt, in addition to processes I and II, process III occurs in the low-frequency range, which is characterized by greater relaxation times τ ∼ 10−7 s and an activation energy ΔU ∼ 28 kcal/mol. In order to establish the nature of process III, temperature dependences of the dipole moments and Kirkwood correlation factor g were studied in both BE[HOP]BA phases. The magnitude of the Kirkwood factor in the isotropic phase of the BE[HOP]BA near the transition temperature from the mesomorphic state (g ∼ 0.88) indicates the retention of the orientational ordering of molecules of the low-molecular liquid crystal and the compensation of their dipole moments intrinsic to the liquid-crystal state. This circumstance suggests that the third process of the relaxation of dipole polarization is due to a cooperative mode of motion of molecules in mesophase nuclei in the isotropic melt.

STRUCTURAL AND OPTICAL ANISOTROPY OF BIAXIAL NEMATICS

It is shown that biaxial thermotropic nematics have anomalously small parameters of biaxial orientational order of molecules, which are one or two orders of magnitude lower than the limit of their detection by NMR. This accounts for the dramatic discrepancy between the optical and NMR data on biaxiality of these compounds. 1. The dramatic discrepancy between the optical data and the results of NMR and molecular statistic studies and computer simulations of biaxial thermotropic nematics N b have recently been an intriguing problem in liquid crystal physics [1]. The optical biaxiality of some nematics N b [2-4] is reliably determined from the presence of force disclinations |s| = 1/2 in textures of disoriented samples and also by conoscopic observations on oriented samples corresponding to an angle α = 5-10° between the optical axes [5]. However, the NMR technique has failed to detect biaxiality of the orientational ordering of the substance or impurity molecules for the same compounds [6, 7]. This also conflicts with the predictions obtained by using molecular statistic theory and computer simulations [8]. The reasons for this disagreement are being the subject of lively debates in the literature [1], but are not clear as yet, undermining the very fact of existence of biaxial thermotropic nematics [1, 6, 7]. Here we establish a relationship between the optical biaxiality of nematics N b and the properties of molecules and their orientational ordering. It is shown that the parameters of the biaxial orientational order of molecules in these compound s are anomalously small, being one or two orders of magnitude lower than the limit of their NMR detection. This may be caused by variation of conformation and anisotropy of the form of biaxial molecules in the nematic phase. 2. Consider a nematic N b with point symmetry group D2h consisting of molecules with the same symmetry. The orientational ordering of the i(xyz) axes of the molecular coordinate system relative to the α(XYZ) axes of the laboratory system is defined by the matrix elements S αβ,ij = 〈3cos θ iα cos θ jβ - δ αβ δ ij 〉 / 2 [9] or by the following order parameters:

Characterization methods for Langmuir-Blodgett films

We report progress in the field of characterization of Langmuir-Blodgett films. Special attention is paid to intermolecular distance determination, film texture and visualization of defects, surface investigation and orientational order of molecules. Some aspects of the characterization of conducting films are also presented.

Landau Theory of Biaxial Nematic Liquid Crystals

Abstract The orientational order of molecules in a liquid crystal is traditionally described by spatial or temporal averages of functions that are quadratic in the direction cosines of molecular axes with respect to laboratory fixed axes. The free energy describing the nematic phase must be independent of the choice of coordinate systems and therefore rotationally invariant with respect to both laboratory and molecular frames. A Landau theory for biaxial and uniaxial nematics based on six fundamental invariants is developed which is properly rotationally invariant. Four possible nematic phases, two uniaxial and two biaxial, result. NMR experiments should be able to identify which of these actually occur in specific materials.