Electrooptical properties of mesogenic chain molecules in solution and in nematic state

Electrooptical characteristics of mesogenic chain molecules in solution and in mesophase can be described in terms of intra- and intermolecular orientational orders. The value and sign of electric birefringence δn in a solution of kinetically rigid chain molecules are determined by the combination of two factors: intramolecular orientational order which depends on the dipolar and anisotropic architecture of the molecule and intermolecular orientational order caused by the action of the external electric field E. The value and sign of the dielectric anisotropy δε of the polymer nematic phase are also determined by the combination of intra- and intermolecular orders. However, in this case the latter is not maintained by the external field but by the nematic potential of the mesophase. Therefore, comparative investigations of electrooptical properties of polymers in solutions and in nematic melts make it possible to obtain information about the intra- and intermolecular orientational orders of the molecules under investigation in these two states. These investigations were carried out using the method of electric birefringence in solutions and the method of orientational deformations of nematic textures in an electric field. The objects being investigated were nematogenic dimers and trimers. Experimental data obtained for these compounds showed the presence of intramolecular order in their molecules, which is manifested in the odd-even oscillations of the value and sign of Kerr constant K≈δn/E² in solution and δε in the nematic phase when the number of C-C bonds in the methylene spacers of these molecules is varied. This effect is particularly dramatic in the mesophase where it is enhanced by intermolecular nematic potential.     

Coupling of orientational and translational motions in viscoelastic fluids. Low frequency depolarized (VH) light scattering

Physical processes in supercooled liquids are discussed and it is explained how elasticity might prevail. Consequently a theory is presented for the VH spectrum of low frequency light scattering from an isotropic solid-like visco-elastic medium containing molecules with orientational degrees of freedom. In contrast with the low frequency VH spectrum from non-elastic fluids the present one contains under specified conditions two central lorentzians instead of only one in addition to two pairs of shifted lorentzians and dispersion curves. The information that can be extracted from comparing this theory to experimental data contains a constant which designates the degree of elasticity and also orientational relaxation times, shear viscosities and coupling constants between the translational and the orientational motions.     

Spontaneous pattern of linear molecules in strongly confined spaces

In this work, we study the tight packing of short linear molecules in confined space by performing molecular dynamic simulations. The short chain-like molecules spontaneously arrange within single-walled carbon nanotubes (SWNTs) and exhibit a variety of chiral and achiral structures, depending on the pore size and molecule length. Simulation results show that the packing structures for these confined short linear molecules are controlled by the competition between positional order and orientational order. For linear molecules with short molecular length, such as the two-site Lennard-Jones molecules, the orientational order gradually decreases as temperature increases, and then the positional order begins to disappear. While for longer molecules, such as four-site Lennard-Jones molecules, the positional order decreases more rapidly than the orientational order as temperature increases. We also investigated the effect of molecular rigidity. For linear molecules with higher rigidity, part of packing structures may slowly rotate as a whole, and the rotation of packing arrangements is found to be induced by the preexisting defects.     

2H NMR Spectroscopic Investigation of para-Nitroazobenzene Liquid Crystals

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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).     

Structure and dynamics of mesogens using intermolecular potentials derived from ab initio calculations

A method for the calculation of the two-body intermolecular potential which can be applied to large molecules is presented. Each monomer is fragmented in a number of moieties whose interaction energies are used to recover the interaction energy of the whole dimer. For these reasons this strategy has been called fragmentation reconstruction method (FRM). By a judicious choice of the fragmentation scheme it is shown that very accurate interaction energies can be obtained. The sampling of the potential energy surface of a dimer is then used to obtain intermolecular force fields at several levels of complexity, suitable to be employed in bulk phase computer simulations. Applications are presented for benzene and for some mesogenic molecules which constitute the principal interest of the authors. A number of properties ranging from phase stability, thermodynamic quantities, orientational order parameter and collective dynamics properties are computed and discussed.     

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 -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.     

Conformational and Orientational Order of Molecules in Nematic Liquid Crystals

The statistical properties of the conformational and orientational distribution of molecules with internal rotation in the isotropic and nematic phases of liquid crystal (LC) are investigated in terms of molecular statistical theory. The paper discusses the effect of mutual correlation between the molecular conformational and orientational degrees of freedom on the conformational, orientational, and mixed order parameters of molecules in LC and on the nematic–isotropic liquid transition temperature. For these order parameters, the recurrent relation method is suggested and used to derive the partial functions of the conformational and orientational distributions of molecules in LC.     

A simulation test of the optical Kerr mechanism for laser-induced nucleation

Recent experiments have demonstrated that intense, nanosecond laser pulses can induce crystal nucleation from supersaturated solutions that are transparent at the incident wavelengths, a phenomenon termed nonphotochemical laser-induced nucleation (NPLIN). Previous work has proposed that this effect is due to the alignment of solute molecules in solution due to the electric field of the applied laser light, promoting crystalline order. We have used simulations of NPLIN to examine how an orientational bias in solution affects nucleation with Monte Carlo simulations of a Potts lattice gas model. We examine this effect within both a classical, one-step nucleation framework as well as in the context of two-step nucleation. Our results indicate that an orientational bias can reduce the free energy barrier to nucleation within the one-step picture as well as promote the crystallization of amorphous precritical nuclei (the rate-determining step in the two-step picture). However, these effects are only present with field strengths that are much greater than those used in experiments.     

Quantitative characterization of orientational order in liquid carbon tetrachloride

A simple geometrical construct is proposed for a clear-cut classification of the relative orientation between two tetrahedral molecules in terms of six orientational classes. When applied to sort out configurations from condensed phase simulations, it leads to a quantitative characterization of orientational order: A definite percentage for each class is obtained as a function of the distance between molecular centers. The basic picture that emerges, for liquid carbon tetrachloride, is that the dominant configuration for each distance is such that the number of chlorines in between both carbons diminishes with increasing separation, with a configuration here termed edge-to-face being the dominant one at contact. Regarding the range of orientational order, remnants are still noticeable at approximately 20 A, i.e., up to the fourth solvation shell. Beyond this distance the distributions are hardly distinguishable from the analytical predictions for random orientation. The analysis of the small fluctuations at such long distances shows that there are no significant differences between the ranges of positional and orientational order.     

Microscopic order and disjoining pressure in lipid films

It is demonstrated how, in a plane film at small film thickness, the orientational order of the hydrocarbon segments of the molecules present in lipid films translates into an increase of the macroscopic normal stress from the film meniscus (film-forming bulk phase) to the proper film phase.     

Confinement Induced Ordering in Fluid of Hard Ellipsoids

The ordering configurations of a fluid of anisotropic ellipsoids under the confinement of two apposing impenetrable walls are studied by Monte Carlo simulations. The excess adsorption of the fluid on the walls with respect to the aspect ratio has a maximum at the critical aspect ratio of 2.9 in high-density ellipsoid fluids, indicating an orientational ordering in the adjacent region of the walls, which is confirmed by probing into the density configurations and the orientational order parameter in the adjacent region of the walls for varying aspect ratios. In addition, the orientational order parameter in the bulk fluid at the same density is calculated, and it indicates an isotropic state as the bulk density is still below the bulk isotropic-to-nematic transition. Therefore, it can be concluded that the anisotropic ordering near the walls in the ellipsoid fluid that exhibits isotropic in the bulk is induced by the confinement effect of the walls.     

Optical properties and orientational order of deoxyribonucleic acid molecules at interfaces

A high degree of planar orientational order of DNA molecules in thin films has been discovered by the oblique-polarized-beam method. The coefficients of surface birefringence of films with different thicknesses are measured. The thickness of optically anisotropic surface layers, which are characterized by macroscopic values higher than the diameter of DNA molecules by three orders of magnitude in terms of thickness, is determined. A difference in the main polarizabilities of the monomer unit of DNA, which is estimated for the films, is compared with the corresponding value for the solution measured by flow birefringence. The effect of temperature on the coefficient of birefringence of the films is examined. A comparison of the data on the native and denatured DNA shows that the degree of orientational order for the single-helix structure is smaller than that for the double helix.     

Effect of molecular structure of a dichroic dye and the ordering of a liquid crystal on the dichroism of the dye-liquid crystal binary mixtures

The orientational ordering of dyes doped into liquid crystals has been investigated. The experimental results show that the ordering of the dyes can be expressed as a function of the order parameter of the liquid crystal host and the molecular structure of the dye. In addition a theory of the ordering of molecules in a binary mixture system has been derived by extending Kimura's theory. The validity of the theory has been confirmed by comparison with the experimental results using various combination of azo dyes and liquid crystals.     

Effect of molecular structure of a dichroic dye and the ordering of a liquid crystal on the dichroism of the dye-liquid crystal binary mixtures

The orientational ordering of dyes doped into liquid crystals has been investigated. The experimental results show that the ordering of the dyes can be expressed as a function of the order parameter of the liquid crystal host and the molecular structure of the dye. In addition a theory of the ordering of molecules in a binary mixture system has been derived by extending Kimura's theory. The validity of the theory has been confirmed by comparison with the experimental results using various combination of azo dyes and liquid crystals.     

Continuum transport model of Ogston sieving in patterned nanofilter arrays for separation of rod-like biomolecules

This article proposes a simple computational transport model of rod-like short dsDNA molecules through a microfabricated nanofilter array. Using a nanochannel consisting of alternate deep wells and shallow slits, it is demonstrated that the complex partitioning of rod-like DNA molecules of different sizes over the nanofilter array can be well described by continuum transport theory with the orientational entropy and anisotropic transport parameters properly quantified. In this model, orientational entropy of the rod-like DNA is calculated from the equilibrium distribution of rigid cylindrical rod near the solid wall. The flux caused by entropic differences is derived from the interaction between the DNA rods and the solid channel wall during rotational diffusion. In addition to its role as an entropic barrier, the confinement of the DNA in the shallow channels also induces large changes in the effective electrophoretic mobility for longer molecules in the presence of EOF. In addition to the partitioning/selectivity of DNA molecules by the nanofilter, this model can also be used to estimate the dispersion of separated peaks. It allows for fast optimization of nanofilter separation devices, without the need of stochastic modeling techniques that are usually required.     

Kronecker-product periodic systems of small gas-phase molecules and the search for order in atomic ensembles of any phase

The periodic law, manifested in the chart of the elements, is so fundamental in chemistry and related areas of physics that the question arises "Might periodicity among molecules also be embodied in a periodic system?" This review paper details how a particular periodic system of gas-phase diatomic molecules, allowing for the forecasting of thousands of new data, was developed. It can include ionized and even quarked-nuclei molecules and it coincides with locality (averaging) and the additivity found in some data; it has interesting vector properties, and it may be related in challenging ways to partial order. The review then explains how periodic systems for triatomic and four-atomic species are evolving along a similar path. The systems rest largely upon exhaustive comparisons of tabulated data, relate to some extent to the octet rule, and include reducible representations of the dynamic group SO(4) in higher spaces. Finally, the paper shows how periodicity can be quantified in data for larger molecules. Data for properties of homologous or substituted molecules, in any phase, are quantified with a vector index, and the index for one set can be transformed into that for another set.     

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.     

Analysis of orientation fluctuation in fluids by small angle X-ray scattering

The small angle X-ray scattering of molecular fluids contains information on particular aspects of their orientational order. Examples are given for the case of the isotropic, nematic and cholesteric phases of mesogenic molecules. It is shown that the distribution of the molecular centers relative to the direction defined by the molecular long axes can be analysed by means of small angle X-ray scattering. An approximate expression for the circulation correlation function is given.     

Deuterium NMR Studies on the Orientational Order Generated in Strained Rubbers

The present paper deals with the induced orientational order of the probe molecules dissolved in the uniaxially strained rubbers measured by using deuterium NMR. The distinctive dependence of the quadrupolar splitting on the swelling, elongation and crosslinking density was observed. The orientational order arising from the correlation between chain segments decreases with the increase of the numbers of both links between junctions and solvent molecules around segments.