Anisotropic Dispersion Forces Research Articles

Toward prediction of surface tension of branched n-alkanes using ANN technique

This study features the application of artificial neural network for prediction of surface tension of branched alkanes. Surface and interfacial tensions of alkanes show a specific interaction between adjacent molecules of the higher n-alkanes which results in an anisotropic dispersion force component of the surface energy. The surface tension of branched alkanes was studied for temperatures between 283.15 and 448.15 K. Two intelligent models named multilayer perceptron model (MLP) and radial basis function (RBF) model were developed and the accuracy of two models was examined by different graphical and statistical methods. Results showed that the both models are accurate and effective in prediction of surface tension of branched alkanes. However, the results were compared with experimental data and it was found that the estimated surface tension by multi-layer perceptron neural network is more accurate than radial basis function network.

A neural network (CSA-LSSVM) model for the estimation of surface tension of branched alkanes

ABSTRACTThe current study highlights the application of a model based on least square support vector machine (LSSVM) for prediction of surface tension of branched alkanes. An optimization algorithm, namely, coupled simulated annealing (CSA) was applied to the model. Surface tensions of alkanes show a specific interaction between adjacent molecules of the branched alkanes which affects the anisotropic dispersion force component of the surface energy. In this paper, surface tension of branched alkanes was studied in temperature range of 283.15 and 448.15 K. To evaluate the performance and accuracy of this model, statistical and graphical error analyses have been used simultaneously. By applying CSA-LSSVM on 600 data points and finding optimum parameters, the estimated values of surface tension of branched alkanes were compared with experimental data which showed a reasonable agreement with the experimental results. Results demonstrate that the model is precise and viable for prediction of solubility data. The model shows an overall R2 and AARD% estimations of 0.9921 and 0.89%, respectively.

Pseudo-molecular approach for the elastic constants of nematic liquid crystals interacting via anisotropic dispersion forces

Abstract The bulk and the surface-like elastic constants of a nematic liquid crystal are calculated for an ensemble of particles interacting via anisotropic dispersion forces using the pseudo-molecular method. The geometrical anisotropy of the molecules is also taken into account in the calculations by choosing a molecular volume of ellipsoidal shape. Analytical expressions for the elastic constants are obtained as a function of the eccentricity in the molecular volume shape. The method allows one to explore the dependence on the molecular orientation with respect to the intermolecular vector by analyzing the magnitude and the behaviour of macroscopic elastic parameters defining the nematic phase.

Anisotropy in the interaction of ultracold dysprosium

The nature of the interaction between ultracold atoms with a large orbital and spin angular momentum has attracted considerable attention. It was suggested that such interactions can lead to the realization of exotic states of highly correlated matter. Here, we report on a theoretical study of the competing anisotropic dispersion, magnetic dipole-dipole, and electric quadrupole-quadrupole forces between two dysprosium atoms. Each dysprosium atom has an orbital angular momentum of L = 6 and a magnetic moment of μ = 10 μ(B). We show that the dispersion coefficients of the ground state adiabatic potentials lie between 1865 a.u. and 1890 a.u., creating a non-negligible anisotropy with a spread of 25 a.u. and that the electric quadrupole-quadrupole interaction is weak compared to the other interactions. We also find that for interatomic separations R < 50a(0) both the anisotropic dispersion and magnetic dipole-dipole potential are larger than the atomic Zeeman splittings for external magnetic fields of order 10 G to 100 G. At these separations the atomic angular momentum can be reoriented. We finish by describing two scattering models for these inelastic m-changing collisions. A universal scattering theory is used to model loss due to the anisotropy in the dispersion and a Born approximation is used to model losses from the magnetic dipole-dipole interaction for the (164)Dy isotope. These models find loss rates that are of the same order of magnitude as the experimental value.

O2 broadening of carbon disulphide 12C32S2 in the v 3 and v 3—v 1 bands

Using a tunable diode laser spectrometer, we have measured O2 broadening coefficients of 12C32S2 for 31 lines in the v 3 fundamental band near 6.5 μm and 12 lines in the v 3—v 1 band near 11.4 μm. The collisional halfwidths are obtained by fitting the spectral lines with a Voigt and a Rautian profile. The broadening coefficients of 12C32S2-O2 are also calculated from semiclassical theory involving, in addition to electrostatic interactions, successively the atom-atom Lennard-Jones model and a simple formulation for the anisotropic dispersion forces, leading to more satisfactory results.

N2 Broadening of Carbon Disulfide 12C32S2 in the ν3 and ν3–ν1 Bands

N2 broadening coefficients have been measured for 33 lines in the ν3 fundamental band and 21 lines in the ν3–ν1 band of C32S2, using a tunable diode–laser spectrometer. These lines with J values ranging from 0 to 78 in the ν3 band and from 4 to 70 in the ν3–ν1 band are located in the spectral ranges 1514–1548 and 853–887 cm−1, respectively. The collisional widths are obtained by fitting the spectral lines with a Voigt and a Rautian profile. Semiclassical calculations of these broadening coefficients have been performed by considering, in addition to electrostatic interactions, successively the atom–atom Lennard–Jones model and a simple formulation for the anisotropic dispersion forces leading to more satisfactory results.

Surface Alignment Effect and Pretilt Angle Generation in NLC with Oblique Non-Polarized UV Light Irradiation on Polymer Surface

In this study, we have investigated the generation of pretilt angle in nematic liquid crystal (NLC) in the cell with oblique non-polarized ultraviolet (UV) light irradiation on two kinds of the polyimide (PI) surfaces. It was found that the monodomain alignment in NLC is obtained having a PI surface without side chain. We successfully observed that the generated pretilt angle of the NLC is about 3° with an angle of incidence of 70° on the PI surface without side chain. It is considered that the pretilt angle generation in NLC is attributed to the interaction between the LC molecules and the polymer surfaces due to photo-depolymerization reaction. Also, the uniform alignment of NLC is attributed to anisotropic dispersion force due to photo-depolymerization of polymer on PI surfaces.

Generation of pretilt angle in NLCs and EO characteristics of a photo-aligned TN-LCD with oblique non-polarized UV light irradiation on a polyimide surface

We have investigated the generation of pretilt angle for a nematic liquid crystal (NLC) alignment in cells with oblique non-polarized ultraviolet (UV) light irradiation on polyimide (PI) surfaces. It was found that monodomain alignment of the NLC is obtained with an incident angle of 70 degrees and 75 degrees on the PI surface. It is considered that this alignment may be attributed to the anisotropic dispersion force due to photo-depolymerization of polymer on PI surfaces. Also, the generated NLC pretilt angles are all about 3 degrees at an incident angle of 70 degrees and 75 degrees for 1 h irradiation. Next, we observed that the voltage-transmittance characteristics for a photo-aligned twisted nematic (TN) LCD with an incident angle of 80 degrees on a PI surface were excellent. Also, we measured that the voltage-holding-ratio (VHR) of a photo-aligned TN-LCD is about 94%; it is almost same as obtained for rubbing-aligned TN-LCDs. Finally, the slow response time of photo-aligned TN-LCDs is attributable to their weak anchoring strength.

Azimuthal ordering in Langmuir monolayers

A simple mean-field model of azimuthal ordering in Langmuir monolayers induced by anisotropic dispersion forces is presented. The approach is removed from previous studies of this interaction in systems of grafted rods insofar as the tilt of amphiphiles is decoupled from azimuthal ordering and treated as an external variable. We examine the phase space of fixed tilt against temperature, finding that azimuthal ordering is second order for most tilts, except in the range 57°–66°, where it is first order. This is related to the existence of three different types of azimuthal order; polar, nematic, and polar–nematic.

Preliminary communication Investigation of pretilt angle generation in nematic liquid crystal with oblique non-polarized UV light irradiation on polyimide films

We investigated new rubbing-free techniques for liquid crystal (LC) alignment with non- polarized ultraviolet (UV) light irradiation on plates coated with two kinds of the polyimide (PI) films. It was found that monodomain alignment of nematic (N) LC is obtained in the cell having a PI surface without a side chain. We successfully observed that the generated pretilt angle of the NLC is about 3 with an angle of incidence of 70 on the PI surface without side chains. This pretilt angle generation is attributed to interaction between the LC molecules and the polymer surfaces; the uniform alignment of NLC is attributed to anisotropic dispersion force effects due to photo-depolymerization of polymer on PI surfaces.

Planar anchoring of nematic 4-n-pentyl-4′-cyanobiphenyl on self-assembled monolayers formed from alkanethiols on gold

We report measurements of planar anchoring of nematic 4-n-pentyl-4′-cyanobiphenyl (5CB) on self-assembled monolayers (SAMs) formed from CH3(CH2)mSH (7⩽m⩽17) on films of gold (prepared without oblique deposition) by using reflection interferometry. The anchoring observed contrasts with past reports of homeotropic anchoring of 5CB by surfaces, such as monolayers formed from octadecyltrichlorosilane on silica, with energies as low as alkanethiols on gold (19 mN/m). The anisotropic dispersion force acting between 5CB and gold was calculated to favor planar anchoring of 5CB on SAMs formed from CH3(CH2)15SH by ∼0.1 mN/m, and can, therefore, override self-interactions of 5CB that lead to homeotropic anchoring. Electric field-induced Freederiksz transitions of 5CB supported on bare gold, and on SAMs formed from CH3(CH2)15SH on gold, confirm weaker (planar) anchoring of 5CB on SAMs on gold as compared to bare gold.

Anisotropic Dispersion Force Effects for Alignment of Liquid Crystals on Rubbed Polystyrene Surfaces

We have studied the anisotropic dispersion force effects for alignment of liquid crystals (LCs) on rubbed polystyrene (PS) surfaces. We previously demonstrated that the nematic (N) LCs are shown to align in the direction perpendicular to the rubbing and polar anchoring strength is very weak compared to rubbed polyimide (PI) surfaces [D.-S. Seo et al.: Jpn. J. Appl. Phys. 31 (1992) 2165]. In microphotographs of the textures, we obtained the NLCs are shown to align in both direction parallel and perpendicular to the rubbing for region up to medium rubbing, however to align in the direction perpendicular to the rubbing for strong rubbing region. We suggest that the anisotropic dispersion force is very important rather than macro-surface groove effect to uniform alignment of LCs. We also observed the surface ordering of the NLCs on rubbed PS surfaces for strong rubbing is disordered. We suggest that the low polarization of the orientation surfaces is attributed to weak anchoring strength.

The anisotropic forces acting on the bonds of benzene in liquid crystals

Abstract The HH and CH dipolar couplings of benzene measured in five different liquid crystal solvents are subjected to an analysis which allows for the correlation between the molecular reorientational and vibrational motions. The number of adjustable parameters is reduced by treating the CH bonds or both the CH and CC bonds as effectively cylindrically symmetric entities. In this way detailed information on the anisotropic forces acting on the bonds of benzene dissolved in liquid crystals is obtained. The behaviour of the CC bonds, but not that of the CH bonds, may be explained by anisotropic dispersion forces. There is an approximately linear relation between the torques acting on the CH bonds of benzene and methane in the same liquid crystal environment. This suggests that these forces stem from a common interaction mechanism, possibly the van der Waals interaction between the atoms of the solute molecule and the liquid crystal surroundings or the interaction between the molecular quadrupole moment an...

Diode-laser measurements of Ar- and CO 2-broadened linewidths in the v1 band of OCS

The pressure-broadening coefficients for a number of spectral lines ranging from| m| = 1to 58 in the v 1 band of 16O 12C 32S, have been measured for collisions with Ar and CO 2 with a tunable diode-laser spectrometer. The results are compared with values calculated on the basis of the Anderson-Tsao-Curnutte theory and an improved semi-classical impact theory. For OCS-Ar, we have used the Tipping-Herman intermolecular potential, as well as an atom-atom interaction model. For OCS-CO 2, we have considered, in addition to electrostatic interactions, successively a simple formalism for the anisotropic dispersion forces and the atom-atom interactions.

The influence of lateral substituents on the phase behaviour of nematogenic liquid crystals

Abstract The influence of lateral substituents or side chains attached to thermotropic nematogens is considered on the basis of the Flory lattice model. The rigid core of the nematogen is characterized by its axial ratio x and the characteristic temperature T∗ which measures the strength of the anisotropic dispersion forces operative between the cores. For the side chain an additional parameter zm which is proportional to its hard core volume enters the partition function. Given these quantities and the free volume as obtained from the thermal expansion coefficient, the nematic-isotropic transition temperature TNI and the temperature dependence of the order parameter may be calculated. The results show that the lowering of T NI by lateral substituents is caused by (1) their interference with molecules packing and (2) the dilution of the anisotropic dispersion forces between the central nematogenic cores. As an additional consequence of this the order parameter is lowered considerably. A survey of recent e...

Optical anisotropies of alkylcyanobicyclohexyls and related compounds

Mean-square optical anisotropies, γ2, of cyanocyclohexane, bicyclohexyl, and the trans-4-cyano-trans-4′-n-alkylbicyclohexyls, NC—C6H10—C6H10—CmH2m+1, with m= 2, 5 and 7, have been determined from depolarized Rayleigh light-scattering measurements conducted on dilute solutions in carbon tetrachloride. The results are treated on the basis of additive contributions of group anisotropies: ΓCC=ΔαCC– 2ΔαCH and ΓCCN=ΔαCCN–ΔαCH, where the Δα denote the anisotropies of the polarizabilities of the indicated bonds. Diverse conformations of the bicyclohexyl system are taken into account. The value Γ′CC= 0.68 Å3 required for bonds comprising this group appreciably exceeds the value of ΓCC= 0.53 Å for the n-alkanes and presumably for the alkyl chains. Similarly, ΓCCN= 3.3 Å3 is required for the cyano group attached to bicyclohexyl, compared with 2.15 Å3 for the same group in acyclic nitriles and in cyanocyclohexane. These differences appear to be due to inductive effects that are known to impair a simple additive scheme. Contributions of members of the alkyl chain beyond the first to the tensor â representing the anisotropy of the polarizability virtually vanish when averaged over the configuration of the chain. Hence, properties such as the electric birefringence should be independent of the length of the alkyl chain under conditions that allow the latter to be in a random configuration. The quadratic tensor invariant, γ2=Tr(ââ), should increase gradually with chain length. The relatively large alternating effects in the values of γ2 for odd and even values of m are explained by the persistence of correlations in the directions of successive bonds of the alkyl chain. The effect of a departure of the principal axis of the polarizability tensor from the major geometric axis of the molecule is illustrated and its relevance to the anisotropic dispersion forces operative in liquid crystals is pointed out. The orientation-dependent energy attributable to the anisotropy of the polarizability is much smaller than would be required by the observed nematic–isotropic transition temperatures. This suggests that the large dipole moment of the molecules may be responsible for the nematic stability of the cyanobicyclohexyls.

Crystal structure and lattice dynamics of chlorine The role of electrostatic and anisotropic atom-atom potentials

An extensive analysis of the intermolecular forces in the chlorine crystal has been performed. Different models of the intermolecular potential, including anisotropic terms and electrostatic contributions from multipoles higher than quadrupole have been taken into account and tested for their ability to reproduce static and dynamical properties of solid chlorine. The inclusion of anisotropic dispersion forces reproduces correctly the orthorhombic unit cell. Anisotropic terms are required also in the short range repulsive atom-atom interactions to obtain a good fit to lattice frequencies and stability of the Cmca structure with respect to a hypothetical cubic phase. A tensor formalism, particularly convenient for dynamical calculations, is developed and used to describe angular dependent atom-atom interactions.

A Theoretical Investigation of the Lattice of Symmetric and Asymmetric Rigid Rods with Anisotropic Dispersion Forces and Rigid Body Repulsions

Abstract The method of symmmetry breaking potential and first order cluster expansion technique for the partition functions adopted for the theory of ordering in liquid crystals has been extended to symmetric asymmetric molecules. The order parameter is calculated as a function of temperature and packing coefficient as a function of chainlength for homologous series of liquid crystals having symmetric and asymmetric molecules. The theoretical calculations account fairly well for the gradient differences in the order parameters of symmetric molecules and packing coefficients.

The phase equilibria in thermotropic liquid crystalline systems

The lattice theory of hard rods with anisotropic dispersion forces presented recently is extended to the case where the rods are dispersed in a diluent. Two systems are treated: (i) rods of axial ratio x diluted with solvent molecules having x=1, and (ii) rods of axial ratio xα mixed with homologs of axial ratio xβ. Treatment of the latter system can be regarded as an extension of the Flory and Abe theory for ’’athermal’’ polydisperse systems, in which dispersion forces are inconsequential. Solutions of sufficiently long (x≳20) rods subject to anisotropic dispersion forces are predicted to exhibit complex phase equilibria involving a re-entrant nematic phase (with a critical point) and a triple point, as well as the usual nematic–isotropic coexistence. These deductions should be relevant to thermotropic solutions of rodlike synthetic polymers with phenylene groups or other groups exhibiting anisotropic interactions. Calculations for mixtures of short rods yield more prosaic phase diagrams. It is suggested that such systems (for example, mixtures of the polyphenyls) can provide comprehensive tests of the underlying theory. Mixtures of quinque- and quaterphenyl are specifically considered.

Interaction energies in nematogens

The effect of steric exclusions between rods in correlating positional with rotational hinderances is used to calculate the mean field a rod sees in a fluid of other rods with which it interacts via anisotropic dispersion forces. The resultant potential involves all Legendre polynomials even within the dipole–dipole approximation. It is also possible to see how specific forces could be incorporated into the derivation to give the often anomalous results seen in Raman scattering. The magnitude of the potential is also estimated using absorption, scattering, and equation of state data. The resultant characteristic energy is somewhat smaller than that required by a recent theory of Flory and Ronca which contains both steric and dispersion effects, and is much smaller than that required by the Maier–Saupe approach which takes no account of steric influences.