101,880 research outputs found
Orientational ordering of a banana-shaped solute molecule in a nematic calamitic solvent by 2H NMR spectroscopy: an indication of glasslike behavior
No full textThe Saupe ordering matrix of a banana-shaped mesogenic molecule as a solute in a common nematic calamitic solvent has been determined by H-2-NMR spectroscopy as a function of temperature. The temperature dependence of the Saupe ordering matrix element associated with the principal molecular axis is consistent with a glassy behavior in the reorientational motion of this particular solute molecule. The Haller expression, appropriately modified, provides a good fit to the experimental data
Computer simulations of pure and mixed systems of disklike particles interacting with the S-function Corner potential
No full textThe S-function potential for single-site particles has been recently introduced as an extension of the Gay-Berne potential. With the S-function potential, we can model particles of other and, possibly, more realistic shapes than Gaussian overlap ellipsoids and we can simulate mixtures in a systematic manner. As an example of both applications, we have studied via Monte Carlo computer simulation assemblies of cylindrically symmetric disklike particles of two different types. They can be considered complementary perturbations of a disk, with, respectively, a depression (red-cell, R particles) and a protuberance (ufo, U particles) at the center. The former is meant to mimic the average effect of side chains of real diskotics, while the latter is a simple representation of diskotic metallomesogens. Four systems of these kind of particles have been studied as a function of temperature: two pure cases and two 1:2 binary mixtures, so that the combined effects of shape and concentration can be observed. We have found the largest variety of phases in the pure U system where also a nematic phase is present. No such phase is observed when 1/3 of the U particles are replaced by R particles, as well as in the mixture 2/3R-1/3U and in the pure R system. At the lowest temperatures investigated all systems show a lattice phase except the mixture 1/3R- 2/3U, whose organization is better described as a disordered columnar phas
Internal Rotation in Alchyl Benzene derivatives: Maximum Entropy and Constrained Maximum Entropy analysis of the Dipolar Couplings of Samples dissolved in a Nematic Liquid Crystal solvent
No full textDiffusion and viscosity of a calamitic liquid crystal model studied by computer simulation
No full textWe report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement
Numerical study of a calamitic liquid-crystal model: Phase behavior and structure
No full textWe have studied an idealized calamitic liquid-crystal model, consisting of a linear rigid array of nine soft repulsive spheres, employing both theory and molecular dynamics simulation. The phase behavior (which includes crystalline, smectic, nematic, and isotropic phases) and structure of a collection of these rodlike particles have been determined by molecular dynamics simulation in an isothermal-isobaric ensemble. The liquid crystalline part of the phase diagram has been compared to that emerging from an Onsager-type density-functional theory. We have found a fair agreement between theory and computer simulation results, with a similar accuracy for the smectic to nematic and nematic to isotropic phase transitions
Atomistic Molecular Dynamics Simulation of Hexakis(pentyloxy)triphenylene: Structure and Translational Dynamics of Its Columnar State
No full textAtomistic molecular dynamics computer simulations have been performed on the columnar state of the discogen hexakis(pentyloxy)triphenylene (HAT5). The parameters of the empirical model potential have been taken from the AMBER and OPLS force fields. A 10 ns simulation run has been carried out in Berendsen's ensemble at pressure P = 0.1 MPa and temperature T = 375 K to study bulk and molecular structure. The bulk structure has been characterized by obtaining structural parameters that compare favorably with those deduced from X-ray diffraction experiments. C-13 and H-2 NMR data suggest two completely different pictures of the position of the pendant chains with respect to the plane of the aromatic core: one, called diablo-like, with the chains in the triphenylene plane; the other, octopus-like, with the chains out of this plane and, alternatively, above and below it. Our simulated data are consistent with an intermediate situation. In agreement with all NMR data, we find that the chains become more and more disordered, moving away from the core. We have observed a decrease of the disorder of the lateral chains in going from gas phase to condensed bulk state. Information on translational dynamics has been obtained by performing a 1 ns simulation in the microcanonical ensemble; these calculations definitely assess the one-dimensional fluid character of the columnar state. We have found that the mean square displacement has a solidlike behavior in the plane perpendicular to the column axis. The translational dynamics along the direction parallel to it is dominated by the reciprocal, fluidlike sliding motion of the columns as a whole. If this collective dynamics is removed, also the parallel diffusion coefficient turns out to be solidlike
On the Distribution Functions of Depletion Interactions
No full textMolecular dynamics computer simulations were performed on model colloidal binary mixtures of two large and many small soft repulsive spheres. Depletion forces arise between the two large spheres, as a function of their distance, because of the nonadditivity of the volume they exclude to the small spheres. The probability distribution functions of both longitudinal and transverse component of the total force exerted by the small particles were calculated and generally turned out non-Gaussian. The distributions of the collective forces were analyzed in terms of the distribution of the force that a single small sphere exerts on a large sphere and of the number of the surrounding small spheres. The reconstructed function matches well the corresponding exact distribution. Residual correlation among small particles, combined with a relatively small number of neighbors, slows the approach to the Gaussian limit. In our fully repulsive model, the direct force between a large and a small sphere is a monotonic function of their distance. On these bases, we propose and successfully test an approach that relates the probability distribution function of the depletion forces to the large-sphere small-sphere radial distribution function. This approach can be extended to experimental data of radial distribution function, thus allowing for an estimate of depletion force fluctuations in real colloidal mixture
Atomistic Molecular Dynamics Simulation of Hexa-pentyloxy-triphenylene: Structure and Translational Dynamics of Its Columnar State
No full textAtomistic molecular dynamics simulations have been performed on the columnar state of the discogen hexa-pentyloxy-tryphenilene
Computer simulation of p-phenyls with interaction potentials from ab-initio calculations
No full textThe potential energy surface (PES) of n-p-phenyls, with n from 2 to 5, has been obtained from ab initio calculations relevant to a few fragments into which the molecule can be decomposed. These PES have been fitted with a single Gay-Berne model that has been used in Monte Carlo NPT and Gibbs ensemble simulations. The results we obtain are in fairly, good agreement with experiment in the case of biphenyl, while only a low density fluid phase (gas) is obtained for p-terphenyl and longer molecules
Computer Simulation of Solid and Liquid Benzene with an Atomistic Interaction Potential Derived from ab Initio Calculations
No full textMolecular dynamics atomistic simulations of solid and liquid benzene have been performed, employing a model intermolecular potential derived from quantum mechanical calculations. The ab initio database includes approximately 200 geometries of the benzene dimer with interaction energies computed at the MP2 level of theory. The accuracy of the modeled force field results is satisfactory. The thermodynamic and structural properties, calculated in the condensed phases, are compared with experimental data and previous simulation results. Single particle and collective dynamical properties are also investigated through the calculation of translational and rotational diffusion coefficients, reorientational dynamics, and viscosities. The agreement of these data with experimental measurements confirms the reliability of the proposed force field
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