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Giorgio Cinacchi

Publications and source records attributed to Giorgio Cinacchi.

14 recordsLinked to original sources
PubMed2006

Smectic order parameters from diffusion data.

Microcanonical molecular dynamics simulations have been performed in the smectic A phase of an elementary liquid-crystal model. Smectic order parameters and diffusion coefficients along directions parallel and perpendicular to the director have been calculated during the same trajectory for a number of state points. This has permitted the satisfactory testing of a procedure, adopted in the analysis of experimental self-diffusion coefficients, leading to an estimate of the temperature dependence of the smectic order parameters. This methodology has been then confidently applied to two smectogenic thermotropic liquid crystals belonging to the 4,4(')-di-n-alkyl-azoxybenzene series. The derived smectic order parameters are larger for the homologue compound with the longest alkyl chains. This is consistent with the well-established increased tendency, for members of a homologue series, to form a smectic phase as their alkyl chains become longer.

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PubMed2006

Orientational ordering of a banana-shaped solute molecule in a nematic calamitic solvent by 2H-NMR spectroscopy: an indication of glasslike behavior.

The Saupe ordering matrix of a banana-shaped mesogenic molecule as a solute in a common nematic calamitic solvent has been determined by 2H-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.

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PubMed2006

Solvation dynamics by computer simulation: coumarin C153 in 1,4-dioxane.

Computer simulation results are presented for an atomistic pair potential model of 1,4-dioxane which takes into account molecular flexibility. The model has been conceived to be applied to the study of solvatochromism and solvation dynamics in the presence of the polar probe coumarin C153. Computer simulations on the pure liquid have produced thermodynamical, structural, and dynamical data in good agreement with available experimental measures. This constitutes a valuable test of the 1,4-dioxane all-atom model employed. The study of solute-solvent interactions for C153 in 1,4-dioxane has been motivated by the aim of casting light, through simulations, on the interesting experimental findings according to which such a solvent behaves as a "polar" solvent with respect to dynamic solvation properties. Molecular dynamics is particularly suitable to model the process and provides an interpretation of the so-called "dioxane anomaly". An investigation of the structure of the solvation shell and of the dynamics of solvation is presented and discussed. In particular, the satisfactory accordance between simulated and experimental solvation response implies that the simulations give a reliable description of both solute and solvent at a molecular level and reinforces the idea that the explicit inclusion of discrete solvent molecules is needed for a realistic treatment of solvation phenomena in which the local structure of the liquid plays a key role.

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PubMed2006

Smectic, nematic, and isotropic phases in binary mixtures of thin and thick hard spherocylinders.

A second-virial Onsager theory, based on Parsons-Lee rescaling and suitably extended to deal with multicomponent systems and smectic phases, has been used to calculate the phase diagram of a collection of binary mixtures of thin and thick hard spherocylinders. In particular, two types of phase diagrams are investigated. First, a number of binary mixtures where the two components have the same total length have been considered; in addition, the phase diagram of a binary mixture where the two components have the same volume has been calculated. For the particles of one of the two components, the length of the cylindrical part and the diameter have always been set equal to 5 and 1, respectively. Spherocylinders of the same total length and different diameter tend to demix considerably as soon as the diameter ratio deviates from unity. This happens especially at high pressures, when at least the phase richer in the thicker component is smectic. In the case where the two components have equal volumes, demixing is further increased due to the disparity not only in particle diameter but also in particle lengths. The incorporation of inhomogeneous layered phases is seen to alter significantly the phase diagrams calculated if only homogeneous phases are allowed, since transitions to a smectic phase often preempt those to a nematic or an isotropic phase. The apparent versatility of the recent experimental techniques suggests that the phase diagram features predicted by the theory might be also observed in real systems.

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PubMed2006

On the analysis of Liquid Crystal NMR dipolar coupling data via mixed a priori-maximum entropy methods.

In this Letter, a general expression is derived for the conformational distribution function of a molecule dissolved in an anisotropic condensed fluid medium by combining an a priori model with the maximum entropy principle applied to treatment of liquid crystalline-NMR data. The recently proposed additive potential maximum entropy (APME) method is recovered as a special case, when the AP is chosen as the a priori model and the orientational order is low.

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PubMed2006

Orientational ordering of solutes in confined nematic solvents: a possible way to probe director distributions.

We propose a method to study the director distribution in a nematic liquid crystal confined in a slab geometry. It is based on the measurement, by NMR spectroscopy, of the Saupe ordering matrices of a collection of biaxial solute molecules dissolved in the confined nematic liquid crystal of interest. Due to the combined action of the surface anchoring and magnetic field interactions, the director is generally not uniformly aligned within the cell. Consequently, the resulting Saupe ordering matrices may be considered as weighted sums of the corresponding Saupe ordering matrices measured in the bulk nematic phase, and modulated by the director distribution. The determined Saupe ordering matrices may then be taken as the set of data in a fitting process where the fitting function, whose form is deduced from molecular mean field and continuum theories, is dependent on the director distribution; the angle that the director forms with the plain surfaces and the corresponding derivative at the surfaces are taken as fitting parameters. The methodology is preliminarily tested on the virtual nematic phase formed by the Lebwohl-Lasher lattice model, confined between two plain surfaces favoring planar anchoring, and where a number of model cuboidal solutes has been dissolved. We comment on the implemention of the method when applied to real experimental systems.

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PubMed2005

Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model.

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density.

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PubMed2005

Density functional theory study of the conformational space of phenyl benzoate, a common fragment in many mesogenic molecules.

The complete conformational space of phenyl benzoate (three coupled rotors) has been studied by B3LYP density functional theory (DFT) at the 6-31+G* basis set level. The overall quality of the DFT results has been checked via Möller-Plesset second-order perturbation theory (MP2) calculations performed on a few significant molecular geometries. Contrary to the general belief, we have found that rotation around the C(=O)-O bond is not more restricted than rotation around the C(=O)-C bond. We have commented on the location and magnitude of the molecular dipole moment and their dependence on conformation. The energy data have been fitted through an expression containing a Fourier expansion plus a Lennard-Jones term, suitable to be used in computer simulations or to assist analysis of experimental data.

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PubMed2005

Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation.

We 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.

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PubMed2005

Atomistic molecular dynamics simulation of benzene as a solute in a columnar discotic liquid crystal.

A molecular dynamics simulation study on a binary liquid-crystalline mixture, where the solvent is the typical discogen hexakis-pentyloxy-triphenylene in its columnar state, while benzene is the solute, is reported. Both discotic and benzene molecules are modeled employing an atomistic force field. Attention has been paid to the structural and dynamic properties of benzene in this unusual environment, comparing these results with available experiments on the same or similar systems and with computer simulation data on neat liquid benzene.

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PubMed2005

Numerical study of a calamitic liquid-crystal model: phase behavior and structure.

We 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.

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PubMed2004

Computer simulation of solid and liquid benzene with an atomistic interaction potential derived from ab initio calculations.

Molecular 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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PubMed2004

Liquid-crystal phase diagrams of binary mixtures of hard spherocylinders.

We have built the liquid crystal phase diagram of several binary mixtures of freely rotating hard spherocylinders employing a second-order virial density functional theory with Parsons scaling, suitably generalized to deal with mixtures and smectic phases. The components have the same diameter and aspect ratio of moderate value, typical of many mesogens. Attention has been paid to smectic-smectic demixing and the types of arrangement that rods can adopt in layered phases. Results are shown to depend on the aspect ratio of the individual components and on the ratio of their lengths. Smectic phases are seen not to easily mix together at sufficiently high pressures. Layered phases where the longer rods are the majority component have a smectic-A structure. In the opposite case, a smectic-A(2) phase is obtained where the shorter particles populate the layers and the longer ones prefer to stay parallel to the latter in the interlayer region.

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