Plate-like Molecules Research Articles

Effects of Delphinidin on Pathophysiological Signs of Nucleus Basalis of Meynert Lesioned Rats as Animal Model of Alzheimer Disease.

Alzheimer's disease (AD) is an advanced neurodegenerative disorder greatly accompanied by cognitive deficits, oxidative stress, inflammation, amyloid plaques deposition, and acetylcholinesterase (AChE) hyper-activation. Growing evidence suggests natural compounds with antioxidant and anti-inflammatory features improve pathophysiological signs of AD. The present study was designed to investigate the effects of Delphinidin (25, 50mg/kg) as an anthocyanidin on spatial memory impairment and AD hallmarks such as hippocampal AChE activity, amyloid plaques deposition, oxidative stress and expression of amyloid precursor protein (APP), AChE, and amyloid beta (Aβ) proteins in nucleus basalis of Meynert (NBM) lesioned rats as the most prevalent animal model of AD. Interestingly, Delphinidin-treated animals showed a significant decrease in escape latency and distance moved. Furthermore, in probe test, NBM lesioned rats treated with both doses of Delphinidin spent more time in the target quadrant zone in Morris water maze task. It could also interact with catalytic site of AChE enzyme and inhibits acetylcholine hydrolysis in in vitro and in vivo conditions. In addition, Delphinidin could scavenge additional produced reactive oxygen molecules dose dependently. Our immunoblotting analysis confirmed high dose of Delphinidin reduced AChE, APP and Aβ contents in AD model. Staining of hippocampus tissue revealed that Delphinidin treatment decreased amyloid plaques formation in NBM lesion rats. It seems that Delphinidin is a plate-like molecule intercalated between β-plated sheets related to Aβ molecules and inhibited amyloid fibril formation. Altogether, Delphinidin and Delphinidin-rich fruits could be suggested as a therapeutic adjuvant in AD and other related cognitive disorders.

New phase diagrams in the mixture of rods and plates of biaxial nematic liquid crystals

Experiments have shown an unusual phase transition between a stable biaxial nematic phase and a coexistence region of the two uniaxial nematic phases in lyotropic liquid crystals. We propose a Landau model to describe this interesting phase behavior. The model considering both rod-like molecules and plate-like molecules is discussed with great details. The possibility of the various phases including the coexistence phases is explored by means of variation of the concentration. The model describes the first theoretical observation of the phase transition between two biaxial nematic phases. The theoretical predictions are found to be in good qualitative agreement with available experimental results.

Nucleation and growth of copper phthalocyanine aggregates deposited from solution on planar surfaces

Copper phthalocyanine (CuPc) dissolved in trifluoroacetic acid (TFA) is deposited on solid SiO2 surfaces by solvent evaporation. The deposited CuPc aggregates are investigated by atomic force microscopy (AFM). The CuPc deposits were prepared by spin casting, dip coating, and spray deposition. Depending on the amount of deposited CuPc the aggregate morphology ranges from small individual domains to mesh-like multilayers. Each domain/layer consists of many parallel stacks of CuPc molecules with the square, plate-like molecules piled face-wise within each stack. The parallel stacks are attached sideways (i.e., edgewise attachment molecularly) to the substrate forming “nanoribbons” with uniform thickness of about 1nm and varying width. The thickness reflects the length of a molecular edge, the width the number of stacks. A nucleation and growth model is presented that explains the observed aggregate and multilayer morphologies as result of the combination of nucleation, transport processes and a consequence of the anisotropic intermolecular interactions due to the shape of the CuPc molecule.

Molecular Graphics of Convex Body Fluids.

Coarse-grained modeling of molecular fluids is often based on nonspherical convex rigid bodies like ellipsoids or spherocylinders representing rodlike or platelike molecules or groups of atoms, with site-site interaction potentials depending both on the distance among the particles and the relative orientation. In this category of potentials, the Gay-Berne family has been studied most extensively. However, conventional molecular graphics programs are not designed to visualize such objects. Usually the basic units are atoms displayed as spheres or as vertices in a graph. Atomic aggregates can be highlighted through an increasing amount of stylized representations, e.g., Richardson ribbon diagrams for the secondary structure of proteins, Connolly molecular surfaces, density maps, etc., but ellipsoids and spherocylinders are generally missing, especially as elementary simulation units. We fill this gap providing and discussing a customized OpenGL-based program for the interactive, rendered representation of large ensembles of convex bodies, useful especially in liquid crystal research. We pay particular attention to the performance issues for typical system sizes in this field. The code is distributed as open source.

The Gas Phase Spectrum of Cyclic C18and the Diffuse Interstellar Bands

The gas phase spectrum of the cyclic C18 molecule recorded in the laboratory at a temperature typical of diffuse interstellar clouds is compared with absorption features toward ζ Oph and HD 204827 in the 5730-5934 A region. For the origin band at 5928.5 A an upper limit to the column density of ≤1.8 × 1011 cm-2 is inferred. The origin band pattern in the laboratory spectrum changes on lowering the internal temperature in the 100-20 K range and bears a striking resemblance to the observed structure of a number of DIBs at other wavelengths. This suggests that platelike molecules or ions, comprising a couple of dozen to hundred carbon atoms, could be responsible for some of the latter absorptions.

Melt Compounding of Polymeric Nanocomposites

Abstract The clay-containing polymeric nanocomposites (CPNC) can be visualized as binary mixtures of strongly interacting, inorganic, plate-like molecules dispersed in a polymeric matrix. To be successful, one must ascertain the thermodynamics, which controls CPNC structure on the molecular level. In this work dispersion of organoclay (Cloisite 15A, C15A) in polyamide 6 (PA6) or in polypropylene (PP) is discussed. The PA-based CPNC's contained two components: polymer and organoclay, whereas those based on PP in addition contained a mixture of two maleated polypropylene's (PP-MA), as a compatibilizer. The melt compounding was carried out either in a single-screw extruder (SSE), or a twin-screw extruder (TSE). Both compounding lines were used with or without the extensional flow mixer (EFM). Furthermore, two versions of EFM were evaluated – one commercial, designed for polymer homogenization and blending, and the other designed for dispersing nano-particles. It was found that addition of EFM significantly improved clay dispersion. Better dispersion was found compounding the CPNC's in a SSE+EFM than in TSE with or without EFM. The best results were obtained using SSE with the new EFM having a relatively small gap between the convergent-divergent plates. C15A was fully exfoliated in PA6 matrix. The results in PP/PP-MA matrix were less spectacular, but again the highest degree of dispersion was obtained using SSE+new EFM with a small gap. Tensile, flexural and impact properties were measured and evaluated.

Do Molecules as Small as Neopentane Induce a Hydrophobic Response Similar to That of Large Hydrophobic Surfaces?

This is the first of two papers aimed at understanding, from an atomistic perspective, hydrophobicity at different length scales and how properties such as local densities and angular profiles change for hydrophobic solutes of different sizes. In a subsequent publication we will describe the hydrophobic hydration and hydrophobic interaction of platelike molecules of nanoscale size. Molecular dynamics is used to compute radial and orientational distribution functions of water around three different molecules: argon, methane, and neopentane. In addition, the potential of mean force between two neopentane molecules is computed. The results for the full OPLS/AA1 force field are compared with the solute−solvent WCA truncated OPLS/AA force field for these systems. This work addresses the question of whether a molecule of the size of neopentane is large enough to induce a hydrophobic response similar to that of large hydrophobic molecules or paraffin walls. We answer this question in the affirmative. The orient...

Global fluid phase behavior in binary mixtures of rodlike and platelike molecules

The phase behavior of a liquid-crystal forming model colloidal system containing hard rodlike and platelike particles is studied using the Parsons–Lee scaling [J. D. Parsons, Phys. Rev. A 19, 1225 (1979); S. D. Lee, J. Chem. Phys. 87, 4972 (1987)] of the Onsager theory. The rod and plate molecules are both modeled as hard cylinders. All of the mixtures considered correspond to cases in which the volume of the plate is orders of magnitude larger that the volume of the rod, so that an equivalence can be made where the plates are colloidal particles while the rods play the role of a depleting agent. A combined analysis of the isotropic–nematic bifurcation transition and spinodal demixing is carried out to determine the geometrical requirements for the stabilization of a demixing transition involving two isotropic phases. Global phase diagrams are presented in which the boundaries of isotropic phase demixing are indicated as functions of the molecular parameters. Using a parameter z which corresponds to the product of the rod and plate aspect ratios, it is shown that the isotropic phase is unstable relative to a demixed state for a wide range of molecular parameters of the constituting particles due to the large excluded volume associated with the mixing of the unlike particles. However, the stability analysis indicates that for certain aspect ratios, the isotropic–nematic phase equilibria always preempts the demixing of the isotropic phase, irrespective of the diameters of the particles. When isotropic–isotropic demixing is found, there is an upper bound at large size ratios (Asakura and Oosawa limit), and a lower bound at small size ratios (Onsager limit) beyond which the system exhibits a miscible isotropic phase. It is very gratifying to find both of these limits within a single theoretical framework. We test the validity of the stability analysis proposed by calculating a number of phase diagrams of the mixture for selected molecular parameters. As the hard rod particles promote an effective attractive interaction between the hard-plate colloidal particles, the isotropic–isotropic demixing usually takes place between two rod-rich fluids. As far as the isotropic–nematic transition is concerned, a stabilization as well as a destabilization of the nematic phase relative to the isotropic phase is seen for varying rod–plate size ratios. Moreover, isotropic–nematic azeotropes and re-entrant phenomena are also observed in most of the mixtures studied. We draw comparisons between the predicted regions of stability for isotropic demixing and recent experimental observations.

Critical behavior of uniaxial–biaxial nematic phase transition

A model considering both rodlike molecules and platelike molecules is presented in this paper. The model is treated in the framework of a Landau mean-field theory and renormalization group theory. Mean-field theory predicts the transition between the isotropic, rod nematic, plate nematic, and biaxial nematic phases. The renormalization group analysis shows that the transition exhibits a multiphase bicritical point for n (dimensionality of the order parameter) ⩽2. However, for n>2, the transition exhibits a tetracritical point and an intermediate biaxial nematic phase can exist. The critical exponents are calculated to first order in ε (=4−d).

Effects of the solute shape on the potential surface of carbon dioxide-aromatic molecule system

Model calculations of the potential surface were carried out for the naphthalene-carbon dioxide (CO 2) system. The resultant potential surface was compared with the cases of benzene-CO 2 system in detail using several representative configurations. At the short intermolecular distance, the interaction of naphthalene-CO 2 system showed strong dependence on the relative configurations. It suggested that the large aromatic molecule tends to attract strongly the neighboring solvent molecules. The resultant potential surface was confirmed to reflect effect of the shape of a plate-like molecule on the solvent structures.

Computer simulation on supercritical carbon dioxide fluid a potential model for the benzene-carbon dioxide system from ab initio calculations

Ab initio quantum chemical calculations were done for a system of a benzene molecule and a carbon dioxide molecule for a lot of relative configurations. Since intermolecular interactions in this system were considered to be small at some configurations, the calculations were carried out with 6–31 G ∗ basis set and the 2nd order Møller-Plesset perturbation method. The resultant potential surface was explained in detail using four representative configurations. At the short intermolecular distance, the interaction showed strong dependence on the relative configurations. On the basis of these data, an analytical pair potential function was proposed for the benzeneCO 2 system. Using this potential model, we have done Monte Carlo calculations for the CO 2 fluid containing one benzene molecule at several supercritical conditions. Our model was confirmed to reflect the shape of a plate-like molecule on the solvent structures.

Phase diagrams of binary mixtures of biaxial nematogens

Abstract A mean field theory is used to describe nematic phases of binary mixtures of biaxial molecules. Using a general pseudopotential consistent with the D2h symmetry of the constituent particles, the theory is used to calculate the elements of the order tensors necessary to describe the orientational order in binary mixtures in both uniaxial and biaxial nematic phases. For a single component, the model only requires one parameter, r 2, a ratio of anisotropic interaction strengths, to predict the temperature dependence of the four order parameters. The temperature dependence of the orientational distribution functions is illustrated for both rod-like and plate-like molecules. For binary mixtures, three anisotropic interaction strengths, r 1, r 2, and r 3, are needed to calculate the order parameters of both components as a function of concentration and temperature. The free energy is evaluated to predict the phase stability of the mixture. By systematically varying the anisotropic interaction strengths...

A Monte Carlo Study of the Argon Fluid Around a Benzene Molecule

Abstract Monte Carlo simulations have been carried out for argon fluid containing one benzene molecule at the supercritical region. The purpose in this study is to examine the effect of plate-like molecule on the structure of neighboring fluid composed of simple spherical molecules of the system. In the first neighbor shell of argon from the center of benzene molecule, the average potential energy of argon atoms is confirmed to have a large density dependence. This potential energy is relatively large in the high density region. It is found that the spatial distribution of argon fluid is significantly affected by the molecular shape of benzene and it has little direct connection with the attractive interaction between benzene and argon.

The elastic constants of nematic and nematic discotic liquid crystals with perfect local orientational order

Explicit expressions for the Frank elasticity coefficients of nematic liquid crystals are obtained using the model of perfect local orientational order. The use of the density functional approach enables one to express the results in terms of the direct correlation function for a liquid of perfectly aligned particles. Assuming an ellipsoidal shape for the equipotential surfaces, the direct correlation function can be related to that of the sphere fluid by means of the affine transformation. Both rod-like and plate-like molecules are considered. The dependence of the elasticity coefficients on the axial ratio of the molecule is discussed, and the results for the ratio of the coefficients K 3/K 1 are found to be in reasonable agreement with experimental data for nematics composed of rigid molecules.

Adsorption of square and rectangular plate-like molecules on a planar surface

Abstract A mean-field statistical thermodynamic analysis of monolayer adsorption of rigid square and rectangular plate-like molecules on a homogeneous planar surface is developed. The analysis is simplified by only considering facewise and edgewise modes of adsorption in restricted orthogonal orientations parallel to the surface. The free energy density, adsorbate population distribution and surface spreading pressure are obtained as a function of adsorbate density and compared for square plate molecules using three different sequences of adsorbate molecule placement on the surface to evaluate the configurational degeneracy. It is found that edgewise adsorbed molecules can be anisotropically ordered if the edge length of square and rectangular plate-like molecules exceeds three length units in the absence of anisotropic dispersion interactions. If intermolecular dispersion interactions are present and of sufficient strength, the spreading pressure-density isotherms can exhibit one or two van der Waals loo...

Diffusion coefficient increases with density in hard ellipsoid liquid crystals.

Molecular-dynamics simulations of highly nonspherical rodlike and platelike molecules reveal that one of the two diffusion coefficients in the nematic-liquid-crystal phase increases with density, reaches a maximum, and decreases, while the other coefficient decreases monotonically in the usual way. This effect seems to be associated with the density variation of the nematic order parameter near the phase transition. A kinetic theory based on isolated binary collisions significantly underestimates the diffusion coefficients, but partially accounts for the enhancement effect.

A Van Der Waals Model for the Nematic Liquuid Crystal Mixtures

Abstract Phase diagrams and excess molar volumes of binary nematogenic mixtures are investigated in the frame of a van der Waals - type theory for systems made up of hard rectangular parallelepipeds. It is shown that the excess volumes can be positive or negative or can change sign depending on temperature and composition. In agreement with experiment the model predicts a coexistence of two nematic phases for mixture of rod-like and plate-like molecules.

The effect of pressure on order destruction and order creation in linear or branched alkane mixtures

The pressure dependence of the excess enthalpy HE, dHE/dP, has been calculated from experimental excess volumes VE and dVE/dT using dHE/dP=VE−TdVE/dT. dHE/dP at zero pressure are reported at 25°C and equimolar concentration for the mixtures: cyclohexane with the series of normal alkanes (n-Cn, where n=6,8,10,12,14 and 16) and with the series of highly branched alkanes (br-Cn, where n=6,8,12 and 16), benzene, toluene and p-xylene +n-Cn and 1-chloronaphthalene +n-Cn and br-Cn. Experimental and Flory theory dHE/dP values are in good agreement for the whole cyclohexane +br-Cn series. For the n-Cn series, dHE/dP becomes increasingly positive deviating from the Flory predictions. This discrepancy is due to the presence of short-range orientational order in the higher n-Cn pure liquids which makes dH/dP more negative and which, upon mixing, is destroyed producing a positive contribution to dHE/dP not accounted for by the theory. The discrepancy between theoretical and experimental dHE/dP is large for benzene, but progressively smaller for toluene, p-xylene and 1-chloronaphthalene. These results are consistent with creation of order between the aromatic plate-like molecule and the long n-Cn in solution. For 1-chloronaphthalene +n-Cn, this order creation process produces a negative contribution to dHE/dP which balances the positive order-destruction contribution originated by the rupture, upon mixing, of short-range orientational order in pure n-Cn.

Induced smectic A1, A2and à phases at low temperatures. Molecule parameters related to phase diagrams and structure

Abstract Binary mixtures combining some rod-like mesogens with non-mesogenic plate-like molecules show surprising induction of smectic A phases close to room temperature. We have systematically varied the nature of both components in order to correlate the phenomenon to the molecular structure. The calorimetric and X-ray analysis of several systems specify the topologies of the phase diagrams and definitely confirm the unusual character of these phases.

Order destruction and order creation in binary mixtures of non-electrolytes

Order-destruction and order-creation during mixing are discussed for binary mixtures of various liquids (component 1) with the n-alkane series (component 2). When component 1 is a spherical molecule liquid, e.g. cyclohexane, anomalous positive contributions occur in H E and S E, but negative ones occur in C P E, Δ C V, d V E/d T and other second-order quantities. The Flory theory is used as a first approximation to the prediction of these quantities. Thermodynamic effects, not included in the theory, are attributed to a net decrease in structure during mixing because of a destruction of short-range correlations of molecular orientations (CMO) in pure n-C n . When component 1 is a plate-like molecule, e.g. 1-chloronaphthalene, these effects are decreased and reversed in sign. This has been ascribed to a net increase in structure because of a hindrance to molecular motion or a change in molecular conformation. When component 1 is a linear or branched alcohol, the n-C n are relatively inert while component 1 is associated in both the pure and solution states leading to positive or negative effects in C P E depending whether the solution or pure component 1 is more structured. The Treszczanowicz-Kehiaian model for self-associated liquids + inert solvents explains these results and also provides a general framework for the discussion of both the process of order-creation and the process of order-destruction in solution.