Diffusive transport by thermal velocity fluctuations

We study the contribution of advection by thermal velocity fluctuations to the effective diffusion coefficient in a mixture of two identical fluids. We find good agreement between a simple fluctuating hydrodynamics theory and particle and finite-volume simulations. The enhancement of the diffusive transport depends on the system size L and grows as ln(L/L?) in quasi-two-dimensional systems, while in three dimensions it scales as L??1 - L?1, where L? is a reference length. Our results demonstrate that fluctuations play an important role in the hydrodynamics of small-scale systems.     

Interaction of NO with Au nanoparticles supported on (100) terraces and topological defects of MgO

The adsorption of nitric oxide (NO) on Au_n (n = 1–3) particles deposited on anionic (O^2?) sites of MgO has been studied using the DFT (Density Functional Theory) approach. The regular O^2? sites of MgO(100) and the sites in edge and corner topological defects with high symmetry of MgO were considered. The adhesion energy of Au_n to MgO is larger for Au₂ and Au₃ due to higher polarization effects. On the other hand, the interaction strength of NO with supported Au_n particles depends mainly on the electronic configuration (open or closed shell) of the particle; the Au particles with odd number of atoms show larger NO binding energies. A comparison was performed with the reactivity of free Au_n particles. From this, it is possible to conclude that the support enhances the NO–Au_n bonding strength for the monomer, weakens this interaction in the case of the dimer, and does not have an effect in the trimers. Besides, the NO–Au_n bonding is essentially insensitive to the coordination of the anionic site where the Au_n particle is linked. A large red-shift of the N–O stretching frequency was obtained, particularly for the Au particles with odd number of atoms, due to a negative charge transfer from Au to NO.     

The effect of high electric fields on the structure and dielectric properties of the B2 phase

Dielectric studies are presented of a banana-shaped compound that exhibits the antiferroelectric B₂ phase. Upon application and subsequent removal of strong electric fields the textures and dielectric properties of the phase drastically change. Most notable is the huge increase of the low frequency permittivity. This behaviour would suggest the induction of ferroelectricity by the electric field.     

Study by fluorescence of calix[4]arenes bearing heterocycles with anions: highly selective detection of iodide

The present work describes a study of complexation efficiency of calix[4]arenes bearing benzoimidazolyl, benzothiazolyl, and benzoxazolyl heterocycles (5–7) towards several anions. The binding ability of calixarene derivatives 5–7 towards selected anions of different molecular geometries such as: F^?, HSO₄ ^?, I^?, N₃ ^?, NO₃ ^?, NO₂ ^?, SCN^?, ClO₄ ^?, Br^?, CN^?, Cl^?, CH₃COO^? CF₃SO₃ ^? in methanol, has been investigated by fluorescence spectroscopic techniques, all anions were used as tetrabutylammonium salts to avoid possible complexation of cationic species by the derivative calix[4]arenes. Fluorescent chemosensor ability of these three calixarene derivatives was highly selective for iodide in contrast with other anions studied. The best chemosensor found, corresponds to compound 7, with an association constant of 2.01 × 10⁴ mol^?1 L and a detection limits of 0.22 ppm for iodide.     

Influence of cylindrical submicrometer confinement on the static and dynamic properties in nonyloxycyanobiphenyl (9OCB)

Broadband dielectric spectroscopy (10(2)-1.9 x 10(9) Hz) and specific heat measurements have been performed on nonyloxycyanobiphenyl (9OCB) in the isotropic (I), nematic (N), and smectic A (SmA) phases confined to 200 nm diameter parallel cylindrical pores of Anopore membranes. Untreated and HTBA-treated membranes have been found to obtain axial and radial confinements, respectively. However, structural or configurational transitions in untreated membranes have been reported to exist in the SmA-mesophase of 9OCB. Both confinements clearly affect the N-I and SmA-N phase transitions. In the axial confinement, the analysis of the specific heat and static dielectric permittivity data leads to a second order SmA-N phase transition, which is known to be weakly first order for bulk 9OCB. Dynamic dielectric measurements have accounted for the different molecular motions in both confinements. On both mesophases, either N or SmA, the relaxation processes in axial configuration are faster than in the bulk. However, in radial confinement, they are either equal or slower than in the bulk. Additionally, there are no differences in the energy barrier hindering the molecular motions between the axial and radial confinements and even in relation to bulk. Likewise, dielectric results suggest that the extension inside the pores of the surface pinned molecular layer (proved to be temperature-dependent) persists at high enough temperature as a residual-thin layer adjacent to the pore wall.     

Carbohydrate-carbohydrate interaction prominence in 3D supramolecular self-assembly

Self-association in water of biologically significant carbohydrate molecules is a controversial topic due to the strong solvation of these molecules in this solvent and the difficulty to experimentally detect these very weak intermolecular forces by biophysical techniques. Herein we report the tremendous ability of amphiphilic carbohydrate molecules to form complex three-dimensional architectures. We have experimentally observed the 3D self-assembly into multilayers of disaccharide neoglycolipid dimers on graphite by means of noncontact AFM and we have also theoretically modeled the interaction between two dimers in order to learn about the structure and composition of these layers. A simple bilayer structure as observed for many amphiphilic lipids was discarded by the experiments. Instead, based on the good agreement between experiments and calculations, we propose that multilayer formation takes place through the assembly of building blocks consisting of two dimers each. The fundamental key in the formation of this supramolecular structure is the complementarity between the van der Waals surfaces of the amphiphilic carbohydrate molecules, a result which differs from the most common idea that H-bonding interactions are prominent in carbohydrate-mediated interactions.     

Role of step sites on water dissociation on stoichiometric ceria surfaces

The adsorption and dissociation of water on CeO₂(111), CeO₂(221), CeO₂(331), and CeO₂(110) has been studied by means of periodic density functional theory using slab models. The presence of step sites moderately affects the adsorption energy of the water molecule but in some cases as in CeO₂(331) is able to change the sign of the energy reaction from endo- to exothermic which has important consequences for the catalytic activity of this surface. Finally, no stable molecular state has been found for water on CeO₂(110) where the reaction products lead to a very stable hydroxylated surface which will rapidly become inactive.     

Thermodynamics of organic mixtures containing amines: V. Systems with pyridines

Binary mixtures containing pyridine (PY), or 2-methylpyridine (2MPY) or 3-methylpyridine (3MPY) or 4-methylpyridine (4MPY) and an organic solvent as benzene, toluene, alkane, or 1-alkanol are investigated in the framework of DISQUAC. The corresponding interaction parameters are reported. The model describes accurately a whole set of thermodynamic properties: vapor-liquid equilibria (VLE), liquid-liquid equilibria (LLE), solid-liquid equilibria (SLE), molar excess Gibbs energies (G^E), molar excess enthalpies, (H^E), molar excess heat capacities at constant pressure () and the concentration-concentration structure factor (S_CC(0)). It is remarkable that DISQUAC correctly predicts the W-shaped curve of the of the pyridine + n-hexadecane system. The model can be applied successfully to mixtures with strong positive or negative deviations from the Raoult's law. DISQUAC improves the theoretical results from UNIFAC (Dortmund version). The replacement of pyridine by a methylpyridine leads to a weakening of the amine-amine interactions, ascribed to the steric effect caused by the methyl group attached to the aromatic ring. This explains that for a given solvent (alkane, 1-alkanol) H^E(pyridine) > H^E(methylpyridine).     

Synthesis and receptor binding evaluation of clavizepine analogues with no ring D substituents

Assembly of the azepine ring of xantheno[9,1-cd]azepines by electrophilic cyclization of sulfonamide acetals provides access to clavizepine analogues in the form of 2,12b-dihydro- or 4-hydroxy-2,3,4,12b-tetrahydro-1H-xantheno[9,1-cd]azepines, in the latter case producing the trans derivative stereoselectively. Binding assays for clavizepine and analogues at adrenergic, dopaminergic, and serotonergic receptors are reported.     

A Comparative Study of Methyl Methacrylate/Butyl Acrylate Copolymerization Kinetics by Atom‐Transfer and Conventional Radical Polymerization

Methyl methacrylate and butyl acrylate monomers are copolymerized by atom‐transfer radical polymerization, affording polymers with well‐controlled molecular weight and low polydispersity. A kinetic analysis of this system is compared with the corresponding free‐radical polymerization system. The copolymerization rate follows an opposite trend to that observed in conventional copolymerization. This fact is attributed to a smaller population of radicals generated in the reaction, since the relative fraction of propagating radicals is the same as that in classical copolymerization.