An efficient Monte Carlo algorithm for the fast equilibration and atomistic simulation of alkanethiol self-assembled monolayers on a Au(111) substrate

A new Monte Carlo algorithm is presented for the simulation of atomistically detailed alkanethiol self-assembled monolayers (R-SH) on a Au(111) surface. Built on a set of simpler but also more complex (sometimes nonphysical) moves, the new algorithm is capable of efficiently driving all alkanethiol molecules to the Au(111) surface, thereby leading to full surface coverage, irrespective of the initial setup of the system. This circumvents a significant limitation of previous methods in which the simulations typically started from optimally packed structures on the substrate close to thermal equilibrium. Further, by considering an extended ensemble of configurations each one of which corresponds to a different value of the sulfur-sulfur repulsive core potential, sigmass, and by allowing for configurations to swap between systems characterized by different sigmass values, the new algorithm can adequately simulate model R-SH/Au(111) systems for values of sigmass ranging from 4.25 A corresponding to the Hautman-Klein molecular model (J. Chem. Phys. 1989, 91, 4994; 1990, 93, 7483) to 4.97 A corresponding to the Siepmann-McDonald model (Langmuir 1993, 9, 2351), and practically any chain length. Detailed results are presented quantifying the efficiency and robustness of the new method. Representative simulation data for the dependence of the structural and conformational properties of the formed monolayer on the details of the employed molecular model are reported and discussed; an investigation of the variation of molecular organization and ordering on the Au(111) substrate for three CH3-(CH2)n-SH/Au(111) systems with n=9, 15, and 21 is also included.     

Perturbation solution of Poiseuille flow of a weakly compressible Oldroyd-B fluid

The isothermal, planar Poiseuille flow of a weakly compressible Oldroyd-B fluid is considered under the assumption that the density of the fluid obeys a linear equation of state. A perturbation analysis for all the primary flow variables is carried out with the isothermal compressibility serving as the perturbation parameter. The sequence of partial differential equations which results from the perturbation procedure is solved analytically up to second order. The effects of the compressibility parameter, the aspect ratio, and the Weissenberg number are discussed. In particular, it is demonstrated that compressibility has a significant effect on the transverse velocity and the first normal stress difference.     

Double large field stereoscopic PIV in a high Reynolds number turbulent boundary layer

An experiment on a flat plate turbulent boundary layer at high Reynolds number has been carried out in the Laboratoire de Mecanique de Lille (LML, UMR CNRS 8107) wind tunnel. This experiment was performed jointly with LEA (UMR CNRS 6609) in Poitiers (France) and Chalmers University of Technology (Sweden), in the frame of the WALLTURB European project. The simultaneous recording of 143 hot wires in one transverse plane and of two perpendicular stereoscopic PIV fields was performed successfully. The first SPIV plane is 1?cm upstream of the hot wire rake and the second is both orthogonal to the first one and to the wall. The first PIV results show a blockage effect which based on both statistical results (i.e. mean, RMS and spatial correlation) and a potential model does not seem to affect the turbulence organization.     

Inclusion compounds of plant growth regulators in cyclodextrins, part VII: study of the crystal structures of 2-naphthylacetic acid encapsulated in β-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin complexes by X-ray crystallography

The crystal structures of the β-naphthylacetic acid (2NAA)/β-cyclodextrin (β-CD) and the 2NAA/heptakis(2,3,6-tri-O-methyl)-β-CD (TMβCD) complexes are reported. The 2NAA/β-CD complex crystallizes in the triclinic system forming a dimer inside the cavity of which two 2NAA molecules disordered over two sites are located. The dimers are stacked along the c axis according to the channel packing mode forming a nanotube which resembles a wireway as it contains guest molecules linked by π–π interactions inside each dimeric cavity and by H-bonds between the adjacent dimers. The 2NAA/TMβCD complex crystallizes in the orthorhombic space group P2₁2₁2₁. Its asymmetric unit contains one host, one guest distributed over two sites and one water molecule having a low occupancy factor. The complexes are packed in a head-to-tail mode forming a screw channel along the b axis. The carboxyl group of the guest protrudes towards the “free” space between the complexes and is H-bonded to the water molecule which in turn is H-bonded to the O5n atom of the host of the subsequent complex. The orientation of the guest molecule in the 2NAA/β-CD complex has been found opposite to that of the guest in the 2NAA/TMβCD complex probably due to the formation of dimers and the π–π interactions between the naphthalene moieties of the encapsulated molecules inside the dimeric cavity.     

Formation of reactive o-quinone methides from the reaction of trimethylsilyl(methyl)-substituted 1,4-benzoquinones with nucleophiles

o-Quinone methides are formed from the reaction of nucleophiles with trimethylsilyl(methyl)-1,4-benzoquinones. These reactive intermediates are trapped by excess nucleophile to form substituted quinones following oxidation. In addition, varying amounts of a symmetrical dimer and a xanthen derivative were observed. The influence of different nucleophiles and ring substituents on the rate of reaction have been studied, and are consistent with rate-limiting formation of a vinylogous enolate initiated by attack of the nucleophile on the silyl group.     

A differential model for the rheological properties of concentrated suspensions with weakly viscoelastic matrices

A model for the rheological properties of a concentrated suspension in weakly viscoelastic fluid matrices is proposed. The model is derived according to the Roscoe differential procedure described in 1952. The analytical results produced recently by Greco et al. (J Non-Newton Fluid Mech 147:1–10, 2007) and Housiadas and Tanner (J Non-Newton Fluid Mech 162:88–92, 2009) for dilute suspensions of neutrally buoyant, non-Brownian rigid spheres in weakly viscoelastic matrix fluids are the key results which are used as a base to predict the properties of concentrated suspensions. The results are compared with the few available experimental data from the literature, showing promising trends for the viscometric properties of the suspensions. In particular, one sees the rapidly increasing value of −N₂/N₁ as concentration increases.     

Study of thermal and mechanical properties of composites based on arc-grown carbon nanotubes and heat-resistant cyanoether binder

Multi-walled carbon nanotubes synthesized by means of arc evaporation of graphite were used to prepare composites with a heat-resistant binder based on cyanoether. To increase the homogeneity of distribution of nanotubes in the polymer matrix, the carbon material was cleaned of graphite particles and amorphous carbon with a potassium permanganate solution in concentrated sulfuric acid. By means of X-ray photoelectron spectroscopy, it was shown that the proposed purification procedure leads to the grafting of oxygen-containing groups to the surface of carbon nanotubes. By means of differential scanning calorimetry, it was revealed that the oxide overcoat on the nanotube surface exerts an influence on the character of binder polymerization. The mechanical properties of a carbon-reinforced plastic with different nanotubes contents were measured. It was shown that the admixture of 0.25–0.50% carbon nanotubes improves the mechanical characteristics of carbon-reinforced plastics by 10–20%.     

Directed assembly of copolymer materials on patterned substrates: Balance of simple symmetries in complex structures

The self-assembly of a lamella-forming blend of a diblock copolymer and its respective homopolymers on periodically patterned substrates is investigated by a concerted experimental and theoretical approach. The substrate pattern consists of square arrays of spots that preferentially attract one component of the blend. The mismatch between the lamellar equilibrium morphology of the copolymer material and the substrate pattern results in the formation of a bicontinuous morphology. At the substrate, a quadratically perforated lamella (QPL) assembles in perfect registry with the substrate pattern. From this, QPL necks emanate and reach the top surface of the film. The detailed structure of these cylindrical nanochannels is analyzed using Voronoi tessellation, orientation correlation functions, and the structure factor of the neck positions on the top surface. The surface morphology is dictated by the antagonism of the square symmetry of the substrate pattern and the tendency of the necks to locally pack in a hexagonal arrangement. The analogy and differences to a system of adsorbed monolayer on corrugated substrates is explored by comparing the arrangement of the necks on the film's top surface with the structure of a soft disk model on a quadratically corrugated substrate. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2589–2604, 2006     

The impact of reverberant self-masking and overlap-masking effects on speech intelligibility by cochlear implant listeners (L)

The purpose of this study is to determine the relative impact of reverberant self-masking and overlap-masking effects on speech intelligibility by cochlear implant listeners. Sentences were presented in two conditions wherein reverberant consonant segments were replaced with clean consonants, and in another condition wherein reverberant vowel segments were replaced with clean vowels. The underlying assumption is that self-masking effects would dominate in the first condition, whereas overlap-masking effects would dominate in the second condition. Results indicated that the degradation of speech intelligibility in reverberant conditions is caused primarily by self-masking effects that give rise to flattened formant transitions.