Autohesion Mechanisms at Interfaces Between Random Copolymer Melts: Mesoscopic Simulations with Realistic Coarse-Grained Models.

The increase in welding time during the interdiffusion of a pair of non reacting random copolymer melts favors the strength rate of healing at the interface. Furthermore, the diffusion kinetic during the interpenetration of copolymer chains across the interface is strongly dependent on molecular weight. In this paper we perform mesoscopic simulations with realistic coarse grain models to study the autohesion mechanism across the interface between slightly entangled styrene-butadiene random copolymer melts.

Consistent and Transferable Force Fields for Statistical Copolymer Systems at the Mesoscale.

The statistical trajectory matching (STM) method was applied successfully to derive coarse grain (CG) models for bulk properties of homopolymers. The extension of the methodology for building CG models for statistical copolymer systems is much more challenging. We present here the strategy for developing CG models for styrene-butadiene-rubber, and we compare the quality of the resulting CG force fields on the structure and thermodynamics at different chemical compositions. The CG models are used through the use of a genuine mesoscopic method called the dissipative particle dynamics method and compared to high-resolution molecular dynamics simulations. We conclude that the STM method is able to produce coarse-grained potentials that are transferable in composition by using only a few reference systems. Additionally, this methodology can be applied on any copolymer system.

The pair distribution function in the planar gas-liquid interface: Application to the calculation of the surface tension.

A Monte Carlo simulation is used to calculate the pair distribution function g(2)r1,r2 for a planar gas-liquid interface. Due to the cylindrical symmetry of the system, g(2) can be stored as a three-dimensional array that can be readily manipulated and used to calculate the surface tension and the single atom density profile directly. The consistency and accuracy of our calculation of g(2)(r1, r2) is demonstrated by a calculation of the single atom density through the first Born-Green-Yvon equation. We show that the surface tension calculated directly from the pair distribution function and from other well-established routes is completely consistent. In the case of the gas-liquid interface for argon modeled with an explicit inclusion of the three-body forces, an accurate pair distribution can be used to estimate the long-range contribution to the three-body part of the surface tension. A detailed analysis of this correction, its dependence on the three-body cutoff, and its overall contribution to the surface tension are presented.

Communication: Slab thickness dependence of the surface tension: toward a criterion of liquid sheets stability.

Microscopic Monte Carlo simulations of liquid sheets of copper and tin have been performed in order to study the dependence of the surface tension on the thickness of the sheet. It results that the surface tension is constant with the thickness as long as the sheet remains in one piece. When the sheet is getting thinner, holes start to appear, and the calculated surface tension rapidly decreases with thickness until the sheet becomes totally unstable and forms a cylinder. We assume here that this decrease is not due to a confinement effect as proposed by Werth et al. [Physica A 392, 2359 (2013)] on Lennard-Jones systems, but to the appearance of holes that reduces the energy cost of the surface modification. We also show in this work that a link can be established between the stability of the sheet and the local fluctuations of the surface position, which directly depends on the value of the surface tension. Finally, we complete this study by investigating systems interacting through different forms of Lennard-Jones potentials to check if similar conclusions can be drawn.

Multiscale Modeling Approach toward the Prediction of Viscoelastic Properties of Polymers.

We report a multiscale modeling approach to study static and dynamical properties of polymer melts at large time and length scales. We use a bottom-up approach consisting of deriving coarse-grained models from an atomistic description of the polymer melt. We use the iterative Boltzmann inversion (IBI) procedure and a pressure-correction function to map the thermodynamic conditions of the atomistic configurations. The coarse-grained models are incorporated in the dissipative particle dynamics (DPD) method. The thermodynamic, structural, and dynamical properties of the cis-1,4-polybutadiene melt are very well reproduced by the coarse-grained DPD models with a significative computational gain. We complete this study by addressing the challenging question of the investigation of the shear modulus evolution. As expected from experiments, the stress correlation functions show behaviors that are dependent on the molecular weights defining unentangled and weakly entangled polymer melts.

Molecular dynamics simulations of ferrocene-terminated self-assembled monolayers.

The present work describes our studies of the Fc(CH(2))(12)S-/C(10)S-Au monolayers to provide a more detailed molecular description. Molecular dynamics simulations of these mixed monolayers are carried out in conditions close to the electrochemical ones. For this purpose, a supporting electrolyte is added (NaClO(4) 1 M) and the electron transfer process is modeled through molecular simulations of ferrocene both in its neutral (initial state) and oxidized form (final state). The heterogeneity of the surface, that is, "clustered " or "isolated" ferrocene moieties, has been considered for the ferrocenylalkylthiolates using the same grafting densities. The structural properties (density profiles and angular distributions) are described in terms of redox induced orientation changes by comparison between the initial and final states. It is established that this orientation change due to the oxidation of the ferrocene to the ferrocenium is mainly observed in the random system, and it is less pronounced in the cluster system. Finally, the energy contributions underline the role played by the supporting electrolyte.

Description of ferrocenylalkylthiol SAMs on gold by molecular dynamics simulations.

Molecular dynamics simulations of mixed monolayers consisting of Fc(CH2)12S-/C10S-Au SAMs are carried out to calculate structural (density profiles, angular distributions, positions of atoms) and energetic properties. The purpose of this paper is to explore the possible inhomogeneity of the neutral ferrocene moieties within the monolayer. Five systems have been studied using different grafting densities for the ferrocenylalkylthiolates. The angular distributions are described in terms of the relative contributions from isolated and clustered ferrocene moieties in the binary SAMs. It is shown that the energetic contributions strongly depend on the state of the ferrocene. The ability of molecular dynamics simulations to enable better understanding the SAM structure is illustrated in this work.

Calculation of the surface tension from Monte Carlo simulations: does the model impact on the finite-size effects?

We report two-phase Monte Carlo simulations of the liquid-vapor interface of the Lennard-Jones (LJ) fluids in order to study the impact of the methodology used for the energy calculation on the oscillatory behavior of the surface tension with the system sizes. The surface tension values are illustrated through the LJ parameters of methane. The first methodology uses a standard truncated LJ potential, the second one adds a long range correction (LRC) contribution to the energy into the Metropolis scheme, and the third one uses a LJ potential modified by a polynomial function in order to remove the discontinuities at the cutoff distance. The surface tension is calculated from the mechanical and thermodynamic routes and the LRCs to the surface tension are systematically calculated from appropriate expressions within these definitions. The oscillatory behavior has been studied as a function of the size of the interfacial area and of the length of the dimension perpendicular to the surface. We show that the methodology has an important effect on the oscillatory variation in the surface tension with the system size. This oscillatory variation in the surface tension with the system size is investigated through its intrinsic and LRC contributions. We complete this work by studying the dependence of the surface tension with respect to the cutoff distance when the LRC part to the energy is considered into the Metropolis scheme.

Surface tensions of linear and branched alkanes from Monte Carlo simulations using the anisotropic united atom model.

The anisotropic united atoms (AUA4) model has been used for linear and branched alkanes to predict the surface tension as a function of temperature by Monte Carlo simulations. Simulations are carried out for n-alkanes ( n-C5, n-C6, n-C7, and n-C10) and for two branched C7 isomers (2,3-dimethylpentane and 2,4-dimethylpentane). Different operational expressions of the surface tension using both the thermodynamic and the mechanical definitions have been applied. The simulated surface tensions with the AUA4 model are found to be consistent within both definitions and in good agreement with experiments.

Molecular dynamics description of grafted monolayers: effect of the surface coverage.

Molecular dynamics simulations of monolayers of metal-chelating ligands grafted onto a graphite surface in water are carried out to calculate structural (density profiles, radius of gyration, and asphericity coefficients), dynamical (diffusion coefficients), and energetical properties as a function of the surface coverage. The purpose is to provide a better understanding of the dependence of various properties of these monolayers on the surface coverage. A critical value of the surface coverage from which all structural properties derive a limiting value has been established. It also appears that the chains rather adopt an elongated conformation along the direction normal to the surface from this critical surface coverage. The hydrogen-bonding structure and dynamics of water molecules are reported. An ordered structure of water in the region close to the terminal groups of the grafted molecules is shown at a relatively high surface coverage. This ordering is similar to that observed in the case of water in interaction with a solid surface.

Expressions for local contributions to the surface tension from the virial route.

The expression of the surface tension using the virial route has been reinvestigated in order to establish a local version of the surface tension and of its long-range corrections. In fact, giving a local surface tension is very important for the simulation from a methodological viewpoint. It is also of basic interest to associate the profile of the intrinsic part of the surface tension with that of the long-range corrections to make the surface tension calculation consistent between the different approaches that can be used. Working expressions for two-phase systems interacting through dispersion-repulsion (Lennard-Jones) and Coulombic (Ewald summation) interactions are proposed. Different operational expressions of the surface tension are compared in the cases of n -pentane, carbon dioxide, and water liquid-vapor equilibria for which the orders of magnitude between the electrostatic and dispersion forces are different.

Surface tension of water and acid gases from Monte Carlo simulations.

We report direct Monte Carlo (MC) simulations on the liquid-vapor interfaces of pure water, carbon dioxide, and hydrogen sulfide. In the case of water, the recent TIP4P/2005 potential model used with the MC method is shown to reproduce the experimental surface tension and to accurately describe the coexistence curves. The agreement with experiments is also excellent for CO(2) and H(2)S with standard nonpolarizable models. The surface tensions are calculated by using the mechanical and the thermodynamic definitions via profiles along the direction normal to the surface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The different profiles of these contributions are proposed in the case of water.

Multiple histogram reweighting method for the surface tension calculation.

The multiple histogram reweighting method takes advantage of calculating ensemble averages over a range of thermodynamic conditions without performing a molecular simulation at each thermodynamic point. We show that this method can easily be extended to the calculation of the surface tension. We develop a new methodology called multiple histogram reweighting with slab decomposition based on the decomposition of the system into slabs along the direction normal to the interface. The surface tension is then calculated from local values of the chemical potential and of the configurational energy using Monte Carlo (MC) simulations. We show that this methodology gives surface tension values in excellent agreement with experiments and with standard NVT MC simulations in the case of the liquid-vapor interface of carbon dioxide.

Molecular simulations of the n -alkane liquid-vapor interface: interfacial properties and their long range corrections.

Monte Carlo simulations have been performed to study the interfacial properties of the liquid-vapor interface of alkanes. We highlight the chemical equilibrium of the liquid-vapor interface by calculating a local chemical potential including the appropriate long-range corrections profiles. We extend the "test-area" (TA) technique developed by Gloor [J. Chem. Phys. 123, 134703 (2005)] on Lennard-Jones and square-well fluids to molecular systems. We establish both operational expressions of the TA approach for the calculation of the surface tension profile and the corresponding long-range corrections by underlining the approximations used. We compare the results between the different operational expressions of the surface tension and focus on the truncation procedures to explain the difference between the different techniques using either the potential or force equations. We make the results of surface tension identical between the different methods by using consistent potential and force equations. In the case of a relatively small cutoff, we propose to show that the Irving-Kirkwood definition and TA methods lead to the same value of the surface tension under condition that appropriate long-range corrections be included in the calculation. We end this paper by calculation of the entropy change profile and a comparison with experiments.

Monoclonal antibodies against human liver cytochrome P-450.

Monoclonal hybridomas which produce antibodies against human liver microsomal cytochrome P-450 were developed. Three similar hybridomas produced antibodies which recognized an epitope specific to a family of human P-450 isozymes (P-450(5)). This epitope was also present on cytochrome P-450 PCN-E (pregnenolone-16 alpha-carbonitrile induced) from rat liver microsomes, but this isozyme differed from the human P-450(5) by its molecular weight. These antibodies enabled us to quantify cytochrome P-450(5) in human liver microsomes and to demonstrate an important quantitative polymorphism in the human liver monooxygenase system.

[Juvenile chronic arthritis. Double-blind study of the efficacy and tolerance of D-penicillamine]. / Arthrite chronique juvénile. Etude en double insu de l'efficacité et de la tolérance de la D-pénicillamine.

Seventy-four children with juvenile chronic arthritis were admitted to a 6-month multicentric double blind comparative study of the efficiency of D-Penicillamine or placebo. The results evaluated in 55 patients who completed the study, indicated an improvement in the total number of stiff joints and in the total index of severity measuring joint pain and inflammation. A significant reduction of the concurrent use of non steroidal antiinflammatory drugs was also observed. The tolerance was good except in two patients. Some children in the placebo group exhibited a definite improvement.

Quantification of two cytochrome P-450 isoenzymes by an enzyme-linked immunosorbent assay (ELISA).

An enzyme linked immunosorbent assay (ELISA) using monoclonal and polyclonal antibodies has been developed to quantify individual cytochrome P-450 isoenzymes in microsomal preparations, namely UT-A and PB-B. This very sensitive method can be used for the rapid processing of large quantities of determinations and requires only limited amounts of antibodies.

Preparation and characterization of monoclonal antibodies against phenobarbital-inducible cytochrome P-450.

Monoclonal antibodies against cytochrome P-450 were prepared from phenobarbital-induced rat liver microsomes. The immunoglobulin classes and subclasses, as well as the binding capacity to cytochrome P-450, of the different antibodies were characterized. Their specificity was verified by various techniques and seemed to correspond to a single form of cytochrome P-450, the major phenobarbital-inducible form. However, the antibodies were unable to inhibit completely the monooxygenase activities investigated. These antibodies may constitute very specific and powerful analytical tools for characterizing and quantifying cytochrome P-450 isoenzymes.

The expression of different monooxygenases supported by cytochrome P-450 in neonatal rats and in primary fetal hepatocytes in culture.

In rat liver, the perinatal development of various monooxygenase activities follows different patterns, depending upon the reaction studied. The ontogeny of the 6 beta-, 7 alpha- and 16 alpha-testosterone hydroxylase activities differs very significantly. Aldrin epoxidase and steroid-metabolizing monooxygenases are expressed in primary fetal rat liver cells in culture after treatment in vitro with dexamethasone. Testosterone is not metabolized by the control cells and is hydroxylated on the 6 beta and 16 alpha positions following the addition of corticoids to the culture medium. The dose and time curves vary according to the hydroxylated position of the steroid. Aldrin epoxidase activity is nearly undetectable in the control cells, but is present and is inducible by phenobarbital following treatment with the corticoid. Phenobarbital induces aldrin epoxidase in the absence of dexamethasone in the culture medium, providing that the cells are pretreated with the corticoid for 48 h. The use of antibodies against the main cytochrome P-450 species purified from adult and phenobarbital-treated rats confirms that a similar cytochrome P-450 can be induced in fetal cells in culture. The perinatal regulation of biological events, such as the expression of the monooxygenases, can be reproduced in fetal rat liver cells in culture; such a model constitutes a unique tool for studying the biochemical mechanisms which control these phenomena.