Neutron molecular simulations

Macromolecules, 35, 7110 (2002). Segmental Dynamics of Atactic Polypropylene as Revealed by Molecular Simulations and Quasielastic Neutron Scattering. 

Dimensions Electron microscopy intrinsic viscosity measurements size exclusion chromatography computer-assisted molecular simulations, comparison with CPK models atomic force microscopy electrophoresis neutron scattering. 

During the last two decades, the wide use of modern experimental tools, especially X-ray, neutron scattering, and modem spectroscopic methods, allowed one to obtain valuable information about the nanostructure of aqueous mixtures containing alcohols or various hydrophobic solutes. The avaUabihty of powerful computers combined with refined methods of molecular simulations, such as molecular dynamics and Monte Carlo, were actively used to investigate the nanolevel scale of aqueous solutions. 

Pore structure analysis methods based upon realistic disordered microstructures may be classified into two types. In one approach, the experimental procedures used to fabricate the material are reproduced, to the greatest extent possible, via molecular simulation, and the resulting amorphous material structure is then statistically analyzed to obtain the desired structural information. In the other approach, adsorbent structural data (e.g., smaU-angle neutron scattering) is used to construct a model disordered porous structure that is statistically consistent with the experimental measurements. As in the first approach, molecular simulations can then be carried out using the derived model structure to obtain the structural characteristics of the original adsorbent. 

Except for the fullerenes, carbon nanotubes, nanohoms, and schwarzites, porous carbons are usually disordered materials, and cannot at present be completely characterized experimentally. Methods such as X-ray and neutron scattering and high-resolution transmission electron microscopy (HRTEM) give partial structural information, but are not yet able to provide a complete description of the atomic structure. Nevertheless, atomistic models of carbons are needed in order to interpret experimental characterization data (adsorption isotherms, heats of adsorption, etc.). They are also a necessary ingredient of any theory or molecular simulation for the prediction of the behavior of adsorbed phases within carbons - including diffusion, adsorption, heat effects, phase transitions, and chemical reactivity. 

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

The space and time scales accessible by the neutron scattering techniques are comparable to the ones covered by molecular simulations so that comparisons between experiment and predictions can be made not only for the diffusivities, but also for the jumps between adsorption sites and for the distribution of adsorbed molecules. 

Cr,Cl ), and (K, K ) (c) a solid, consisting of crystalline (curve c) and amorphous (curve a) silicon and (d) a solvated polymer, where the pair consists of a polymer segment and a solvent molecule. The plotted function in (c) would be roughly compatible with the others if it were divided by Anr. Curves (b) and (c) are based on neutron-scattering data, while (a) and (d) are the result of MD calculations. [Curve (a) is from Ref. 60 Molecular Simulation), (b) is from Ref. 61 Journal de Physique), (c) is from Ref. 62 (US Atomic Energy Commission), (d) is from Ref. 63 (American Institute of Physics), and they appear by permission of the authors and publishers.]... 

This chapter discusses the form and parameterization of the potential energy terms that are used for the atomistic simulation of polymers. The sum of potential terms constitutes a molecular force field that can be used in molecular mechanics, molecular dynamics, and Monte Carlo simulations of polymeric systems. Molecular simulation methods can be used to determine such properties as PVT data, selfdiffusion coefficients, modulus, phase equilibrium, x-ray and neutron diffraction spectra, small molecule solubility, and glass transition temperatures with considerable accuracy and reliability using current force fields. Included in the coverage of Chapter 4 is a review of the fundamentals of molecular mechanics and a survey of the most widely used force fields for the simulation of polymer systems. In addition, references to the use of specific force fields in the study of important polymer groups are given. 

Our understanding of supercritical water and its properties has improved dramatically over the past decade. This is due, in no small part, to a constructive interplay between molecular-based simulations and neutron scattering experiments on supercritical water that has resulted in a thorough re-examination of both techniques. Better models for simulating water and better techniques for processing raw scattering data have resulted. Important implications have also resulted for studies of ambient water. Molecular simulation has played an indispensable and unprecedented role in these developments. 

Chialvo, A. A. Cummings, P. T. Simonson, J. M. Mesmer, R. E. Cochran, H. D. (1998) The Interplay Between Molecular Simulation and Neutron Scattering in Developing New Insights into the Structure of Water, Industrial and Engineering Chemistry Research 37, 3021-3025... 

Chialvo A A, Yezdimer E, Driesner T, Cummings PT, Simonson JM (2000) The structure of water from 25°C to 457°C Comparison between neutron scattering and molecular simulation. Chem Phys... 

One of the recent successes in computCT simulation of clay mina-al hydrates is that the structure of interlayer wate in toms of quantitatively obtainable content called total radial distributionfunction was achieved by both Monte Carlo simulations and neutron diffraction methods (111,112). The accuracy of the method and the predicting power of the molecular simulation of this complex solid-liquid interface was first confirmed for the intolayCT structure of 2 1 clay mineral smectite. Computational studies on the clay mina-al-water interface using quantum mechanics, molecular mechanics, Monte Carlo, and molecular dynamics simulations are summarized in Table 5. 

The interplay between molecular simulations and neutron-scattering experiments on supercritical or ambient water was realized by Chialvo et al. (155). They claimed that the excellent agreement between two techniques is an indication of the increasing reliability of the intermolecular potential models and the accuracy of the simulation results giving us greater confidence in our abilities to measure and predict the microstructural properties of water at all condition. 

The solvation of cation and electrical double layer structure near clay surface was studied by neutron diffraction methods (156-158). The intaplay between molecular simulations and neutron diffraction techniques also has been also applied to this clay mineral-water-cation interface system. Park and Sposito (112) simulated the total radial distribution function (TRDF) of interlayer water from Na-, Li-, and K-montmorillonite hydrates as a physical quantity from molecular simulations. They obtained TRDF values from Monte Carlo simulations and directly compared with previously obtained H/ D isotopic difference neutron diffraction results (9,10). 

The starting point of molecular simulation methods is - as in the density functional theory - the well-defined microscopic description of the system studied. This macroscopic (molecular) specification includes (1) the equations of statistical thermodynamics describing the fluid/fluid and solid/fluid interactions, and (2) the molecular model of solid adsorbent. This model should take into account all possible and reliable information on the solids, most of which can be developed from various modern surface science techniques [417]. For instance, some important data on the bulk crystalline structures are given by the X-ray diffraction or neutron diffraction, but the scanning tunelling microscopy is a valuable source of information on the topography of a surface solid. For solving... 

By the time COlL-2 took place in 2007, the nanostructured nature of the ionic liquids had been postulated using molecular simulation [50] and evidenced by indirect experimental data [54, 85] or by direct X-ray or neutron diffraction studies [56]. This microscopic vision of these fluids changed the way their physico-chemical properties could be explained. The concept of ionicity was supported by this microscopic vision, and indirect experimental evidence came from viscosity and conductivity measurements, as presented by Watanabe et al. [54, 86]. This molecular approach pointed towards alternative ways to probe the structure of ionic liquids, not by considering only the structure of the conponent ions but also by using external probes (e.g. neutral molecular species). Solubility experiments with selected solute molecules proved to be the most obvious experimental route different molecular solutes, according to their polarity or tendency to form associative interactions, would not only interact selectively with certain parts of the individual ions but might also be solvated in distinct local environments in the ionic liquid. 

A. Paliwal, D. Asthagiri, D. P. Bossev, E. Paulaitis, Pressure denaturation of staphylococcal nuclease studied by neutron small-angle scattering and molecular simulation, Biophys. J. 87 (2004) 3479-3492. 

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