There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Polymer/layered silicate nanocomposites: a review from preparation to processing

New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications. With exceptionally high surface areas and chemically-tunable structures, microporous metal–organic frameworks have recently emerged as some of the most promising candidate materials. In this critical review we provide an overview of the current status of hydrogen storage within such compounds. Particular emphasis is given to the relationships between structural features and the enthalpy of hydrogen adsorption, spectroscopic methods for probing framework–H2 interactions, and strategies for improving storage capacity (188 references).

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Hydrogen storage in metal–organic frameworks

[Liu, Chang; Li, Feng; Ma, Lai-Peng; Cheng, Hui-Ming] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China.;Cheng, HM (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;cheng@imr.ac.cn

Advanced Materials for Energy Storage

A thermodynamic treatment of surfactant self-assembly in aqueous media is developed that allows an a priori quantitative prediction of the aggregation behavior of surfactants, starting from their molecular structures and the solution conditions. The treatment combines the general thermodynamic principles of self-assembly with detailed molecular models for various contributions to the free energy of aggregation. The treatment is employed to predict the aggregation behavior of surfactants that generate narrowly dispersed, small, spherical or globular aggregates; the transition from spherical to large, polydispersed, rodlike aggregates; the aggregation behavior of surfactants containing polymeric head groups such as the widely used poly(oxyethy1ene)-type nonionic surfactants; the solubilization of hydrophobic molecules in aggregates; the solubilization-induced transition from rodlike to spherical aggregates; the solubilization of binary mixtures of hydrocarbons; and the temperature dependence of micellization and solubilization. For illustrative purposes, calculated results concerning nonionic, zwitterionic, and ionic surfactants are presented and compared with experimental results generated by more than 20 different laboratories. The calculations show that, as a result of solubilization, microemulsion structures are formed with a single surfactant. However, the core of the solubilizate in these structures is rather small. In developing a molecular model for the free energy of aggregation, we consider contributions arising from (i) the transfer of the surfactant tail from water into the hydrophobic core of the aggregate assumed to behave like a liquid hydrocarbon, (ii) the conformational free energy of the tails inside the aggregates due to the constrained location of the head groups on the aggregate surface, (iii) the free energy of formation of the aggregatewater interface, and (iv) the interactions between the head groups of the surfactant at the aggregate surface. In the case of solubilization, the free energy of mixing of the solubilizates with the surfactants also makes a contribution. Expressions for each of the above free energy contributions are derived here. The transfer free energy is expressed as an explicit function of temperature. Analytical free energy expressions are derived for the dependence of the conformational free energy of the surfactant tails inside the aggregates as a function of the size and shape of aggregates. The interfacial tension characteristic of the aggregate-water interface is calculated using the Prigogine theory when more than two distinct molecular species are present as in the case of micellization involving poly(oxyethy1ene) surfactants and for the case of solubilization of one or more additives into the aggregates. The interactions among the poly(oxyethylene) head groups in nonionic surfactants are calculated on the basis of two limiting models which differ from one another in the description of the mixing and elastic deformation free energies of the poly(oxyethy1ene)-water region surrounding the aggregates. Also, the head group interactions between ionic Surfactants are calculated using an approximate analytical solution for the Poisson-Boltzmann equation available in the literature. The thermodynamic treatment developed here is strictly predictive in the sense that it is free of information extracted from the aggregation behavior of surfactants. In addition, no arbitrary or empirical factors are involved. The only parameters that appear are molecular constants that can be reasonably determined on the basis of known molecular structures and properties.

Theory of surfactant self-assembly : a predictive molecular thermodynamic approach

The rupture of a liquid film on a solid surface and of a free liquid film have been studied using hydrodynamic stability theory. The films are not thicker than several hundred Angstrom. A small perturbation applied to the free interface generates motions in the film, and the assumption is made that the Navier–Stokes equations can be used to describe them. The difference in forces acting upon an element of liquid in a thin film and in a bulk fluid is accounted for by introducing a body force in the Navier–Stokes equations. This force is calculated from the potential energy per unit volume in the liquid caused by the London–van der Waals interactions with the surrounding molecules of the liquid and with those of the solid. If the perturbation grows, it leads to the rupture of the film. The range of wavelengths of the perturbation for which instability occurs is established and the time of rupture is evaluated. The effect of insoluble and soluble surface active agents is analyzed. Available experimental data concerning condensation on a solid surface and coalescence of bubbles are explained on the basis of the obtained results.

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Spontaneous rupture of thin liquid films

Sorption by solids with bidisperse pore structures

UV-induced graft polymerization of acrylic acid (AAc) on the surface of silicon nanoparticles was used to prepare a stable aqueous luminescent silicon nanoparticle solution. By grafting a water-soluble polymer on the particle surface, the dispersions in water of the silicon nanoparticles became very stable and clear aqueous solutions could be obtained. XPS and NMR spectroscopy confirmed that PAAc was covalently grafted to the silicon nanoparticles. The grafted PAAc on silicon particles increased not only the dispersibility but also improved the photoluminescence stability of the silicon nanoparticles against degradation by water. The surface-modified nanoparticles were used as biological labels for cell imaging. The Si quantum dot labels exihibited bright fluorescence images and provided higher resistance to photobleaching than the commonly used organic dyes.

Water-Soluble Poly(acrylic acid) Grafted Luminescent Silicon Nanoparticles and Their Use as Fluorescent Biological Staining Labels

The CO2 reforming of CH4 over reduced NiO/alkaline earth metal oxide catalysts was investigated. A CO yield of 95% was obtained from a stoichiometric feed mixture of CH4 and CO2, at high GHSV (60 000 cm3 g−1 h−1), over a reduced NiOMgO with a weight ratio of 0.2. In addition, the catalyst had excellent stability, since the CO yield remained unchanged during 120h. Compared to the reduced NiOMgO catalyst, the reduced NiOCaO,NiO SrO NiO BaO catalysts had low CO yields and very low stabilities. The TPD of CO over the reduced NiOMgO catalyst indicated a lower decomposition of CO to CO2 than over the other catalysts investigated, hence that MgO inhibits the disproportionation reaction 2CO → C + CO2 over Ni. The behavior of the reduced NiOMgO catalyst is probably due to the formation of a NiOMgO solution, as a result of the similar crystalline structures of NiO and MgO.

Eli Ruckenstein

Papers

Theory of surfactant self-assembly : a predictive molecular thermodynamic approach

Spontaneous rupture of thin liquid films

Sorption by solids with bidisperse pore structures

Water-Soluble Poly(acrylic acid) Grafted Luminescent Silicon Nanoparticles and Their Use as Fluorescent Biological Staining Labels

Carbon dioxide reforming of methane over nickel alkaline earth metal oxide catalysts