Organic monolayers at the surfaces of aqueous aerosols play an important role in determining the mass, heat transfer rate and surface reactivity of atmospheric aerosols. They can potentially contribute to the formation of cloud condensation nuclei (CCN) and are involved in a series of chemical reactions occurring in atmosphere. Recent studies even suggest that organic-coated interfaces could have played some role in prebiotic biochemistry and the origin of life. However, creating reproducible, well-characterized aqueous aerosol particles coated with organic films is an experimental challenge. This opens the opportunity for computer simulations and modeling of these complex structures. In this work, molecular dynamics simulation was used to probe the structure and the interfacial properties of the dicarboxylic acid coated aqueous aerosol. Low molecular weight dicarboxylic acids of various chain lengths and water solubility were chosen to coat a water droplet consisting of 2440 water molecules. For malonic acid coated aerosol, the surface acid molecules dissolved into the water core and formed an ordered structure due to the hydrophobic interactions. The acid and the water are separated inside the aerosol. For other nanoaerosols coated with low solubility acids, phase separation between water and acid molecules was observed on the surface of the particle. To study the water processing of the coated aerosols, the water vapor accommodation factors were calculated.

Using the classical molecular dynamics method we simulate the mechanochemical behavior of small ͑i.e., core diameterϽ 10 nm͒ oxide coated aluminum nanoparticles. Aluminum nanoparticles with core diameters of approximately 5 and 8 nm are simulated with 1 and 2 nm thick oxide coatings or shells. In addition to thickness the shells are parametrized by varying degrees of crystallinity, density, and atomic ratios in order to study their effect on the ignition of nanoparticle oxidation. The oxide shells are parametrized to consider oxide coatings with the defects that commonly occur during the formation of an oxide layer and for comparison with a defect free crystalline oxide shell. Computed results include the diffusion coefficients of aluminum cations for each shell configuration and over a range of temperatures. The observed results are discussed and compared with the ignition mechanisms reported in the literature. From this effort we have found that the oxidation ignition mechanism for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. This is in contrast to the currently assumed pressure driven diffusion process. This induced electric field accounts for approximately 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experimental results.

Molecular dynamics simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. We observed that the surface of gold nanoparticles becomes highly corrugated by the adsorption of the SAMs. Furthermore, as the temperature is increased, the SAMs dissolve into the gold nanoparticle, creating a liquid mixture at temperatures much lower than the melting temperature of the gold nanoparticle. By analyzing the mechanical and chemical properties of gold nanoparticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we determined that the system is metastable. The model and computational results that provide support for this hypothesis are presented.

Molecular dynamics simulations are used to simulate the energetic reaction of Ni and Al particles at the nanometer scale. The effect of particle size on reaction time and temperature for separate nanoparticles has been considered as a model system for a powder metallurgy system. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles embedded within a matrix. The differences in melting temperature and phase change behavior, e.g., the volumetric expansion of Al between Al and Ni, are expected to produce differing results for the coated nanoparticle systems. For instance, the volumetric expansion of Al upon melting is expected to produce large tensile stresses and possibly rupture in the Ni shell for Ni-coated Al. Simulation results show that the sintering time for separate and coated nanoparticles is nearly linearly dependent on the number of atoms or volume of the sintering nanoparticles. We have also found that nanoparticle size and surface energy are important factors in determining the adiabatic reaction temperature for both systems at nanoparticle sizes of less than 10 nm in diameter.

No abstract

Fracture of nanosize contacts formed between spherical probes and flat surfaces is studied using an atomic force microscope in an ultrahigh vacuum environment. Analysis of the observed deformation during the fracture process indicates significant material extensions for both gold and silica contacts. The separation process begins with an elastic deformation followed by plastic flow of material with atomic rearrangements close to the separation. Classical molecular dynamics studies show similarity between gold and silicon, materials that exhibit entirely different fracture behavior at macroscopic scale. This direct experimental evidence suggests that fracture at nanoscale occurs through a ductile process.

The binding energy, density, and solubility of functionalized gold nanoparticles in a vacuum are computed using molecular dynamics simulations. Numerous parameters including surface coverage fraction, functional group (-CH(3), -OH, -NH(2)), and nanoparticle orientation are considered. The analysis includes computation of minimum interparticle binding distances and energies and an analysis of mechanisms that may contribute to changes in system potential energy. A number of interesting trends and results are observed, such as increasing binding distance with higher terminal group electronegativity and a minimum particle-particle binding energy (solubility parameter) based upon surface coverage. These results provide a fundamental understanding of ligand-coated nanoparticle interactions required for the design and processing of high-density polymer composites. The computational model and results are presented as support for these conclusions.

No abstract