Volume 111, Issue 14, 08 October 1999
Index of content:
- SURFACES, INTERFACES, AND MATERIALS
111(1999); http://dx.doi.org/10.1063/1.480010View Description Hide Description
We present the results of molecular dynamics simulations of Langmuirmonolayers of a semifluorinated hydrocarbon molecule Our simulations show that highly ordered structures can be formed at low temperature after quenching from a random structure at high temperature. The structuralformation process of the monolayer is characterized by a decrease in the gauche defects of the hydrocarbon block and an increase in the global bond-orientational order throughout the chain molecules. The behavior of the order parameter exhibits a sudden change, signaling a structuralphase transition.Structuralformation of the semifluorinated monolayer is found to depend on temperature, suggesting that the time scale for the ordering process is determined by the rate of conformational change. It is also found that the monolayerstructure consists of clusters of different chain orientations relative to the interface for the head-group-free molecule, with a slightly larger fraction for a hydrocarbon-down, fluorocarbon-up configuration.
111(1999); http://dx.doi.org/10.1063/1.480008View Description Hide Description
Collisional dissociation induced by impact of a cluster anion, onto a siliconsurface was studied by measuring the branching fractions of the dissociation and the translational energies of the product anions as functions of n and the collision energy (per of in an apparatus based on a tandem time-of-flightmass spectrometer. It was found that the branching fraction of the dissociation did not change with n at the collision energies of 30–70 eV. Molecular dynamics simulation showed that the impinging core ion, tends to orient with the molecular axis of the core ion being at the angle of 55° with respect to the surface normal, whereas in the I) collision, the molecular axis of the core ion being in parallel to the surface plane. This finding together with prominent wedge effect in the collision leads us to conclude that the molecules do not work as a “wedge” in the collision because of the unfavorable orientation of the incoming cluster anion. No discernible cage effect could also be related to the orientation. The translational energies of the product anions were interpreted in terms of energy redistribution of the collision energy among the degrees of freedom of the cluster anion and the surface atoms involved in the collision. It was also found that the I and Cl product ions reach quasiequilibrium with the surface by measuring these translational energy distributions.
Structure of binary hard-sphere mixtures near a hard wall: A simple weighted-density-functional approach111(1999); http://dx.doi.org/10.1063/1.479948View Description Hide Description
The structure of binary hard-sphere mixtures near a hard wall is studied using a density functional theory. The formalism is based on a simple weighted density approach for the one-particle correlation functions of the nonuniform system, and requires as input only the one- and two-particle direct correlation functions of the corresponding uniform system. The approach is designed in a way, where the weight function is decoupled from the weighted density. Numerical results on the density profiles are shown to compare well with available simulation data.
111(1999); http://dx.doi.org/10.1063/1.479949View Description Hide Description
We analyze a model for CO oxidation on surfaces which incorporates both rapid diffusion of adsorbed CO, and superlattice ordering of adsorbed immobile oxygen on a square lattice of adsorption sites. The superlattice ordering derives from an “eight-site adsorption rule,” wherein diatomic oxygen adsorbs dissociatively on diagonally adjacent empty sites, provided that none of the six additional neighboring sites are occupied by oxygen. A “hybrid” formalism is applied to implement the model. Highly mobile adsorbed CO is assumed randomly distributed on sites not occupied by oxygen (which is justified if one neglects CO–CO and CO–O adspecies interactions), and is thus treated within a mean-field framework. In contrast, the distribution of immobile adsorbed oxygen is treated within a lattice–gas framework. Exact master equations are presented for the model, together with some exact relationships for the coverages and reaction rate. A precise description of steady-state bifurcation behavior is provided utilizing both conventional and “constant-coverage ensemble” Monte Carlo simulations. This behavior is compared with predictions of a suitable analytic pair approximation derived from the master equations. The model exhibits the expected bistability, i.e., coexistence of highly reactive and relatively inactive states, which disappears at a cusp bifurcation. In addition, we show that the oxygen superlattice ordering produces a symmetry-breaking transition, and associated coarsening phenomena, not present in conventional Ziff–Gulari–Barshad-type reaction models.
Investigations of surface forces between gypsum crystals in electrolytic solutions using microcantilevers111(1999); http://dx.doi.org/10.1063/1.479950View Description Hide Description
This paper introduces a new approach to the study of the interactions between gypsum faces in electrolyticsolutions of calcium sulfate. A systematic study with respect to the orientation of crystals, the concentration of the solution, the type of electrolyte, and the duration time of the contact leads to an improved understanding of both the DLVO theory and the mechanism involved in the development of the ionic correlation force. Using an atomic force microscope, direct local measurement of forces between crystals defines the ideal conditions favorable to the adhesion. The most important factor in crystal coagulation was found to be the effective surface charge of each face.