Mirror symmetry breaking through an internal degree of freedom leading to directional motion

We analyze here the minimal conditions for directional motion (net flow in phase space) of a molecular motor placed on a mirror-symmetric environment and driven by a center-symmetric and time-periodic force field. The complete characterization of the deterministic limit of the dissipative dynamics of several realizations of this minimal model, reveals a complex structure in the phase diagram in parameter space, with intertwined regions of pinning (closed orbits) and directional motion. This demonstrates that the mirror-symmetry breaking which is needed for directional motion to occur, can operate through an internal degree of freedom coupled to the translational one.Comment: Accepted for publication in Phys. Rev.

Structure of the Electric Double Layer in Hydrothermal Systems. Molecular Simulation Approach and Interpretation of Experimental Results

When an aqueous electrolyte solution is put in contact with a solid surface the solid-liquid interface develops a net electric polarization resulting from the formation of an electric double layer. While this phenomenon has been known for more than a century, giving rise to a variety of theories including the first one by Helmholtz followed by Gouy-Chapman (Israelachvili 1992), and Bockris-Devanathan-Miller (Bockris, Devanathan et al. 1963). The existence of the electric double layer has a strong impact on many chemical (electrochemical metal deposition, corrosion, catalysis), biochemical (ion channels), and geological (mineral dissolution/deposition and reactivity, crystallization) processes due to charge-shielding resulting from the movement of ions toward the interface to balance the charged surface. Typically, the theoretical investigation of the microscopic structure of the medium near the (charged or uncharged) interface has been done with simplified models, involving a structure less surface in contact with point (or hard-sphere) ions immersed in a dielectric continuum (Schmickler and Henderson 1986). Obviously, these models neglect important physicochemical aspects of the system, such as the solvent structure around the species in solution (solvation effects), as well as the reactive interaction between the solvent and the solid surface. In order to interpret the actual electrochemical behavior of ionic species in solution, their interaction with the solid surface and their mobility we need to develop molecular-based tools. These tools must take explicitly into account the discrete nature of all species in solution and the essential features to describe the solid surface and its interactions with the contact solution (ultimately, it will include the discrete nature of the surface). The main goal of our research is the analysis of the microscopic behavior of high-temperature aqueous electrolyte solutions in contact with metal oxide surfaces, to gain an improve d understanding of the configurational behavior of the electric double layer. For that purpose we develop molecular dynamics protocols to characterize formation of the electric double layer through the determination of the profiles of species' concentrations, electric field, species diffusivity, and solvent polarization normal to the oxide surface

Ion Association in High-Temperature Aqueous HCl Solutions. A Molecular Simulation Study

The profiles of the potential of mean force for the Cl- - H3O+ pair, as predicted by two ab initio models, are determined by constraint molecular dynamics simulation at a near-critical condition. The corresponding association constants are then determined and compared with that from conductance measurements to test the reliability of the current simulation models for HCl

Thermodynamics and kinetics of ion speciation in supercritical aqueous solutions: A molecular based study

Molecular simulation of infinitely dilute NaCl aqueous solutions are performed to study the Na{sup +}/Cl{sup -} ion pairing in a polarizable and a nonpolarizable solvent at supercritical conditions. The Simple Point Charge, Pettitt-Rossky, and Fumi-Tosi models for the water-water, ion-water, and ion-ion interactions are used in determining the degree of dissociation, its temperature and density dependence, and the kinetics of the interconversion between ion-pair configurations in a nonpolarizable medium. To assess the effect of the solvent polarizability on the stability of the ion-pair configurations, we replace the Simple Point Charge by the Polarizable Point Charge water model and determine the anion-cation potential of mean force at T{sub r}=1.20 and {rho}{sub r}=1.5

Fundamental chemistry and thermodynamics of hydrothermal oxidation processes. 1998 annual progress report

'The objective of this research program is to provide fundamental scientific information on the physical and chemical properties of solutes in aqueous solutions at high temperatures needed to assess and enhance the applicability of hydrothermal oxidation (HTO) to the remediation of DOE hazardous and mixed wastes. Potential limitations to the use of HTO technology include formation of deposits (scale) from precipitation of inorganic solutes in the waste, corrosion arising from formation of strong acids on oxidation of some organic compounds (e.g., chlorinated hydrocarbons), and unknown effects of fluid density and phase behavior at high temperatures. Focus areas for this project include measurements of the solubility and speciation of actinides and surrogates in model HTO process streams at high temperatures, and the experimental and theoretical development of equations of state for aqueous mixtures under HTO process conditions ranging above the critical temperature of water. A predictive level of understanding of the chemical and physical properties of HTO process streams is being developed through molecular-level simulations of aqueous solutions at high temperatures. Advances in fundamental understanding of phase behavior, density, and solute speciation at high temperatures and pressures contribute directly to the ultimate applicability of this process for the treatment of DOE hazardous and mixed wastes. Research in this project has been divided into individual tasks, with each contributing to a unified understanding of HTO processing problems related to the treatment of DOE wastes. This report summarizes progress attained after slightly less than two years of this three-year project.