In this article we describe a novel molecular dynamics simulation of a single mechanically stable water-containing reverse micelle in an apolar solvent. It takes explicit account of the forces between component molecules and achieves a degree of realism hitherto unobtained but without attempting to reproduce the detailed properties of any specific surfactant system. Steric stabilization was achieved by using a model cationic surfactant with single Lennard-Jones (LJ) interaction centers to represent the hydrophobic tails, each being attached to its cation head group by a harmonic bond. Specific chain structures are not necessary. Neutrality is maintained by mobile anions. The simple point charge (SPC) model is used for the water molecules, and single-site LJ interaction centers are used to represent the apolar solvent molecules. The numbers of surfactant, water, and solvent molecules are 36, 72, and 1079, respectively, and these are contained in a spherical cavity of a solvent continuum. Periodic boundary conditions are not used. The radial density profiles of the equilibrated assembly show that the surfactant forms a "coat" but that there is significant roughening of the interface. Its mean shape and fluctuations are analyzed by using a spherical harmonic expansion. The anions are strongly adsorbed at the cation head group surface, and there is significant penetration of water into the hydrophobic region. The outermost water molecules are not H-bonded but are strongly coordinated to anions in the ionic layer. We believe that this latter effect may have some relevance to the role of cosurfactants in stabilizing micelle formation.

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Subsurface contaminants such as coal tar, creosote, diesel fuel, and other petroleum-derived materials typically exist as very complex chemical mixtures. Risk assessment is useful for site management if a single metric can represent the composition-dependent risk profile of the mixture. This paper examines the factors governing human health risk assessment for multicomponent nonaqueous phase liquids (NAPLs) containing polycyclic aromatic hydrocarbons (PAHs). A model is presented describing the interdependence of the dissolution rates of individual compounds and the shifts in the NAPL composition that occur due to the large differences in aqueous solubilities. The model also accounts for solidification of the less soluble NAPL constituents. Thirty-year numerical simulations describe composition dynamics for natural environmental processes as well as three remediation processes: pump-andtreat, bioremediation, and solvent extraction. Carcinogenic risk due to ingestion of contaminated groundwater at the source is estimated, and its dependence on contaminant removal and NAPL composition shifts is described. When composition dynamics are slow, a compound like naphthalene has great potential to contribute to risk because it may persist in groundwater. When there is significant depletion of the lower molecular weight compounds, the risk is dominated by contributions from compounds such as benzo[a]pyrene. Remediation technologies have the greatest potential for risk reduction if they are effective in removing the more carcinogenic, high molecular weight compounds. Because PAHs can contribute to risk for different reasons and because of the interdependence of their behaviors, compositional approaches lead to better risk predictions for PAHs than simple lumped metrics such as total petroleum hydrocarbon (TPH).

The electrostatic behavior of the charge-regulated surfaces of Gram-negative Escherichia coli and Gram-positive Bacillus brevis was studied using numerical modeling in conjunction with potentiometric titration and electrophoretic mobility data as a function of solution pH and electrolyte composition. Assuming a polyelectrolytic polymeric bacterial cell surface, these experimental and numerical analyses were used to determine the effective site numbers of cell surface acid-base functional groups and Ca(2+) sorption coefficients. Using effective site concentrations determined from 1:1 electrolyte (NaCl) experimental data, the charge-regulation model was able to replicate the effects of 2:1 electrolyte (CaCl(2)), both alone and as a mixture with NaCl, on the measured zeta potential using a single Ca(2+) surface binding constant for each of the bacterial species. This knowledge is vital for understanding how cells respond to changes in solution pH and electrolyte composition as well as how they interact with other surfaces. The latter is especially important due to the widespread use of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in the interpretation of bacterial adhesion. As surface charge and surface potential both vary on a charge-regulated surface, accurate modeling of bacterial interactions with surfaces ultimately requires use of an electrostatic model that accounts for the charge-regulated nature of the cell surface.

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