Water-surface effects modeling

The Langevin model has been employed extensively in the literature for various numerical and physical reasons. For example, the Langevin framework has been used to eliminate explicit representation of water molecules [22], treat droplet surface effects [23, 24], represent hydration shell models in large systems [25, 26, 27], or enhance sampling [28, 29, 30]. See Pastor s comprehensive review [22]. 

Legislation enacted by both Canada and the United States (see the US-Canada Air Quality Accord, 1991) will, when implemented, reduce the North American emissions of sulphur dioxide by about 50% based upon the 1980 baseline. These projected emission fields have been appplied in the atmospheric source-receptor models that were described above, to provide a projected deposition field for acidic sulphate that would be expected (14). The predicted sulphate deposition fields have then subsequently been appUed in aquatic effects models that provide estimates of regional surface water acidification distributions (50). The regional acidification profiles have then been used in a model of fish species richness (51) that results in an estimate of the expected presence of fish species as compared to that expected in an unacidified case. 

K( results predict that flushing only a few pore volumes of clean water through the aquifer can displace the contamination, suggesting pump-and-treat remediation will be quick and effective. Models constructed with the surface complexation model, in contrast, depict pump-and-treat as a considerably slower and less effective remedy. 

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

When a region includes a large body of water, the roughness cannot be characterized by simply associating with the wave height. Unlike the land, the effective roughness of the water surface is a dynamic variable whose magnitude is influenced by factors such as the wave state and wind stress. There is a variety of models of the air-sea interaction and... 

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

Each GCC model differs in the set of assumptions made and therefore concentrates on different effects. For instance, a simple numerical model of the gas exchange at the ocean-atmosphere boundary in the case of wind-driven roughness of the sea at wind speeds of 7 m s 1 makes it possible to formulate, in the global model, a unit to calculate the persistent C02 flux between the water surface and the atmosphere. This can be exemplified by models of the ocean carbonate system described by many authors. Also, there are other models of the C02 cycle in natural systems (Riedo et al., 2000 Zonneveld, 1998). 

Carbon dioxide is absorbed in alkaline water from a mixture consisting of 30% CO2 and 70% N2, and the mass transfer rate is 0.1 kmol/s. The concentration of CO2 in the gas in contact with the water is effectively zero. The gas is then mixed with an equal molar quantity of a second gas stream of molar composition 20% C02, 50%, N2 and 30% H2. What will be the new mass transfer rate, if the surface area, temperature and pressure remain unchanged It may be assumed that a steady-state film model is applicable and that the film thickness is unchanged. 

It is known that first principles molecular dynamics may overcome the limitations related to the use of an intermolecular interaction model. However, it is not clear that the results for the structure of hydrogen bonding liquids predicted by first principles molecular dynamics simulations are necessarily in better agreement with experiment than those relying on classical simulations, and recent first principles molecular dynamics simulations of liquid water indicated that the results are dependent on the choice of different approximations for the exchange-correlation functional [50], Cluster calculations are an interesting alternative, although surface effects can be important and extrapolation to bulk phase remains a controversial issue. 

Other examples of such mixed potential models include that developed by Macdonald and Urquidi-Macdonald to predict water radiolysis effects in thin condensed water layers on metal surfaces (24), and the models of Marsh and Taylor (25), and Kolar and King (22) to predict the corrosion of carbon steel and copper waste containers surrounded by a low permeability material such as clay. 

The second approach was to employ periodic boundary conditions and molecular mechanics (COMPASS) to model the solvated SFA.55 73 These simulations were performed with Cerius2 4.2 (Accelrys, Inc.). Periodic boundary conditions create a bulk system with no surface effects and hence, this situation is more realistic compared to the experimental system of SFA dissolved in water. H20 molecules, however, must diffuse to allow motion of the SFA model, so that the SFA model conformations may be restricted due to this limited motion of the surrounding H20 molecules. Note also that periodic simulations must be charge neutral within the... 

Shablakh et al. (1984) investigated the dielectric properties of bovine serum albumin and lysozyme at different hydration levels, at low frequency. Besides a relaxation attributed to the electrode—sample interface, they detected a further bulk relaxation that can be confused with a d.c. conduction effect. The latter relaxation was explained by a model of nonconductive long-range charge displacement within a partially connected water structure adsorbed on the protein surface. This model has nonconventional features that differ from the assumptions of other more widely accepted models based on Debye relaxations. 

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