Interfacial water simulations

Molecular predictions of the properties of interfacial systems are now becoming possible as a result of rapid advances in liquid state chemical physics and computer technology. The objectives of this paper are 1) to review the general approaches and models used in Monte Carlo (MC) and molecular dynamics (MD) simulations of interfacial systems, 2) to describe and discuss results from selected simulation studies of interfacial water, and 3) to discuss the major limitations of these techniques and to offer suggestions for overcoming them. 

Equations 3-4 show that the form of the interaction potentials used in simulating interfacial water is critical. Of interest for interfacial systems are both the interaction potential between water molecules and that between the surface and a water molecule. 

In simulating interfacial water, it is important to use a model for water-water interactions which yields accurate results in simulations of bulk water. Each of the models discussed here have obvious advantages and disadvantages. The CF model is generally more... 

An interaction potential between the surface and ions may also be needed in simulating counterion diffusion for the smectite and mica surface models. The form of such an interaction potential remains to be determined. This may not pose a significant problem, since recent evidence (40) suggests that over 98% of the cations near smectite surfaces lie within the shear plane. For specifically adsorbed cations such as potassium or calcium, the surface-ion interactions can also be neglected if it is assumed that cation diffusion contributes little to the water structure. In simulating the interaction potential between counterions and interfacial water, a water-ion interaction potential similar to those already developed for MD simulations (41-43) could be specified. 

Several MC and MD studies of interfacial water near hydrophobic surfaces have been reported (33-36,44-48). Both of the MC studies (35,45). as well as the four MD studies (33,34,36,47) reporting detailed observations of interfacial water are discussed here. This comparison will show that choice of the water-water potential is critical for such studies. It will also illustrate the wide range of interfacial properties which can be studied using computer simulations. Results from the early pioneering MC studies for interfacial water are summarized in Table IV. 

These results indicate that, compared to bulk water, interfacial water exhibits unique oscillations in density with distance from the surface and preferential dipolar orientations. Both simulations report density values which are unreasonable. Part of this problem arises from attempting to fix the water density based on the average cell volume and the number of water molecules an approach which... 

The results in Table V illustrate that MD studies, compared to the MC results in Table IV, facilitate the investigation of transport and time-dependent properties. Also, they show that use of the MCY potential leads to very large density oscillations and increasing water density near the surfaces. This appears to be a serious drawback to the use of the MCY potential in simulations of interfacial water. Results from the investigations using the ST2 potential show that interfacial water density is approximately 1.0 g/cc, with a tendency for decreased density and hydrogen bonding near the surfaces. As in the MC simulations, orientations of the water dipole moment are affected by the presence of a solid/liquid interface, and an... 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

Thus, effects of the surfaces can be studied in detail, separately from effects of counterions or solutes. In addition, individual layers of interfacial water can be analyzed as a function of distance from the surface and directional anisotropy in various properties can be studied. Finally, one computer experiment can often yield information on several water properties, some of which would be time-consuming or even impossible to obtain by experimentation. Examples of interfacial water properties which can be computed via the MD simulations but not via experiment include the number of hydrogen bonds per molecule, velocity autocorrelation functions, and radial distribution functions. 

On the whole, the advantages and strengths of MC and MD simulations of interfacial water outweigh their disadvantages and weaknesses. Even if quantitative prediction of interfacial water properties is not possible in some cases, a knowledge of qualitative trends as a function of distance from the surfaces or relative to results from simulations of bulk water are often extremely i11uminating. 

Fig. 2 Schematic description of the free energy (solid line) and internal energy (dashed line) profiles of the interfacial water species. The energy differences are obtained from simulations.

Simulation. In this study, VSFS and molecular dynamics calculations were employed to examine the structure and dynamics of the hydrogen bonding network of water at the hexane/water, heptane/water and octane/water interfaces in detail [66]. The complementary nature of the approaches has allowed a more detailed understanding of the interface. The calculations provide information not available in the spectroscopic studies, namely the interactions between interfacial water molecules that are isotropically oriented. The direct and iterative comparison of experiment with theory allows for the improvement of the models used to describe water-water and water-solute interactions. 

P. Ahlstrom, O. Teleman, and B. jonsson,/. Am. Chem. Soc., 110, 4198 (1988). Molecular Dynamics Simulation of Interfacial Water Structure and Dynamics in a Paravalbumin Solution. 

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