Simulations of interfacial water

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... 

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. 

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. 

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. 

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... 

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. 

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. 

A related, relatively unexplored topic is the importance of many-body forces in the simulations of interfacial systems. The development of water-polarizable models has reached some level of maturity, but one needs to explore how these models must be modified to take into account the interactions with the metal surface atoms and the polarizable nature of the metal itself... 

The proximity of this liquid-liquid transition to the protein-glass transition temperature is suggestive. Clearly, at temperatures below 220 K or so, the dynamics of water and protein are highly coupled. A recent computer simulation study has shown that the stmctural relaxation of protein requires relaxation of the water HB network and translational displacement of interfacial water molecules. It is, therefore, clear that the dynamics of water at the interface can play an important role. This is an interesting problem that deserves further investigation. 

Korb el al. proposed a model for dynamics of water molecules at protein interfaces, characterized by the occurrence of variable-strength water binding sites. They used extreme-value statistics of rare events, which led to a Pareto distribution of the reorientational correlation times and a power law in the Larmor frequency for spin-lattice relaxation in D2O at low magnetic fields. The method was applied to the analysis of multiple-field relaxation measurements on D2O in cross-linked protein systems (see section 3.4). The reorientational dynamics of interfacial water molecules next to surfaces of varying hydrophobicity was investigated by Stirnemann and co-workers. Making use of MD simulations and analytical models, they were able to explain non-monotonous variation of water reorientational dynamics with surface hydrophobicity. In a similar study, Laage and Thompson modelled reorientation dynamics of water confined in hydrophilic and hydrophobic nanopores. 

H. Watarai and Y. Onoe, Molecular dynamics simulation of interfacial adsorption of 2-hydroxy oxime at heptane/water interface, Solv. Extr. Ion Exch., 19(1), 155-166 (2001). 

The status of computer simulations of electric double layers is briefly summarized and a road map for solving the important problems in the atomic scale simulation of interfacial electrochemical processes is proposed. As examples efforts to simulate screening in electric double layers are described. Molecular dynamics simulations on systems about 4 nm thick, containing up to 1600 water molecules and NaQ at IM to 3M concentration, displayed the main features of double layers at charged metal surfaces including bulk electrolyte zone, diffuse ionic layer that screens the charge on the electrode and a layer of oriented water next to the surface. 

FIGURE 4.5 Schematic representations (from simulation snapshots) describing different orientations of interfacial water molecules at different alkali chloride surfaces. (From Du, H. and Miller, J. D., J. Phys. Chem. C, 111 10013, 2007c. With permission.)... 

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