Structure and dynamics of water near

E. Spohr. Computer simulaton of the structure and dynamics of water near metal surfaces. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid-Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 3, pp. 31-44. 

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. 

Computer Simulation of the Structure and Dynamics of Water Near Metal Surfaces... 

Adsorption energy, effect on density, computer simulation of structure and dynamics of water near metal surfaces, 34-36... 

E. Spohr, G. Toth, and K. Heinzinger, Electrochem. Acta, 41, 2131 (1996). Structure and Dynamics of Water and Hydrated Ions Near Platinum and Mercury Surfaces as Studied by MD Simulations. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

We now arrive at a rather complete picture of the structure and dynamics of the water molecules near an ion. In Fig. 6 an I ion surrounded by water molecules is shown. The... 

Understanding the structure and dynamics of pure water on a molecular level is only the beginning. Simulations of electrolyte solutions near metallic interface are much more demanding in terms of computer time than those of bulk water, because the relatively small number of ions even in a highly concentrated electrolyte solution mandates the treatment of systems with a much larger total number of particles than in pure water for a longer time span. Furthermore, as was discussed in section 3, much less is known from quantum chemistry about nature and strength of the ion-metal interaction than about the water-metal interactions, so that the interpretation of the results obtained from the simulations is less clear. 

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. 

The study of liquids near solid surfaces using microscopic (atomistic-based) descriptions of liquid molecules is relatively new. Given a potential energy function for the interaction between liquid molecules and between the liquid molecules and the solid surface, the integral equation for the liquid density profile and the liquid molecules orientation can be solved approximately, or the molecular dynamics method can be used to calculate these and many other structural and dynamic properties. In applying these methods to water near a metal surface, care must be taken to include additional features that are unique to this system (see later discussion). 

Polarizability and Water Density Constraint on the Structure of Water Near Charged Surfaces Molecular Dynamics Simulations. 

Regarding (1), the most academic approach, at the time of writing only embryonic attempts have been made. In sec. 2.2c the structure of water near Interfaces has been discussed and in sec. 3.9 the same has been done for charged surfaces. As to the dynamics, fig. 2.5 may be reconsidered. Although this figure does not specifically apply to water, for this liquid the dynamics may be similar. It may be inferred that residence times of fluid molecules are relatively... 

Realistic three-dimensional computer models for water were proposed already more than 30 years ago (16). However, even relatively simple effective water model potentials based on point charges and Leimard-Jones interactions are still very expensive computationally. Significant progress with respect to the models ability to describe water s thermodynamic, structural, and dynamic features accurately has been achieved recently (101-103). However, early studies have shown that water models essentially capture the effects of hydrophobic hydration and interaction on a near quantitative level (81, 82, 104). Recent simulations suggest that the exact size of the solvation entropy of hydrophobic particles is related to the ability of the water models to account for water s thermodynamic anomalous behavior (105-108). Because the hydrophobic interaction is inherently a multibody interaction (105), it has been suggested to compute pair- and higher-order contributions from realistic computer simulations. However, currently it is inconclusive whether three-body effects are cooperative or anticooperative (109). 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

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