Water density, computer simulation surfaces

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

Data obtained on the electrode-oxygen distances and the reorientation of water in the inner layer are consistent with the results of computer simulations. The relative density profile for oxygen at the water/platinum interface shows two distinct maxima at 2.5 and 5.4 A, respectively. The first maximum corresponds to water directly bound to the surface whereas the less pronounced second maximum can be ascribed to the second layer of water linked through hydrogen bonds to the first layer. 

The calculations of the stmcture of water between charged flat walls show that the density profile becomes asymmetric and that there is enhanced structuring. This enhanced structuring is intimately connected with the possibility of a continuous phase transition in quasi two-dimensional systems, a subject of recent intense interest. ° Most of the molecular dynamics computer simulations on the effects of an external field have been carried out in an attempt to clarify the field-induced restructuring of water molecules at the metal surface, for which recent experimental data have become available. ... 

The Monte Carlo computer simulations by Kim and Landau [88] prove that also in the second and higher adsorbed layers the critical surface coverage is equal to one half. This means that in water adsorption on oxides, the density of the slab-like multilayer phase is below one half at a statistical coverage of three adsorbed layers. 

The concept of activation volumes has also become a valuable tool in studies of exchange reactions by ab initio computer calculations and in classical computer simulations. In these theoretical studies activation volumes can be estimated by bond-length variations or by calculating volume differences using Connolly surfaces. In MD simulations pressure can be applied by variation of the density of the simulated water box. In that way reaction volumes are accessible by following for instance the change in coordination number. 

In short, the structural order observed for the hydration shell of the Li+ ion at various distances from the mercury surface results from the strong tendency of this ion to form an octahedral arrangement with its six nearest neighbor water molecules in bulk solution. This structure adjusts to the densities and inhomogeneities determined by the metal surface. This example demonstrates how mechanistic information about the process of ion adsorption can be extracted from computer simulations. 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

During the last few years, the latter view has received more supporting evidence. Already the early experimental work of Giraultand Schiffrin [71], who determined the surface excess of water at the interface with 1,2-dichloroethane, had indicated the existence of a mixed boundary layer. Recent X-ray scattering experiments [72] indicate an average interfacial width of the order of 3 to 6 A. These experiments are in line both with model calculations based on the density functional formalism [73] and with computer simulations [74, 75]. Accordingly, the interface is best visualized as rough on a molecular scale as indicated in Fig. 13. 

We start with the simplest model of the interface, which consists of a smooth charged hard wall near a ionic solution that is represented by a collection of charged hard spheres, all embedded in a continuum of dielectric constant c. This system is fairly well understood when the density and coupling parameters are low. Then we replace the continuum solvent by a molecular model of the solvent. The simplest of these is the hard sphere with a point dipole[32], which can be treated analytically in some simple cases. More elaborate models of the solvent introduce complications in the numerical discussions. A recently proposed model of ionic solutions uses a solvent model with tetrahedrally coordinated sticky sites. This model is still analytically solvable. More realistic models of the solvent, typically water, can be studied by computer simulations, which however is very difficult for charged interfaces. The full quantum mechanical treatment of the metal surface does not seem feasible at present. The jellium model is a simple alternative for the discussion of the thermodynamic and also kinetic properties of the smooth interface [33, 34, 35, 36, 37, 38, 39, 40]. 

The mean residence times, MRT, of water molecules in the immediate vicinity of ions were studied extensively by means of these quantum-mechanical combined with molecular-mechanical computer simulations as reviewed at the time by Hofer et al. [70], The computational program employed has evolved over the years as was the minimal time t, above which a molecule is deemed to have left its position in the immediate vicinity of an ion, from 2ps in the earlier studies to 0.5 ps used in the later ones. The MRT of water molecules in the bulk solvent, r =1.7 ps, is only one-tenth of the time it takes the molecule to diffuse completely away. The relative mean residence times of water molecules in the second hydration shell to that in bulk water, RMRT = /t w (in %) at 25°C, are shown in Table 5.4. The MRT of water in the first hydration shells of multivalent ions are longer than could be studied by the computations. The RMRTs of water molecules near the ions are roughly proportional to the surface density of the charge on the ions, o. RMRT=0.22+l.l4(oJC mrr ), but exceptions are noted. 

Near strongly attractive surfaces, liquid structure differs noticeably from the bulk one. This is caused by the packing effect due to the localization of molecules in a plane(s) parallel to the wall and by specific fluid-wall interactions, such as H-bonds. Density oscillations of liquids near solid substrates were observed in experiments [143, 144, 417-419] and in numerous computer simulations of confined fluids. Besides, fluids with strongly anisotropic interactions (such as water) unavoidably undergo orientational ordering near the wall. It is important to know the character of this ordering and its intrusion into the bulk liquid. In the present section, we consider structural properties of adsorbed water layers in the liquid, bilayer, and monolayer phases. 

Different theories have been proposed to explain hydrophobic attraction. Like on hydrophilic surfaces, the structure of water at hydrophobic surface is different from the bulk structure. Computer simulations [1211, 1212], sum-frequency vibrational spectroscopy [1163], X-ray [1078, 1213, 1214], and neutron reflectivity [1076, 1077] show a layer of up to 1 nm with a reduced density and an increased order. When two hydrophobic surfaces approach each other at some point, the surface layers overlap and lead to an attractive force [1212,1215,1216]. This force is, however, short ranged and can certainly not explain the long-range component. 

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