Bulk water computer simulations

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. 

The structure of the adsorbed ion coordination shell is determined by the competition between the water-ion and the metal-ion interactions, and by the constraints imposed on the water by the metal surface. This structure can be characterized by water-ion radial distribution functions and water-ion orientational probability distribution functions. Much is known about this structure from X-ray and neutron scattering measurements performed in bulk solutions, and these are generally in agreement with computer simulations. The goal of molecular dynamics simulations of ions at the metal/water interface has been to examine to what degree the structure of the ion solvation shell is modified at the interface. 

Recent studies showed that amphiphilic properties have to be taken into account for most water-soluble monomer units when their behavior in water solutions is considered. The amphiphilic properties of monomer units lead to an anisotropic shape of the polymer structures formed under appropriate conditions, which is confirmed both by computer simulation and experimental investigations. The concept of amphiphilicity applied to the monomer units leads to a new classification based on the interfacial and partitioning properties of the monomers. The classification in question opens a broad prospective for predicting properties of polymer systems with developed interfaces (i.e., micelles, polymer globules, fine dispersions of polymer aggregates). The relation between the standard free energy of adsorption and partition makes it possible to estimate semiquantitatively the distribution between the bulk and the interface of monomers and monomer units in complex polymer systems. 

Simulations of three representative Cs-smectites revealed interlayer Cs+ to be strongly bound as inner sphere surface complexes, in agreement with published bulk diffusion coefficients [78]. Spectroscopic and surface chemistry methods have provided data suggesting that in stable 12.4 A Cs-smectite hydrates the interlayer water content is less than one-half monolayer. However, Smith [81] showed using molecular simulations of dry and hydrated Cs-montmorillonite that a 12.4 A simulation layer spacing was predicted at about one full water monolayer. The results of MD computer simulations of Na-, Cs-and Sr-substituted montmorillonites also provide evidence for a constant water content swelling transition between one-layer and two-layer spacings [82]. 

One sees that the corresponding peaks for hydration water in protein are also shifted upward slightly compared to the bulk water at the same temperature and as it is observed for water confined in Vycor (Figure 6). The up-shift of the librational peak increases either as the temperature is lowered or as the level of hydration is decreased, which reflects the amplified effect of confinement [49]. This indicates that both the translational and librational motions of water molecules near or at the protein surface are slightly more hindered, in agreement with observation from computer simulations. 

Figure 10. Electronic absorption line shape of N,N -diethyl-p-nitroaniline in several bulk and interfacial systems, calculated by molecular dynamics computer simulation at 300K. (a) The spectrum in bulk water (solid line) and at the water liquid/vapor interface (dashed line), (b) The spectrum in bulk 1,2-dichloroethane (solid line) and at the water/1,2-dichloroethane interface.

Computer simulations were also used to show that the crystallization nucleus is more likely to form in the subsurface than in the bulk phase of the water slab. This result can have far reaching atmospheric implications. It has been suggested that formation of an ice nucleus at the interface would be hampered by contamination of the surface by organic surfactants. The effect of the adsorbed material will surely propagate towards the subsurface as well, however it will be smaller than in the topmost layer. Therefore, the anthropogenic emissions should have an effect on the radiative balance of the Earth atmosphere. This effect should, however, be smaller than predicted using the assumption of surface nucleation. 

With more detailed information from computer simulations, on the hydrophobic hydration shells the ideas about hydrophobic hydration gradually changed. It became apparent that the hydrogen bonds in the hydrophobic hydration shell are not , or only to a minor extent, stronger than in normal water. These results are confirmed experimentally through neutron scattering - 2- 4 nd X-ray studies (EXAFS) These studies revealed that the water molecules in the hydrophobic hydration shell remain essentially fully hydrogen-bonded. For each water molecule in contact with the non-polar solute one O—H bond is oriented parallel to the non-polar surface the other bonds point into bulk water. 

We have investigated the microscopic nature of water molecules in the hydration shell. The experimental, theoretical, and computer simulation studies clearly demonstrate that the dynamics of hydration water molecules are significantly different from those of bulk water. 

The strength of the water-metal interaction together with the surface corrugation gives rise to much more drastic changes in water structure than the ones observed in computer simulations of water near smooth nonmetallic surfaces. Structure in the liquid state is usually characterized by pair correlation functions (PCFs). Because of the homogeneity and isotropy of the bulk liquid phase, they become simple radial distribution functions (RDFs), which do only depend on the distance between two atoms. Near an interface, the PCF depends not only on the interatomic distance but also on the position of, say the first, atom relative to the interface and the direction of the interatomic distance vector. Hence, considerable changes in the atom-atom PCFs can be expected close to the surface. 

Hydrogen bonds are the most characteristic element of liquid water structure. Water models used in computer simulations are able to describe the properties of the hydrogen bond network in a realistic way, contrary to many of the dipolar model fluids used in analytical theories. Much has been learned about bulk water and solutions through an analysis of the hydrogen bond network (e.g.. Ref. 156, 157). 

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. 

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