Water interfacial

The potentials of zero charge considered in this chapter are those in the absence of specific adsorption of ionic as well as nonionic species. There has been no attempt to review the enormous amount of data on the effect of specific adsorption on Ea+j, except for the few cases where extrapolation back to zero specific adsorption has been used as a more accurate way to determine <7-o- However, specific adsorption is difficult to relate quantitatively to the structure of interfacial water as well as to the effect of the metal. 

The apparent molar volume of interfacial water in AOT-reversed micelles is lower and its refractive index is greater than that of pure water. These findings, together with other experimental evidence, emphasize that these water molecnles are destructured, immobilized, and polarized by the ionic head of AOT [2,84,89]. In particular, it has been reported that the... 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

Previously, we have proposed that SFG intensity due to interfacial water at quartz/ water interfaces reflects the number of oriented water molecules within the electric double layer and, in turn, the double layer thickness based on the p H dependence of the SFG intensity [10] and a linear relation between the SFG intensity and (ionic strength) [12]. In the case of the Pt/electrolyte solution interface the drop in the potential profile in the vicinity ofelectrode become precipitous as the electrode becomes more highly charged. Thus, the ordered water layer in the vicinity of the electrode surface becomes thiimer as the electrode is more highly charged. Since the number of ordered water molecules becomes smaller, the SFG intensity should become weaker at potentials away from the pzc. This is contrary to the experimental result. 

Interfacial Water Structure at Polyvinyl Alcohol (PVA) Gel/Quartz Interfaces Investigated by SFC Spectroscopy... 

Hydropolymer gel has been considered as a possible candidate for an artificial articular cartilage in artificial joints because it exhibits very low friction when it is in contact with a solid. The origin of such low friction is considered to be associated with the water absorbed in the gel [83-86], some of which is squeezed out from the gel under the load and serves as a lubricant layer between the gel and solid surface, resulting in hydrodynamic lubrication [87, 88]. Although the structural information about the interfacial water is important to understand the role of water for the low frictional properties of hydrogel in contact with a solid and the molecular structure of lubricants other than water at solid/solid interfaces have been investigated theoretically [89-91] and experimentally [92-98], no experimental investigations on water structure at gel/solid interfaces have been carried out due to the lack of an effective experimental technique. 

Here, the structures of interfacial water at a fused quartz surface with and without contact of polyvinyl alcohol (PVA) were investigated by in situ SFG spectroscopy and their role in low friction between PVA and a fused quartz surface is discussed. 

Interfacial Water Structure at Po/yw ny/ Alcohol (PVA) Cel/Quartz Interfaces 91... 

This trend is shown more clearly in Figure 5.15 where the intensity ratio between the two peaks, which can be considered as an indicator of the order of the interfacial water, is plotted against the pressure applied to the PVA gel. 

Uosaki, K., Yano, T. and Nihonyanagi, S. (2004) Interfacial water structure at as-prepared and UV-rnduced hydrophilic Ti02 surfaces studied hy sum frequency generation spectroscopy and quartz crystal microhalance. J. Phys. Chem. B, 108, 19086-19088. 

Noguchi, H., Minowa, H., Tominaga, T., Gong, J. P., Osada, Y. and Uosaki, K. (2008) Interfacial water structure at polymer gel/quartz interfaces investigated by sum frequency generation spectroscopy. Phys. Chem. Chem. Phys., 10, 4987-4993. 

Effect of Underpotentially Deposited Lead on the Surface-Enhanced Raman Scattering of Interfacial Water at Silver Electrode Surfaces... 

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... 

Results of selected MD studies of interfacial water are reported in Table V. 

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... 

According to the Kirkwood theory of polar dielectrics, simple relations (23) between molecular dipole moment vectors and the mean-square total dipole moment of water clusters can be used to compute the static dielectric constant of water. As the normalized mean-square total dipole moment increases towards unity, theory predicts decreases in the static dielectric constant. Since MD results indicate that the mean-square total dipole moment of interfacial water is greater than that for bulk water (48), the static dielectric... 

Another transport property of interfacial water which can be studied by MO techniques is the dipole relaxation time. This property is computed from the dipole moment correlation function, which measures the rate at which dipole moment autocorrelation is lost due to rotational motions in time (63). Larger values for the dipole relaxation time indicate slower rotational motions of the dipole... 

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. 

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