Structure of Water at the Interface

An understanding of the structure of water at the electrochemical interface is of fundamental importance in electrochemistry, as water is such a commonly used solvent. Previously, SXS had been used to study the effects of the applied potential on the orientation and spatial distribution of water molecules at Ag electrode surfaces [11, 17]. Models of the electrochemical interface predict that water molecules form an ice-like structure at the 

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Various dynamic processes have been investigated using computer simulations of phospholipids. These include the dynamics of the alkyl chain movement of the phospholipid, the structure of water at the interface, diffusion of small molecules, interactions of phospholipids with water, dmgs, peptides, and proteins, and the effect of unsaturation or the presence of cholesterol on the phospholipid conformation. 

MD simulations of the structure of water at the interface between pure water and its vapor have shown [27]-[31] that the molecular dipoles of water tend to lie parallel to the surface, though with a net dipole moment of the interfacial layer which points into the liquid and persists several molecular layers into the bulk. Similar preferred orientations have been observed experimentally [32]. Besides, MD calculations have been used [33] to interpret [34] (electrostatic) surface potential measurements. 

M. L. Berkowitz, I.-C. Yeh, E. Spohr. Structure of water at the water/metal interface. Molecular dynamics computer simulations. In A. Wieckowski, ed. Interfacial Electrochemistry. New York Marcel Dekker, 1999, (in press). 

The structure of water at the PVA/quartz interface was investigated by SFG spectroscopy. Two broad peaks were observed in the OH-stretching region at 3200 and 3400 cm , due to ice-like and liquid-like water, respectively, in both cases. The relative intensity of the SFG signal due to liquid-like water increased when the PVA gel was pressed against the quartz surface. No such increase of the liquid-like water was observed when the PVA gel was contacted to the hydro-phobic OTS-modified quartz surface where friction was high. These results suggest the important role of water structure for low friction at the polymer gel/solid interfaces. 

Osawa, M Tsushima, M Mogami, H., Samjeske, G. and Yamakata, A. (2008) Structure of water at the electrified platinum-water interface a study by surface-enhanced infrared absorption spectroscopy. J. Phys. Chem. C, 112, 4248- 256. 

The microscopic structure of water at the solution/metal interface has been the focus of a large body of literature, and excellent reviews have been published summarizing the extensive knowledge gained from experiments, statistical mechanical theories of varied sophistication, and Monte Carlo and molecular dynamics computer simulations. To keep this chapter to a reasonable size, we limit ourselves to a brief summary of the main results and to a sample of the type of information that can be gained from computer simulations. 

These differences are, e.g. a lowered melting point and a lowered dielectric constant [97]. If lipases are forced into this structured layer of water at the interface between water and oil by reducing the size of the reverse micelles (wi < 5), a lipase catalysed hydrolysis can be accelerated. On the other hand, the activity of oxidoreductases and their stabilities can decrease dramatically inside such small reverse micelles. 

Zhang et al. (1996) suggest that this preactivation changes the crystal structure of SAT at the interface to more closely match the NAT lattice. This nucleation and growth of NAT on solid SAT was proposed as one potential mechanism for Type I PSC formation (e.g., see Tolbert, 1994), although this process now appears to be less important because of the low probability for binary nucleation of nitric acid and water on SAT (MacKenzie et al., 1995). 

When amphipathic molecules are dispersed in water, their hydrophobic parts (i.e., hydrocarbon chains) aggregate and become segregated from the solvent. This is a manifestation of the hydrophobic effect which comes about because of exclusion and hence ordering of water at the interface between these distinct types of molecule. Aggregates of amphipathic molecules can be located at a water-air boimdary (monolayers) (Fig. 3-24) however, only a small quantity of an amphipathic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film). The bulk of the lipid must then be dispersed in water as micelles (Fig. 3-24). In both of these structures the polar parts, or heads (O), of the lipid make contact with the water, while the nonpolar parts, or tails (=), are as far from the water as possible. Micelles can be spherical as shown in Fig. 3-24, but can also form ellipsoidal, discoidal, and cylindrical stmctures. 

Fig. 7.4 Proposed models for the structure of water at the Au/solution interface at various potential regions relative to the potential of zero charge, Ep c (A-D). In D, adsorbed sulfate ions are included in the drawing. Adapted with permission from Ref. [90].

Much of the theoretical effort to understand molecular structure at the metal-water interface is contained in the work of Halley [21,22,28,29,55]. A tight-binding molecular dynamics method was developed in which the electrons in the metal are treated via first principles calculations. The water phase was treated classically and coupled to the metal such that the electronic structure is matched at the metal water interface and the double-layer was appropriately accounted for. The structure and orientation of water at the interface and the nature of the potential drop were investigated [21,22,55]. The results agree with experimental observations of the extent of water orientation at the electrode lectrolyte interface [20]. 

FIGURE 2.30 Chemical shift of protons in water bound to different oxides as a function of temperature compared to bulk water. (Adapted with permission from Gun ko, V.M., and Turov, V.V., Structure of hydrogen bonds and H NMR spectra of water at the interface of oxides, Langmuir, 15, 6405-6415, 1999, Copyright 1999 American Chemical Society.)... 

It has been recognized that the structure of water near the interface determines the adsorption behavior of ions on the metal surface in specific ways [24, 30, 31]. Therefore, realistic models of the metal phase are needed in order to describe the inhomogeneity and orientational anisotropy in the aqueous phase adequately. Contrary to the situation for bulk liquid where reliable interaction potentials, from empirical parametrizations or from ab initio calculations, are available, the quantum chemical description of interactions between molecular adsorbates and metal substrates poses substantial problems due to the complexity of the system. Systematic studies contribute to the understanding of the key factors that determine the structure and dynamics at the electrochemical interface. In the present work the influence of water adsorption energy (for many transition metal surfaces a known experimental quantity [32]), surface corru-... 

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