Water orientation

The ABMA Commercial BW Requirements is very strong on practical engineering and operations (similar to the ASME Consensus) but contains almost no chemistry, whereas the ASB BW Guide is a chemistry-oriented, water treatment primer. 

It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li", Na and are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg " or Ba +, cause a much greater disruption to the secondary hydration sheath. 

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. 

Fig. 7e. Distinct neutron structure functions, H (s), for amorphous solid (...) and for liquid D2O. Calculated curves are for randomly oriented water molecules with molecular center correlations derived from X-ray diffraction. (From Ref. 27>)...

Fig. 1. Orientations of molecules used for determining the stopping power of oriented water.

There has been considerable discussion on the validity of docking and scoring functions in structure-based design because of the complex issues involved such as ligand orientation, water participation, or flexibility of the target protein itself [186-190]. 

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

Not all ceramic materials behave the same at a given pH, however. As the material begins to dissolve, ions form at the snrface, water molecnles orient themselves accordingly, and an electrical double layer is established, as shown in Eigure 3.18. The first layer of charged ions and oriented water molecules is called the inner Helmholtz plane (IHP), and the second layer of oppositely charged particles is called the outer Helmholtz... 

However, it must not be imagined that the water molecules act by themselves and that they are unaffected by the presence of their neighbors. After all, dipoles interact with dipoles. Hence, the oriented water molecules also experience lateral interaction— a phenomenon that affects the net number of water molecules oriented in one direction and therefore the value of the dipole potential, gj-ipole (Section 6.7.6). Once the dipole potential is affected, the total potential difference across the interface gets affected, and consequently, the properties of the interface. 

One Effect of the Oriented Water Molecules in the Electrode Field Variation of the Interfacial Dielectric Constant... 

Except near the potential of zero charge, the first layer of water molecules near the electrode (first hydration layer) is completely oriented the molecules form a saturated dielectric. These water molecules do not affect the dielectric constant of the medium because they are not able to orient more in the presence of an electric field. The value of such an oriented water layer is approximately 6 (Fig. 6.74). 

S. Trasatti, in Trends in Interfacial Electrochemistry, A. Fernando Silva, ed., Vol. 179, NATO ASI Series 179, Reidel, Dordrecht (1986). The potential of oriented water at a metal/solution interface. 

Every ion in an aqueous solution is surrounded by a shell of oriented water molecules held by the attraction of the water dipoles to the charged ion. 

The Ion-Dipole Model. In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in the immediate vicinity of an ion (Fig. 2.11). One end of the water dipole is attached electrostatically to the oppositely charged ion. The result of this orienting force is that a certain number of water molecules in the immediate vicinity of the ion are preferentially oriented, forming a primary hydration shell of oriented water molecules. These water molecules do not move independently in the solution. Rather, the ion and its primary water sheath is a single entity that... 

Primary region with completely oriented water... 

The solvated electron thus formed is no longer centered on the parent atom. To form two distinct entities an asymmetrization occurred either by movement of the I atom from the center or of the electron from the spherical symmetry. The electron e q is now the center of solvation, at first near to, but distinct in its diffusive properties from its parent atom. It is bound by oriented water molecules. This process must have occurred in less than the lifetime of the spectroscopic excited state less than 10 1° sec. Fluorescence is not observed in solutions of I q, showing the short lifetime of the primary excited state. 

Devaux and McConnell (39) measured a lateral diffusion coefficient of about 2 X 10"8 cm2/sec at 25°C for phosphatidylcholine (PC) diffusing along bimolecular leaflets in an oriented water-PC lamellar phase. Spin-labeled PC was used in this work. 

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