Water-cage

Figure 7.6 Principle components analysis (PCA) of PCB congener concentrations in technical Aroclor mixtures, contaminated water, caged brown trout, SPMDs, and hexane filled dialysis bags. The plot shows that 77% of the variance of samples within the 95% confidence ellipse is explained by PCI and PC2 and that caged fish and SPMDs are clustered together (PCA plot courtesy of Kathy Echols, USGS-CERC, Columbia, MO, USA).

Short-time water caging and transient electron-OH couplings in liquid phase... 

Here, we focus our attention on the interplay that exists between solvation processes and ultrafast redox reaction in the vicinity of the strong oxidant hydroxyl radical (OH). This diatomic radical represents one of the most efficient oxidant of cellular components (proteins, lipids, DNA), contributes to Haber-Weiss reaction and plays some important role in fundamental radiation or stratospheric chemistry. Presently, we have investigated short-time water caging effect on transient electron delocalization-relocalization in the vicinity of nascent aqueous OH radicals. This specific electronic channel is represented by Eq.(l). 

Tmskett and Dill (2002) proposed a two-dimensional water-like model to interpret the thermodynamics of supercooled water. This model is consistent with model (1) for liquid water. Cage-like and dense fluid configurations correspond to transient structured and unstructured regions, observed in molecular simulations of water (Errington and Debenedetti, 2001). Truskett and Dill s model provides a microscopic theory for the global phase behavior of water, which predicts the liquid-phase anomalies and expansion upon freezing. 

It should be noted that the use of the KVSj charts implies that both the gas phase and the hydrate phase can be represented as ideal solutions. This means that the Kvsi of a given component is independent of the other components present, with no interaction between molecules. While the ideal solution model is approximately acceptable for hydrocarbons in the hydrate phase (perhaps because of a shielding effect by the host water cages), the ideal solution assumption is not accurate for a dense gas phase. Mann et al. (1989) indicated that gas gravity may be a viable way of including gas nonidealities as a composition variable. 

Positive-ion-to-negative-ion attractions are straightforward and strong interactions so strong, in fact, that they tend to dominate when they are present. As we pointed out in our discussion of precipitation, when opposite ions find each other, they can form an ionic bond and come out of solution. Ionic attractions do not, however, always prevail. Water molecules can keep ions apart by forming cages around the ions. These water cages are the result of dipole-to-dipole and dipole-to-ion attractions. 

Clathrate hydrates form when small (<0.9 nm) non-polar molecules contact water at ambient temperatures (typically <3(X) K) and moderate pressures (typically >0.6 MPa). On a molecular scale, single small guest molecules are encaged (enclathrated) by hydrogen-bonded water cavities in these non-stoichiometric hydrates. Guest repulsions prop open different sizes of water cages, which combine to form three well-defined unit crystals shown in Figure 1. 

There are no interactions of the solute molecules, i.e., the energy of each encaged guest molecule is shielded hy the surrounding water cage, and is therefore independent of the number and types of other solute molecules. 

Gorbitz, C.H. /3 Turns, water cage formation and hydrogen bonding in the structures of L-valyl-L-phenylalanine, Acta Crystallogr. B58 (2002) 512-518. 

Short-time water caging and transient electron-OH couplings in liquid phase B. Brozek-Pluska, A. Hallou, D. Glijer, B. Charles, Y. Gauduel 233... 

Clathrate hydrates are crystalline inclusion compounds composed of host water cages that trap guest molecule-. [-.The three most common types of clathrate hydrates are known as structure I, II, and H, which differ in the type of water cage they contain. The type of clathrate hydrate structure formed depends mainly on the size of the guest molecules present (for further details, see the article Clathrate Hydrates and Refs. [8,9]). The structures of these compounds were first determined from x-ray diffraction studieshowever, as mentioned previously, vibrational spectroscopy can provide important complementary information on the structures and dynamics of these compounds and can also detect the presence of any guest molecule-host lattice interactions. 

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