Water layer, dynamics approximated

Teppen et al. [89] have used a flexible model for clay minerals that allows full movement of the M-O-M bonds in the clay structure, where M represents Si, Al, or other cations in the octahedral sheet. This model was used in MD simulations of interactions of hydrated clay minerals with trichloroethene [90, 91]. The simulations suggest that at least three distinct mechanisms coexist for trichloroethene sorption on clay minerals [90], The most stable interactions of trichloroethene with clay surfaces are by full molecular contact, coplanar with the basal surface. The second type more reversible, less stable is adsorption through single-atom contact between one chlorine atom and the surface. In a third mechanism, trichloroethene interacts with the first water layer and does not interact with clay surface directly. Using MC and MD simulation the structure and dynamics of methane in hydrated Na-smectite were studied [92], Methane particles are solvated by approximately 12-13 water molecules, with six oxygen atoms from the clay surface completing the coordination shell. 

The response range of the local environment to the excited Trp-probe is mainly within 10 A because the dipole-dipole interaction at 10 A to that at —3.5 A of the first solvent shell drops to 4.3%. This interaction distance is also confirmed by recent calculations [151]. Thus, the hydration dynamics we obtained from each Trp-probe reflects water motion in the approximately three neighboring solvent shells. About seven layers of water molecules exist in the 50-A channel, and we observed three discrete dynamic structures. We estimated about four layers of bulk-like free water near the channel center, about two layers of quasi-bound water networks in the middle, and one layer of well-ordered rigid water at the lipid interface. Because of lipid fluctuation, water can penetrate into the lipid headgroups, and one more trapped water layer is probably buried in the headgroups. As a result, about two bound-water layers exist around the lipid interface. The obtained distribution of distinct water structures is also consistent with —15 A of hydration layers observed by X-ray diffraction studies from White and colleagues [152, 153], These discrete water stmctures in the nanochannel are schematically shown in Figure 21, and these water molecules are all in dynamical equilibrium. 

The analysis of the dynamics assumes the water layer to be infinitely thin, approximated by a two-dimensional space. The dynamics of a diffusion-controlled reaction in a two-dimensional space is given by Hardt s analytic expression (relaxation rate (7) will vary almost linearly with the two-dimensional concentration [4>0 ](2) of the reactants ... 

Figure 2a shows the density of wata between two parallel Pt surfaces (the 100 crystal face) at T = 300 K. The distance between the two surfaces is chosen so that the density of water in the middle region (which we consider as bulk water) is near 1 g/cm. One can clearly see three peaks near each surface, which represent approximately three disordered layers of water molecules. The data in Fig. 2a are based on an approximately 500-ps molecular dynamics trajectory with the Heinzinger-Spohr poten-... 

The dynamics of turbulent plumes relevant to most crustaceans are complicated by the fact that many are produced in boundary layer flows. A crustacean moving across the substratum does so in a velocity gradient characterized by no motion of fluid in contact with the substratum and a nominal or ffee-stream velocity at some distance away. The region in between is characterized by a roughly logarithmic velocity profile that comprises approximately 30% of the water depth (Schlichting 1987). 

---