Local Diffusion Coefficient of Water

The local diffusion coefficient profile of water in DPPC bilayer revealed the followings  

1) The diffusion coefficient in the middle of the water layer (z 33 A) is comparable to that observed in pure water (-0.77 x W cm s at 323 K Note that modified TIP3P water [76] used here overestimates the experimental diffusion coefficient by a factor of 2). If we construct the system with a larger concentration of water, it will be expected that the diffusion coefficient in the water layer completely coincides with that of bulk water. Although a slightly smaller diffusion coefficient observed in the middle of water layer is, in this sense, a finite 

From the bulk water to the interfacial region, z ISA, the local diffusion coefficient of water decreases monotonously as water molecules approach the interfacial region. 

3) The translational mobility of the water located in the region of z = 9-15 A, where rather highly packed hydrophobic chains and ester groups are found, is significantly low. 

4) In the hydrophobic bilayer center, z 9 A, water diffusion is promoted in the vicinity of the sHp plane between the two leaflets of the bilayer. 

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Methanol diffusion coefficient in the CCL (cm s ), Equation 4.200 Oxygen diffusion coefficient in the CCL (cm s ), Equation 4.200 Long-range diffusion coefficient of water in membrane (cm s ) Local diffusion coefficient of water in membrane (cm s )... 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

The diffusion coefficient of oxygen in sucrose solution decreases much more rapidly with increasing sucrose concentrations than does the diffusion coefficient of water (King, 1988). According to King (1988) when diffusional limitations exist, the local oxygen concentrations may be different from those that would be expected if the head space or surrounding air were instantly and continuously maintained in equilibrium. 

Figure 9.7 Local diffusion coefficients of a water molecule in the normal direction of the bilayer, D. The circles and triangles are the Dz values determined using the force...

At the opposite side of the timescale (in the range of seconds to minutes), the macroscopic diffusion coefficient of water in swollen Nafion membranes, as determined by the diffusion of tritiated water through the membrane, is lower by a factor of 10 compared to the local diffusion coefficient or the self-diffusion in bulk water. This high value integrates all the restricted motions, which shows that the Nafion morphology is favorable to obtain a high ionic conductivity [160]. One important issue is the identification of the typical... 

Nitric acid is a strong electrolyte. Therefore, the solubilities of nitrogen oxides in water given in Ref. 191 and based on Henry s law are utilized and further corrected by using the method of van Krevelen and Hofhjzer (77) for electrolyte solutions. The chemical equilibrium is calculated in terms of liquid-phase activities. The local composition model of Engels (192), based on the UNIQUAC model, is used for the calculation of vapor pressures and activity coefficients of water and nitric acid. Multicomponent diffusion coefficients in the liquid phase are corrected for the nonideality, as suggested in Ref. 57. 

Based on the study of Sugano et al. (2000) and our predictive VolSurf model for this series, it can be concluded that factors like size and shape previously reported to affect paracellular permeability are indeed important to explain the local structure-permeability relationship of this chemotype. Usually, permeability via paracellular aqueous pore diffusion depends on the size of the solute and its diffusion coefficient in water. Another important factor is lipophilicity. Between intestinal absorption and both volume and lipophilicity, a negative correlation was reported for this series of thrombin inhibitors. In addition, hydrogen bonding properties and dipolarity are factors that determine... 

To follow the dynamics of the ions near the protein, molecular dynamics simulations were carried out in the presence of 6639 water molecules, and Na+ and Cl ions were added to a formal concentration of 30 and 120 mM. The simulations were carried out for a period of 10 ns, and the diffusion coefficients of the ions were calculated to be comparable with the values determined by experimental methods [85]. A great advantage of molecular dynamics calculations is the possibility to visualize the motion of each ion. On inspecting the various ions, it became evident that their spatial distribution was not random. There was a clear tendency of some anions and cations to remain in the vicinity of the protein, as if the local forces detained them next to the protein s surface. 

What is the stmcture of neat ionic liquids. Even though the stmcture of liquids, such as water, has been studied for many years, the study of room-temperature ionic liquids is still in its infancy [58]. Purified [BMIM][PFj], probably the most studied IL, has been shown to be purely monophasic, with no aggregates, but to have a local stmcture [47]. Imidazolium groups are positioned in pairs with a plane-to-plane separation of 4.5 A. The stmcture of ILs is also characterized by a degree of anion-cation association. Thus, in [EMIM][NTf2] and [BP][NTf2], the self diffusion coefficients of the anions and the cations measured by pulsed-gradient spin-echo N M R are superior to the diffusion coefficients deduced from the molar conductivity, which demonstrates the existence of ion pairs which cannot contribute to the ionic conduction [53]. 

The last assumption means that local electroosmotic flux of water in membrane is exactly counterbalanced by back diffusion. Recent studies [14,27] have shown that in a wide range of operating conditions total transfer coefficient of water from the anode to the cathode does not exceed 0.2. Since electroosmotic drag coefficient in Nafion is 1.5 [28], we conclude that the average over the cell surface electroosmotic flux in the membrane is almost fully compensated for by back diffusion. Note that the local value of total water flux in the membrane may significantly deviate from the surface-averaged value, e.g. close to the outlet of the oxygen channel [27]. Nevertheless, assumption 5 seems to be a reasonable approximation. 

Physically, r is proportional to the ratio of mass transfer coefficient of liquid water in membrane to mass transfer coefficient of water vapour in the backing layer. The parameter r thus describes the competition of two opposite water fluxes back diffusion, which wets the anode side of the membrane and leakage through the backing layer to the channel, which facilitates membrane drying. Physically, r controls the local water-limiting current density (see below). 

The model also ignores local 2D effects due to non-uniformity of oxygen and water distribution under the channel/rib [10]. This non-uniformity also reduces the effective diffusion coefficient of oxygen in ID or ID+ID models. There is evidence that local 2D effects can be approximately accounted for in 1D+ ID models by simple correction of oxygen diffusion coefficient [32]. 

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