Water dragging effect

W. Fan, A New Experimental Hydration Parameter the Water-Dragging Effect, Ph.D. Thesis, Universite de Lausanne, 1992. 

Fan, W. Tsai, R. S. El Tayar, N. Carrupt, P.-A. Testa, B, Solute-water interactions in the organic phase of a biphasic system. 2. Effects of organic phase and temperature on the water-dragging effect, J. Phys. Chem. 98, 329-333 (1994). 

Fan, W., El Tayar, N., Testa, B. and Kier, L.B. (1990). Water-Dragging Effect A New Experimental Hydration Parameter Related to Hydrogen-Bond-Donor Acidity. J.Phys.Chem., 94, 4764-4766. 

Once the organic solvent has been selected and the optimum volume ratio of organic to aqueous phases is determined, the water content of the solvent must be considered. There are several different reasons for this. First, using the same solvent from different suppliers leads to different results due to dissimilar water contents of the solvents (Yokozeki et al., 1982). To remedy the situation, water-saturated solvent can be used. It has also been found that when solvents are poorly hydrated, the solutes may drag water molecules into the organic phase (Tsai et al., 1993 Fan et al., 1994). A theoretical analysis of this water-dragging effect in biphasic systems has also been attempted. 

R. S. Tsai, W. Fan, N. El Tayar, B. Testa, P.-A. Carrupt, and L. B. Kier, /. Am. Chem. Soc., 115, 9632 (1993). Solute-Water Interactions in the Organic Phase of a Biphasic System. I. Structural Influence of Organic Solutes on the Water-Dragging Effect. 

As briefly mentioned in Section 4.3.S.2, Atiyeh et al. [152] performed water balance measurements and calculations to determine the effect of using DLs with MPLs (on either or both cathode and anode sides). In their fuel cell test station, water collection systems were added in order to be able to collect and measure accurately the water leaving both anode and cathode sides of the fuel cell. Based on the operating conditions (e.g., pressures, temperatures, relative humidities, etc.) and the total amount of water accumulated at the outlets of the test station, water balance calculations were performed fo defermine the net water drag coefficient. Janssen and Overvelde [171] used this method to observe how different operating conditions and fuel cell maferials affected... 

Under fuel cell operation, a finite proton current density, 0, and the associated electro-osmotic drag effect will further affect the distribution and fluxes of water in the PEM. After relaxation to steady-state operation, mechanical equilibrium prevails locally to fix the water distribution, while chemical equilibrium is rescinded by the finite flux of water across the membrane surfaces. External conditions defined by temperature, vapor pressures, total gas pressures, and proton current density are sufficient to determine the stationary distribution and the flux of water. 

The mobility estimates of Grozema et al. [52] are quite optimistic, at least so far as hole transport in solution is concerned. For what are called realistic rotational force constants and static energy disorder they predict a hole mobility in DNA of 0.1 cm /Vs. However, taking into account the drag effect of the water we obtained a mobility of 3.5x10 cm /Vs, as detailed in Sect. 2.5. This is an upper limit because we included neither scattering nor energetic disorder. 

Water will also flow. Thus, the cations and anions bring their hydration waters with them and since the hydration number (Vol. 1, Section 2.7) of a cation generally exceeds that of an anion, the net water transport will be toward the cathode. Apart from this effect as the origin of flow, there is a drag effect of the ion on its secondary solvation waters, the motion of which moves water outside the primary solvation sheath. 

The compression or decompression of bovine serum albumin monolayers spread on an aqueous substrate at a pH near the isoelectric point can effect surface tension. The surface pressure changes depend on the distance between the position of the surface pressure measuring device and the compression barrier. This effect is minimal at a pH above or below the isoelectric point and undetected for small molecules (myristic acid and eicosyl sodium sulfate) even when the substrate contains substituted alkyl amines. A theory is proposed which attributes the above observation to surface drag viscosity or the dragging of a substantial amount of substrate with the BSA monolayer. This assertion has been experimentally confirmed by measuring the amount of water dragged per monolayer using the technique of surface distillation. 

A comprehensive water transport model was developed to account for effect of liquid water on the performance of the cell. This involved solution of an additional transport equation for liquid water saturation. Effects of convection, surface tension, electro-osmotic drag, gravity and surface tension are taken into account in this model. 

Operation mode of fuel cell is strongly determined by water balance. Water production by electrochemical process and also water transport due to proton migration and diffusion were measured with use of special complex. For MEA based on MF-4SK proton exchange membrane with hydrophobic catalytic layer an effective water drag coefficient =0.28 for air and =0.53 for pure oxygen, water diffusion coefficient trough membrane T) , =l.55x10 mVs. 

One of the frequently advertised advantages of the phosphoric acid imbibed polybenzimidazole systems is their zero water drag coefficient and their possibihty to operate with dry hydrogen and oxygen. However, a vast literature has been devoted to the study of the proton conduction and the effect of relative humidity on the conductivity of the PBl-phosphoric acid system. The promoting effect and the physicochemical interactions of water vapors with the polymer electrolyte and on the fuel cell performance have been explicitly shown for the PBl/PPy(50)coPSF 50/50 polymer blend imbibed with phosphoric acid under fuel cell conditions. ... 

Darcy s law is an empirical relationship based on experimental observations of a one-dimensional stream of water through a sand medinm [33]. The Darcy equation (5.178) is linear for the flow velocity v, which is conserved when v is sufficiently small. In practice, sufficiently small means that the Reynolds number of the flow. Rep, based on the pore diameter, is of the order of nnity or smaller. As V increases, there is no abrupt transition for Rep from 1 to 10. A change in Eq. (5.179) to consider drag effects gives... 

Proton conduction in these fuel cells has an undesired side effect of osmotic water drag, also known as the electro-osmotic drag, from anode to cathode which causes the necessity for water recycling from anode back to the cathode at the system level. 

Fascine mattresses are often kept 1 or more days on the preparation site before they are dragged to the spot where they are to be sunk. This waiting position may have problems, especially when the mattress is spread in a tidal area, where during high water it is partly covered with water. Two effects have to be considered ... 

It may be of interest to the user to know that the use of Equations 12.7 and 12.9 are consistent with the general observations that measured water-side MTCs increase with increasing surface roughness see Equations 12.1 and 12.2. The use of either formula presented above for estimating drag effects (Cd) reflects roughness through the n values. Since Equation 12.8 applies and the MTC is directly proportional to m ... 

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