Electrolytic effective pore volume

Table II also shows the effect of electrolyte concentration on Rf and kj. Both effects reflect the fact that at the higher ionic strengths particle/substrate repulsion is decreased, thus effectively increasing the available pore volume at a given particle size. These results are illustrated in Figure 3. Included in this figure are data from work by Nagy (14) with a column set similar in configuration to that employed here.

The influence of the pore size distribution of carbon on NOM uptake has been recognized by several researchers [57, 63]. Likewise, Karanfil and coworken [64] and Kilduffand coworkers [65] concluded that the adsorption of humic substances was largely governed by molecular size distribution in relation to pore size. Moreover, a good linear relationship (Fig. 25.6) was foundbetween the amount adsorbed by different carbons and their pore volume between 0.8 and 50 nm [61, 66] when the adsorption was carried out at pH 3. This is because electrostatic effects are minimized under these experimental conditions and nonelectrostatic interactions predominate. The adsorption mechanism would be due to hydrophobic and/or TT-TT-electron interactions, and in this case as with other electrolytes (see above). 

The values of k, D, and a used in Equations 3, 12, and 4, lespeetively, are reduced within the porous electrode relative to their bulk values due to the tortuous path which the ions in solution must make around the solid particles or which the electrons must make around the electrolyte-fiUed pores. Generally, the effect of volume fraction e, and tortuosity of the phase of interest on property P in that phase is accounted for by the Bmggeman relation [82], P = EpPoo/T where x is the tortuosity and is usually related to the porosity by t = s . Because the flux N is defined with respect to superficial area as opposed to electrolyte area, D already gets multiplied once by e in the mass balance, so D = Dj ... 

In another vein, double layers play a role In the salt-sieving phenomenon, mentioned In the Introduction to Volume I, and already known to Aristotle. When seawater percolates over a compact sediment of slllcate-like particles, under some conditions the effluent Is potable. Basically the phenomenon is attributable to the negative adsorption of (in this case) anions, leading to the Donnan expulsion of electrolyte, see sec. 3.5b. Over-demand may lead to salt penetration the screening of the double layers around the silica particles (reduction of x ) makes the pores between them effectively wider. For this problem technical solutions had to be found. 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

The above-discussed acid retardation and base retardation in the immobiUzed Uquid phase could be compared with the so-called salting-out effects. However, this term is hardly applicable to the case of salt retardation, the first example of which was demonstrated by a successful removal of small amounts of NH4CI from a concentrated brine of (NH4)2S04. This practically important problem arises in the manufacture of caprolactam, where large amounts of sulfuric acid are converted into ammonium sulfate used for the preparation of the crystalline fertilizer. The new process of ISE on nanoporous NN-381 resin allowed an effective purification of very large volumes of concentrated sulfate brine, due to the fact that small ions of NH and Cl are efficiently squeezed into and retained in the finest pores of the sorbent [172]. We consider this salt retardation process as a convincing proof of our interpretation of the mechanisms of the new electrolyte separation process. 

In Section 9.1, Figure 9.1, we have seen that adjacent to a charged surface there is an excess of counterions and a deficit of co-ions. Both contribute to the neutralization of the surface charge. Let us now focus on the expulsion of the co-ions. The expulsion of co-ions implies a reduced volume available for electrolyte, or, in other words, there is an excluded volume with respect to the presence of electrolyte. This is known as the Donnan effect. For the same reason salt (= electrolyte) cannot penetrate in narrow capillaries and pores having charged walls. Based on this phenomenon, porous membranes that are permeable for water but not for salt may be used in reversed osmosis (also called ultrafiltration). Practical applications of reversed osmosis are found in, for example, the production of potable water from seawater, in hemodialysis using artificial kidneys, and in the concentration of solutions such as fruit juices. 

Recently, pore network modeling has been applied to simulate the accumulation of liquid water saturation within the porous electrodes of polymer electrolyte membrane fuel cells (PEMFCs). The impetus for this effort is the understanding that liquid water must reside in what would otherwise be reactant diffusion pathways. It therefore becomes important to be able to describe the effect that saturation levels have on reactant diffusion. Equally important is the understanding of how the properties of porous materials affect local saturation levels. This requirement is in contrast to most continuum modeling of the PEMFC, where porous materials are treated with volume-averaged properties. For example, the relationship between bulk liquid saturation and capillary pressures foimd through packed sand and other soil studies are often employed in continuum models. ... 

The quantity and the size of Si02 have the most important effect on battery performance when a colloid electrolyte is used. The 3D structure of the electrode active layers will become stronger as the quantity of Si02 is increased. The size of colloid particles can affect the volume of the pores and the average pore size of the electrode active masses. Therefore, the volume of the pores and the average pore size will be smaller when the quantity of Si02 is increased. The effect of Si02 quantity on electrolyte structure is summarized in Table 5.9. 

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