Separators hydration shorts

Cellophane or its derivatives have been used as the basic separator for the silver—ziac cell siace the 1940s (65,66). Cellophane is hydrated by the caustic electrolyte and expands to approximately three times its dry thickness iaside the cell exerting a small internal pressure ia the cell. This pressure restrains the ziac anode active material within the plate itself and renders the ziac less available for dissolution duriag discharge. The cellophane, however, is also the principal limitation to cell life. Oxidation of the cellophane ia the cell environment degrades the separator and within a relatively short time short circuits may occur ia the cell. In addition, chemical combination of dissolved silver species ia the electrolyte may form a conductive path through the cellophane. 

In the case of trace metals, adsorption is typically much faster than the time intervals for which it is practically possible to separate the cells. Therefore, in practice, values of kf and kr are most often estimated by assuming that water loss from the hydrated cation is rate-limiting (Eigen-Wilkins mechanism, see Section 4.3.1 above). In some cases, uptake transients can be observed at the start of a short-term uptake experiment or by using pulse-chase experiments for which a metal solution containing a radioactive tracer is replaced by a solution... 

The earliest concerted effort in the research and development of Nafion perfluorosulfonate ionomers was directed toward their use as a permselective membrane separator in electrochemical cells used in the large scale industrial production of NaOH, KOH, and CI2. In short, the membrane in this application, in addition to keeping CI2 and H2 gases separated, prevents the unfavorable back migration of hydrated OH ions from the catholyte (concentrated aqueous NaOH or KOH) chamber, while allowing for the transport of hydrated Na+ ions from the anolyte chamber in which is aqueous NaCl. 

These various relationships between force and particle separation imply that the attractive force between particles will become infinite when they touch. In reality, other short-range forces will modify this relationship when r is very small, in particular the repulsion from overlap of atomic orbitals. The van der Waals attraction will then be balanced by this overlap repulsion. At these short distances (a few tenths of a nanometer), the van der Waals attraction will be strong enough to hold the particles fairly strongly together. This balance between van der Waals forces of attraction and overlap repulsion forces is shown schematically in Fig. 1.4, where the very steep repulsive interaction at atomic distances is due to the overlap repulsion. Hydration forces (see section 1.3.3) may also result in repulsion between surfaces at somewhat greater separations. 

That the hydrated electron is a separate chemical entity has been demonstrated by the technique of pulse radi l sis This consists of subjecting a sample of pure water to a very short pulse of accelerated electrons. The energetic electrons have the same effect upon water as a beam of y-ray photons. Shortly after the pulse of electrons has interacted with the water, a short flash of radiation (ultraviolet and visible radiation from a discharge tube) is passed through the irradiated water sample at an angle of 90° to the direction of the pulse to detect the absorption spectra... 

For neutral bilayers, there are no long-range doublelayer forces which, coupled with the van der Waals attraction, could explain the stability of the lamellar structure. At small separations, the required repulsion is provided by the hydration force, which was investigated both experimentally6-8 and theoretically.9,10 However, it was experimentally observed that the lipid bilayers could be swollen in water up to very large interlayer distances,11 where the short-range exponential hydration repulsion becomes negligible. 

We showed previously that a simple model for the ion-hydration interactions, which separates the ion-hydration forces in a long-range term due to the behavior of water as a continuous dielectric (the screened image force) and a short-range term due to the discreetness of the water molecules (SM/SB), can explain almost quantitatively a number of phenomena related to the electrolyte interfaces.6 In this article, we examined the limitations of the model in predicting the distributions of ions near the air/water interface, by comparison with molecular dynamics simulations. It is clear that the real ion-hydration forces are more complicated than the simple model employed here however, the interfacia] phenomena (including specific ionic effects) can be understood, at least qualitatively, in terms of this simple approach. 

The forth issue is the increase in the repulsion between bilayers at short distances. In Fig. 1, the osmotic pressure is plotted as a function of separation distance (data from Ref. [13]) for no added salt, for l M KC1 and for 1 M KBr. They reveal an increase in repulsion at short separation distances upon addition of salt. While the relatively small difference between 1 M KC1 and 1 M KBr can be attributed to the charging of the neutral lipid bilayers by the binding of Br (but not C.1-) [14], the relatively large difference between no salt and 1 M KCl is more difficult to explain. Even a zero value for the Hamaker constant (continuous line (2) in Fig. 1), in the 1 M KCl case, is not enough to explain the increase in repulsion, determined experimentally. The screening of the van der Waals interaction, at distances of the order of three Debye-Hiickel lengths (about 10 A) should lead, according to Petrache et al. calculations, to a decrease of only about 30% of the Hamaker constant (from 1.2kT to about 0.8kT, see Fig. 5C of Ref. [14]). Therefore, an additional mechanism to increase the hydration repulsion or the undulation force (or both) upon addition of salt should exist to explain the experiments. 

Fig. 1. Experimental values for the osmotic pressure as a function of separation distance from Ref. [13] (stars water circles 1 M KCl triangles 1 M KBr) are compared with calculations based on the simple equations ((1), (3) and (9)), with parameters reported in Ref. [13] (note that in Ref. [13] it was suggested that hydration interaction increases upon addition of salt) Ah = 1.6 x 10s N/m2, H = 2.1 A, H = 9.2 x KT21 J. b = 39 A, Kc = 5.8 x 10 2(1 J, An = 1.06 A 2, >.fl = 6.0 A (Line 1). Even for H = 0 and the rest of the parameters as before (Line 2), the repulsion at short separations is weaker than in the experiment. This points out that either hydration and/or undulation forces must increase upon addition of electrolyte.

In the previous section, it was shown that a hydration force with a short decay length and almost independent of the electrolyte concentration can be achieved with neutral lipid bi layers at short separation distances. At separations larger than about 20 A, the attractive van der Waals interactions dominated the interactions. 

The experimental data reported by Pashley [17] raise two important issues. The first one is why the critical electrolyte concentration at which the hydration force emerges, depends so strongly on the type of electrolyte (6x 10 2 for LiCl, 3x10 / M for KC1). The second issue is why the decay length of the interactions at short separations is about 10 A, about five times largo- than that corresponding to the hydration force between lipid bilayers. 

Fig. 5. All experimental data exhibit a hard-wall-like repulsion at veiy short separations, regardless of pH or ionic strength. Note that the hydration forces increases at higher ionic strength cE= 1 M (triangles), compared with cK 0.01 M, pH==3 (squares). At pH=9 (circles) and < , = 0.01 M, the surface charges create a long range double layer repulsion.

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