Effective diffusivity ionic

The aqueous diffusivities of charged permeants are equivalent to those of uncharged species in a medium of sufficiently high ionic strength. The product DF(r/R) is the effective diffusion coefficient for the pore. It is implicit in k that adsorption of the cations does not occur, so that the fixed surface charges on the wall of the pore are not neutralized. Adsorption is more likely to occur with multivalent cations than with univalent ones. 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

The Effect of Ionic Atmosphere on the Tracer Diffusion of Miceiles Toshihiro Tominaga and Masayuki Nishinaka... 

Figure 9.33. (a) Schematic description of the effects of ionic strength (I) and pH on the conformations of a humic molecule in solution and at a surface. Rh denotes the hydrodynamic radius of the molecule in solution and 6h denotes the hydrodynamic thickness of the adsorbed anionic poly electrolyte. (Adapted from Yokoyama et al., 1989 and O Melia, 1991). (b) The influence of ionic strength of pH on diffusion coefficient, Dl, and on Stokes-Einstein radius of a humic acid fraction of 50,000-100,000 Dalton. (From Cornel et al., 1986). 

Where I), is the diffusion coefficient (m2s-1) in 0.86M NaCl solution at 298K, C, is the molar concentration (mol dm"3) and u is the ionic mobility (m2s"1V"1) of species Side faces of the computational mesh are insulated whereas the top and bottom faces are both fixed at the initial concentration value. For corroding cells at the interface that contain corrosion products an effective diffusion coefficient, must be calculated. In the absence of accurate data this is done using a quadratic form [3] so that the effect of corrosion product at the surface is to increasingly prevent diffusion occurring through that cell. Finally, once the diffusion step is completed for all four species, both [OH-] and [H+] are adjusted to enforce the condition [OH ].[H+]=1014 at all elements containing electrolyte. 

It was believed that the effect of ionic strength on the electrokinetic potential might produce the value of the slipping plane separation. The well known method by Eversole, Lahr and Boardman is based on the assumption of constancy of the potential at the onset of the diffuse layer (j>i [35,36]. Since the < d value should decrease with ionic strength one can only determine the upper limit of the value of the separation distance [4]. The following form of the Gouy-Chapman theory is used for that purpose ... 

Additional calculations of the effective ionic diffusivities are shown in Figure 2.10 as a function of the square root of the concentration ratio r. The experimentally determined effective diffusivities are shown in the same figure for comparison. The agreement between theory and experiment is very good, especially for the Cl and Ba ions. The theory overestimates the effective diffusivity of the H ions but the decrease in the effective diffusivity of the H ions as the concentration ratio increases is predicted correctly. 

The monolayers chosen included cephalins and lecithins as examples of molecules expected to have a high polarizability normal to the interface (9, 26). Long chain sulfate and quaternary ammonium ions were studied as examples of monolayers with diffuse ionic double layers. Other experiments were made with protein films, with a long chain /5-alanine, and with monolayers of equimolar mixtures of long chain sulfates and quaternary ammonium ions. The various results can be explained by the effects illustrated below for long chain sulfates and lecithin. 

Tivant P, Turq P, Drifford M, Magdelenant H, Menez R. Effect of ionic strength on the diffusion coefficient of chondroitin sulfate and heparin measured by quasielastic light scattering. Biopolymers 1983 22 643-662. 

In order to describe the stability of fine disperse systems stabilized by diffuse ionic layers, one has to use the total free energy of interaction between particles, instead of the energy per unit film area, and compare the barrier height,, to the thermal energy, kT. For us to be able to use the solution derived for the case of plane-parallel surfaces, let us introduce some effective area of particle contact, Se[. Then the potential barrier height for the particles can be expressed as = A5 max St(. When diffuse part of electrical double... 

The effect of ionic environment on the rate of diffusion controlled e"aq reactions is revealed when one compares the experimental rate constants with the calculated values. In Table I the highly charged bis-pentacyano cobaltic peroxide (I) is much more reactive than expected for a pentavalent anion (1.11). It has been claimed (19) that polyvalent anions exhibit a lower effective charge in their kinetic behavior than expected from their structural formulae. We have checked the salt effect on the reaction of I + e m and compared it with the NOjf e m reaction. The results presented in Table IV and Figure 1 show that nitrate ions possess a normal salt effect, a result previously obtained by competition kinetics (15, 17). On the other hand, the salt effect of the I + e"aq reaction shows that this bis-pentacyano cobaltic peroxide ion has an... 

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