Charged particles concentration polarization

Diffuslophoresis is the counterpart of electrophoresis in that In the former case a concentration gradient is applied, it leads to polarization and motion in the latter case an electric field Is applied, which leads to concentration polarization and motion. When the particles are immobile, as in a porous plug, or when the concentration gradient is applied over a charged capillary the liquid starts to move. This leads to plug or capillary osmosis, the counterpart of electro-osmosis in plugs or capillaries. 

Charged particles influence the net conductivity in several ways (1) the presence of particles having dielectric constant and conductivity different from those of the medium affects the local electrical field and the conditions for ion transport (e.g., nonconducting particles act as obstacles to the electromigrating ions and polarize the incident electric field) (2) the inaeased ionic concentration in the diffuse ion... 

The study of colloidal crystals was initiated as part of research into the determination of phase diagrams for colloids, which itself was perceived as a means to model phase behaviour in molecular systems [22]. Extensive literature is available on the dynamics of colloidal crystal formation, as a function of several parameters, such as the nature of the solvent, surface charge, particle size and concentration. The results described here refer to the formation of colloidal crystals from dispersions of silica-coated gold nanoparticles in ethanol, after silica surface functionalization with 3-(trimethoxysilyl)propyl methacrylate (TPM). Earlier studies by Philipse and Vrij [23] showed that TPM adsorption leads to a reduction in surface charge, so that the particles are stable in organic solvents with low polarity, such as ethanol, toluene or DMF. This means that the particle be-... 

The potential difference can be seen to be made up of two terms. The first term represents the ohmic potential drop due to the flow of current through a medium of given electrical conductance. The second term, called the diffusion potential drop, is associated with a region in which there is a concentration gradient (concentration polarization region). This term does not disappear in the absence of a current and is due to unequal rates of diffusion of the charged particles, thus giving rise to a diffusional electric field. 

The transport of charged particles to the anode or cathode through a set of ion-exchange membranes leads to a concentration decrease of countejjons in the laminar boundary layer at the membrane surface facing the diluate cell and an increase at the surface facing the brine cell. The effect of concentration polarization due to a concentration increase in the... 

The expression for potential difference consists of two terms. The first term has the meaning of ohmic potential drop caused by the resistance of the medium to propagation of electric current of density i. The second term, called the diffusion potential drop, is related to the gradient of concentration, that is, to the presence of regions of concentration polarization. This term is caused by the difference in diffusion rates of charged particles and the occurrence of diffusion flux (the second term in Eq. (5.98)). 

The interaction between two charged particles in a polar media is related to the osmotic pressure created by the increase in ion concentration between the particles where the electrical double-layers overlap. The repulsion can be calculated by solving the Poisson-Boltzmann equation, which describes the potential, or ion concentration, between two overlapping double-layers. The full theory is quite complicated, although a simplified expression for the double-layer interaction energy, V dl( ) between two spheres, can be written as follows ... 

The objective of this work was to characterise the sodium dodecyl allyl sulphosuccinate (Eliminol JS-2) stabilised poly(butyl acrylate) (PBA) lattices produced in a semibatch reactor. The PBA latex particles were prepared using a semibatch pulsion polymerisation process. It was found that the concentration of JS-2 or sodium dodecyl sulphate present in the initial reactor charge is very important in determining the final latex particle size. The higher the particle size polarity is, the larger is the saturated particle surface area covered by one JS-2 molecule. It was also found that at a common surfactant concentration, the JS-2 stabilised latex displays reduced chemical stability than the sodium dodecyl sulphate stabilised latex. The results of such research should be useful to tape and label manufacturers. 3 refs. 

For originally non-charged particles dispersed in polar media, the surface preferential adsorption of ions, surface molecular group dissociation, isomorphic substitution, adsorption of polyelectrolytes, and even surface polarization will make them behave similarly to charged particles. The Stem layer and diffuse layer comprise what is commonly known as the electrical double layer, the thickness of which depends on the type and concentration of the ions in the suspension as well as on the particle surface. A parameter, called the Debye-Huckel parameter k, is used to characterize electrical double layer thickness. K has the dimension of reciprocal length. For smooth surfaces in simple electrolytes. 

In the previous chapter we discussed the polarization curve and all of the losses associated with the generation of current that result in decreased operating efficiency and generation of heat. At a fundamental level, all of these polarizations are a result of transport limitations. The ohmic polarization is a result of ion and electron transport losses, and the concentration and activation polarization is a result of mass transport limitations of the reactant to the catalyst surface and charged particles across the double layer, respectively. Even the crossover and internal short current loss from the expected Nemst potential is a result of transport. Optimization of the fuel cell design therefore must include an optimization of the (desired) modes of transport and minimization of the undesired modes of transport. In this chapter, the modes of transport relevant to fuel cells are described in greater detail. 

FIG U RE 5.2 Schematics of the fluxes of counterions (+) and coions (-) around a negatively charged colloidal particle in the presence of an electric field ( o)- la (a) the concentration polarization (electrolyte concentration, c r), increased on the right and depleted on the left) provokes diffusion fluxes (y ) that superimpose to the electromigration fluxes (J. In panel (b) the frequency is above the oc-relaxation, and only electromigration can take place. (Reprinted from Jimenez, M.L. et al. 2007. /. Colloid Interface Sci. 309 296—302. With permission from Elsevier.)... 

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