Counterion transport, direction

The chief phenomenon to be considered here is non-uniform distribution of electric charge in charged membranes. The effect of this on ionic sorption and transport properties is of considerable practical interest, because membrane permselectivity for the counterion against the coion (or for uncharged species vs electrolytes), and hence membrane performance in important technical applications (such as electrodialysis) is directly involved. 

Until now we have ignored an important factor. The electric field affects not only the surface charges of the particle, but also the ions in the electrical double layer. The counterions in the double layer move in a direction opposite to the motion of the particle. The liquid transported by them inhibits the particle motion. This effect is called electrophoretic retardation. Therefore the equation is only valid for D [Pg.77]

Most of the redox centers in a polymer film cannot rapidly come into direct contact with the electrode surface. The widely accepted mechanism proposed for electron transport is one in which the electroactive sites become oxidized or reduced by a succession of electron-transfer self-exchange reactions between neighboring redox sites [22]. However, control of the overall rate is a more complex problem. To maintain electroneutrality within the film, a flow of counterions and associated solvent is necessary during electron transport. There is also motion of the polymer chains and the attached redox centers which provides an additional diffusive process for transport. The rate-determining step in the electron site-site hopping is still in question and is likely to be different in different materials. 

In summary, there is evidence that the skin presents a weak cation permselectivity [25,76,77,80,93,125], which can be reversed by acidifying the pH of the solutions bathing the skin [10,23,76,77]. At pH>p/, the skin is negatively charged and electroosmotic flow proceeds in the anode-to-cathode direction. At pH < pi, the skin becomes positively charged and electroosmotic flow reverses to the cathode-to-anode direction. Under the application of an electric field, counterions (cations at physiological pH) are preferentially admitted into the skin. As a consequence, the sodium and chloride transport numbers are 0.6 and 0.4, respectively, during transdermal iontophoresis (in contrast to their values in a neutral membrane tNa = 0.45 rCi = 0.55) [126]. 

Study on the rapid transport of a polymer in dextran solutions, first observed by Preston et al., is extended into two directions. They arc (1) enhancement effect on the transport rate of polyvinylpyrrolidone (PVP) by the addition of a simple salt, and (2) extension to the transport of linear polyelectrolytes. The enhancement effect was observed on the structured flow as well as on the transport rate. The enhancement effect was correlated with the densities of the solutions in the lower compartment of the diffusion cell. The correlation was improved when the rate was corrected for the differences in viscosities. We have found that effects of charges on the polymers favor the rapid transport of polyacrylates (PA) and sodium hyaluronate. Counterion condensation was manifested in the transport rate of PA. Transport rates of several salts of PA in the absence of added salt increased linearly with their partial specific volumes in water. 

Organic solutes such as nutrients (amino acids, sugars, vitamins, and bile acids), neurotransmitters, and drugs are transferred across cellular membranes by specialized transport systems. These systems encompass integral membrane proteins that shuttle substrates across the membrane by either a passive process (channels, facilitated transporters) or an active process (carriers), the latter energized directly by the hydrolysis of ATP or indirectly by coupling to the cotransport of a counterion down its electrochemical gradient (e.g., Na, ... 

Mutations in the Saccharomyces cerevisiae H+-ATPase gene, PMA1, that confer cellular growth resistance to hygromycin B cause a generalized depolarization of cellular membrane potential. The normal hyperpolarized membrane potential in yeast is maintained by the H+-ATPase, and it is believed that the pmal mutations alter electrogenic proton transport by the enzyme. Electroneutral H+ transport by the mutant enzymes may involve the countertransport of K+, but other ions including H+ could participate. More direct evidence is needed to confirm the role of K+ as a counterion and to probe its putative transport mechanism. It will be important to determine whether H + and K+ use the same mechanistic pathway for transport. 

The electrically induced contraction of the gel is caused by transport of hydrated ions and water in the network, and the contractile behaviors observed are essentially electrochemical phenomena. When an outer electrical field is applied across the gel, the macro- and microions experience electrical forces in opposite directions. However, the macroions are a stationary phase because they are chemically fixed to the polymer network, whereas the counterions are mobile, capable of migrating along the electric field and dragging water molecules with them. In... 

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