Passive ionic transport

Thus, the ideas above do not suffice for an interpretation of all experimental results. These ideas include the assumption that the ions move in the membrane only under the effect of concentration and potential gradients (diffusion and migration), and that transport of one sort of ions is independent of the transport of other sorts of ions. This transport of ions under the effect of external forces has been named passive ionic transport. 

This occurs in strongly acid or strongly alkaline solutions, but there are specific exceptions. Thus in concentrated nitric acid the metal is passive and the kinetics of the process are controlled by ionic transport through the... 

Basic issues such as surface reactions, surface film formation, passivation, ionic and electronic transport phenomena through surface films, problems in uniformity of deposition and dissolution processes, correlation between surface chemistry, morphology, and electrochemical properties are common to all active metal electrodes in nonaqueous solutions and are dealt with thoroughly in this chapter. It is believed that many conclusions related to Li, Mg, Ca, and A1 electrodes can be extended to other active metal electrodes as well. 

The initiation of an electrogenic process causes an adjustment of the passive ionic fluxes across the membrane. In particular, the net charge actively brought in is soon electrically compensated by appropriate passive movements of that ion and other ions into or out of the cell. The actual electrical potential difference across the membrane then results from the diffusion potential caused by these new passive fluxes plus a steady-state contribution from the electrogenic process involving the active transport of various charged species. We can represent the electrical potential difference generated by the active transport of species/, atj, by... 

To study the effects of electrochemical properties on passive ion transport processes, we developed a model that focuses on ionic processes at membrane and channel surfaces (14). The surface compartment model (SCM) is based on a Helmholtz electrical double layer, where the enhanced concentration of counterions and the depletion of co-ions at charged surfaces is described by straight line gradients. Treatment of the electrical double layer as a compartment greatly simplifies the calculation of ion transport. 

The electrogenic potential may be determined by measuring the change in potential due to temperature change of the system or introduction of specific inhibitors (such as ouabain) for Na transport. Since that part of the membrane potential due to passive ionic processes across the membrane (as in the squid axon membrane) is less sensitive to temperature than that due to an active transport process, one can deduce from the temperature coefficient of the membrane potential the contribution due to an active transport process or since ouabain is supposed to inhibit the active transport process, one can measure the active transport contribution to the membrane potential as the difference in membrane potential in two environments (one, a normal physiological solution the other, a normal physiological solution containing ouabain). 

If a solid and compact continuous film of corrosion products is formed on the metal surface, having the characteristics of ionic conduction, the anodic process can take place at the interface between the corrosion products and the solution. However, at low temperatures, the low ionic mobility in solids and the low charge carrier concentration limit the ionic transport through the corrosion products, and the rate of the anodic process may be reduced to negligible values from a corrosion point of view, such as in the case of materials in a passivity condition. If the corrosion products are not conductors, the anodic process can be limited to the free metal surface through the porosity of the corrosion product. 

Taurine conjugates are not absorbed in the upper intestine of human subjects (31,32), the major transport taking place in the lower ileum by both an active mechanism and passive ionic diffusion. Glycine conjugates, particularly those of dihydroxy bile acids, on the other hand, are absorbed also in the jejunum by passive ionic diffusion (33). Negligible amounts of free bile acids are normally found in the upper small intestine (23), while deconjugation is known to occur in the lumen of the terminal ileum. Absorption of free bile acids appears to take place by both ionic and nonionic diffusion, the transport for dihydroxy bile acids being particularly rapid even in the upper intestine (33). 

To validate the Pxy-TFSI- RTIL mixtures as electrolytes for batteries Li-ion application, tests of cycling ability are required, after the determination of their electrochemical window. The quality of the passivating protective layer, and its stability at the graphite electrode, must be checked by mean of galvanostatic chronopotentiometric measurements. The Pxy-TFSI- RUL contents lower than 20 % (w/w) are not of a real interest, in spite of an ionic conductivity over the standard electrolyte one, because the phenomenon of self-extinguished flame becomes striking only from 20 % of Pxy-TFSI. For between 20 % and 30 % Pxy-TFSI- RUL, the performances are optimized from the point of view of the ionic transpxjrt, of the thermal stability and of the material wettability Beyond these contents, up to 50 %, the increase of the viscosity of mixtures can limit their applications at room temperature (mainly because of the decrease of the performances of ionic transport), but their use can be considered for higher temperature applications. 

In the deposition zone, corrosion products will accumulate, following their nucleation on the substrate. The corrosion products formed under thin-film atmospheric conditions are closely related to the formation of naturally occurring minerals. Over long periods of time, the most thermodynamically stable species will tend to dominate. The nature of corrosion products foimd on different metals exposed to the atmosphere is shown in Fig. 2.6. The solution known as the inner electrolyte can be trapped inside or under the corrosion products formed. The deposited corrosion product layers can thus be viewed as membranes, with varying degrees of resistance to ionic transport. Passivating films tend to represent strong barriers to ionic transport. 

As a general rule, dissolution occurs in strongly acid or strongly alkaline solutions (as per the Pourbaix diagram), but there are specific exceptions. For instance, in concentrated nitric acid, the metal is passive and the kinetics of the process are controlled by ionic transport through the oxide film. Also, inhibitors such as silicates permit the use of some alkaline solutions (up to pH 11.5) to be used with Al. Even where corrosion may occur, A1 may be preferred to other metals because its corrosion products are colorless. 

Research in lithium batteries began in 1912 under GJS[. Lewis, but the breakthrough came in 1958 when Harris noticed the stabihty of Li-metal in a number of nonaqueous (aprotic) electrolytes such as fused salts, hquid SO2, or hthium salt into an organic solvent such as LiC104 in propylene carbonate (C4H6O3). The formation of a passivation layer that prevents the direct chemical reactimi between hthium metal and the electrolyte but stih allows for ionic transport is at the origin of the stabihty of hthium batteries [17]. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

It is possible that the stationary-state situations leading to an active ion transport occur only in localized regions of the membrane, i.e., at ATPase molecule units with diameters of about 50 A and a length of 80 A. The vectorial ion currents at locations with a mixed potential and special equipotential lines would appear phenomenologically like ionic channels. If the membrane area where the passive diffusion occurs is large, it may determine the rest potential of the whole cell. 

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