Ionic drift under electric fields

Remark The ionic species that undergo the electrochemical reactions move under the influence of several transport phenomena ionic drift under an electric field (a transport phenomenon also called conduction or migration), drift under a chemical-potential gradient (diffusion transport), and/or a convection phenomenon. The origin of the transport of electroactive species plays a very important part in the principles of the electrochemical methods of analysis. 

When applied to the motion of ions in a crystal, the term drift applies to motion of ions under the influence of an electric field. Although movement of electrons in conduction bands determines conductivity in metals, in ionic compounds it is the motion of ions that determines the electrical condu-ctivity. There are no free or mobile electrons in ionic crystals. The mobility of an ion, ji, is defined as the velocity of the ion in an electric field of unit strength. Intuitively, it seems that the mobility of the ion in a crystal should be related to the diffusion coefficient. This is, in fact, the case, and the relationship is... 

Why are ion-ion interactions important Because, as will be shown, they affect the equilibrium properties of ionic solutions, and also because they interfere with the drift of ions, for instance, under an externally applied electric field (Chapter 4). 

Another way of looking at ionic drift is to consider the fate of any particular ion under the field. The electric force field would impart to it an accelaation according to Newton s second law. Were the ion completely isolated (e.g., in vacuum), it would accelerate indefinitely until it collided with the electrode. In an electrolytic solution, however, the ion very soon collides with some other ion or solvent molecule that crosses its path. This collision introduces a discontinuity in its speed and direction. The motion of the ion is not smooth it is as if the medium offers resistance to the motion of the ion. Thus, the ion stops and starts and zigzags. However, the applied electric field imparts to the ion a direction (that of the oppositely charged electrode), and the ion gradually works its way, though erratically, in the direction of this electrode. The ion drifts in a preferred direction. 

The second problem concerns an understanding of the sharing of transport duties (e.g., the carrying of current) in pure liquid electrolytes. In aqueous solutions, it was possible to comprehend the relative movements of ions in the sense that one ionic species could drift under an electric field with greater agility and therefore transport more electricity than the other until a concentration gradient was set up and the resulting diffusion flux equalized the movements when the electrodes were reached. In fused salts, this comprehension of the transport situation is less easy to acquire. At first, it is even difficult to see how one can retain the concept of transport numbers at all when there is no reference medium (such as the water in aqueous solutions) in which ions can drift. 

We have recently developed a gas-phase ion chromatography technique and applied it to carbon cluster cations " " and anions""". A pulse of mass-selected cluster ions is injected into a high-pressure drift cell filled with 2-5 torr of helium. The ionic mobilities of different isomeric structures depend on their different collision cross-sections with He, and the isomers are therefore separated while drifting through the cell, under the influence of a weak electric field. The absolute value of the ionic mobility for a given cluster together with computer simulations often allows unambiguous determination of the cluster... 

The particle mobility bi is defined as the average drift velocity t j per unit force. In this way, explicit statements about collisions and the microscopic paths of the particles i are avoided. Under the assumption of uncorrelated collisions for particles which are originally in random motion, it can be shown that the application of a force leads to a drift velocity which is proportional to the acting force F. For the purpose of illustration, consider an ionic crystal to which an electric field is applied. The force F acts as F--= oCc - -eod<5 >/dx upon particles which carry unit charge. Then ... 

Figure 9-7. Schematic cross-section of a portion of a modulator. In this example, positive bias is applied to the signal plate above the waveguide region, while the other electrode is grounded. Positive ions in the spacer oxide drift under the influence of the electric field. The drift of ions toward the waveguide region results in a decrease of the electric field in the spacer and an increase of field in the waveguide region. This kind of ionic motion would cause the required DC bias field to decrease with time.

Another mechanism is the phenomenon first called the Wien effect.A drifting ion is surrounded by a solvatation well. Under the motion the structure of this well undergo changes and then reconstruction. The time of this process, called the relaxation time, T, is characteristic for a given liquid. Above the threshold electric field the ionic motion becomes greater in comparison with t, reconstruction does not take place, and the mobility increases greatly. The onset of the electric field strength for this process is -1 MV. ... 

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