Electrical drift contribution

Figure 6.—Continued. C, Predicted electrical drift contribution to molecular transport across one of the two cubic cell membranes. (Weaver, J. C. Barnett, A. Wang, M. W. B/iss, J. G., unpublished). A hypothetical series of molecules, a// with unit charge (zs = l) was used to test the relative importance of different size pores in the pore population. More realistic predictions would use estimates of the size (radius rs), shape (a form factor), and the Bom energy repulsion (zs>eff — zm, where m is a number in the range 1 < m < 2).

Our initial estimates of molecular transport based on electrical drift should be extended by including convection [e.g., electroosmosis (31)] and diffusion (52). The same general strategy is reasonable A dynamic pore population will be computed, in which electrical interactions are the dominant source of pore creation and expansion. In the case of a pl nar membrane with no osmotic or hydrostatic pressure gradient, the final stages of pore population expansion and collapse should also be governed by purely electrical interactions. By following the pore population over its development, the contribution of each transport mechanism can be estimated. For cell membranes, a nonzero pressure difference will usually exist. In this case, pores of... 

In the steady state, when the drift contribution dominates, the ionic current obeys the Ohm s law. In the absence of either drift or barriers, the behavior of ions is according to the Pick s law. The GHK equations offer a convenient way to describe the crossover behavior between the diffusion-, drift-, and barrier-dominated regimes. We have also shown the utility of the numerically solved results from the PNP equations for the ionic currents through the GA channel and the aHL protein pore. The PNP calculations show that the steepest gradient in the electrical potential is only very near and across the pore. We have... 

Let us now consider the situation when this balance has been upset by the presence of a weak electric field perpendicular to AB. The motion of the ions will no longer be completely random, but a tendency to drift will be superimposed on the random motion. If in unit time there has been an appreciable excess flow of negative ions across AB in one direction, we can be certain that there has been an appreciable excess flow of positive ions across AB in the opposite direction. These two separate contributions will together constitute the electric current. 

Since a small trace of water suffices to produce a large effect, the equilibrium of (43) evidently lies far in favor of the right-hand side (see also Sec. 115), indicating that a water molecule dissolved in ethyl alcohol provides a vacant level for an additional proton that lies lower than the level occupied by the protons in the OH2 group of the (C2H3OH2)+ ion. A proton captured in this lower level of (HaO)+ will have to wait until it receives the necessary energy before it can move back to an alcohol molecule. In the meantime the (H30)+ ion can merely contribute to the electrical conductivity by drifting slowly in the field only when the proton has returned to an alcohol molecule can the rapid proton jumps be resumed. 

The dependence of the drift mobility p on the electric field is represented by formula p (p-E1/2/kTcf) which corresponds to the Pool-Frenkel effect. The good correspondence between experimental and theoretical quantity for Pool-Frenkel coefficient 3 was obtained. But in spite of this the interpretation of the drift mobility in the frame of the Coulombic traps may be wrong. The origin of the equal density of the positive and negative traps is not clear. The relative contribution of the intrinsic traps defined by the sample morphology is also not clear [17,18]. This is very important in the case of dispersive transport. A detailed analysis of the polymer polarity morphology and nature of the dopant molecules on mobility was made by many authors [55-58]. 

In the phenomenological treatment of the directed drift that the field brings, we take the attitude that there is a stream of cations going toward the negative electrode and anions going toward the positive one. We now neglect the random diffusive movements they do not contribute to the vectorial flow that produces an electrical current. 

A limitation of the magnetic prism-based EELS detection system is the spectrum drift in the energy dispersion direction. Various sources of instability in the microscope contribute to this effect High voltage fluctuation, magnetic field creep etc. This places a fundamental limit on the useful exposure time. Experimental efforts in electron spectroscopy consist largely in reducing unwanted electrical noise and specimen drift. 

Figure 1 is a schematic representation of Frenkel s notion an atom or ion can get dislodged from its normal site to form etn interstitial-vacancy pair. He further proposed that they do not always recombine but instead may dissociate and thus contribute to diffusional transport and electrical conduction. They were free to Wcuider about in a "random walk" mcuiner essentially equivalent to that of Brownian motion. . . this meant they should exhibit a net drift in an applied field. 

For biological samples four different contributions to the standard potential Ef can be observed the contribution of the internal standard potential of the reference electrode ERef the diffusion potential over the liquid junction Ej generated between the sample solution and the reference electrode a potential difference (electrical asymmetry) of the ion-selective membrane after preparation and conditioning Eei and a sample-induced asymmetry of the membrane Eas. For measurements in human blood samples directly the adsorption of sample components at the membrane surface creates a drift associated with the affinity of the membrane to lipids as well as proteins. 

---