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Planar lateral diffusion

Radial diffusion — Diffusion converging to a point is called radial diffusion, and is applied for diffusion at small microelectrodes when the radius of the electrode is much larger than the diffusion layer thickness estimated from (Dtj1/2. Genuine radial diffusion occurs at a spherical electrode. However, radial diffusion is sometimes used for diffusion at an edge of a planar electrode, which is also called lateral diffusion. See also -> hemispherical diffusion. [Pg.154]

Figure 16.13 Suggested mechanism for endosome formation of short DNA and CUV membranes, (a) DNA adsorption to the planar CUV membrane (dashed circles represents transverse sections of the DNA molecules), (b) Lateral diffusion and increase of the Sph+ concentration, decoupling of both monolayers, and external monolayer rolling up on the DNA molecules, (c) Topological transformation of the external lipid monolayers and encapsulation of DNA within a cylindrical inverted micellar structure. Membrane asymmetry is created (S xt < SJ. (d) Membrane invaginates at a scale of a few micrometers, (e) Formation of the endosome. Reprinted with kind permission of Springer Sciences-Business Media... Figure 16.13 Suggested mechanism for endosome formation of short DNA and CUV membranes, (a) DNA adsorption to the planar CUV membrane (dashed circles represents transverse sections of the DNA molecules), (b) Lateral diffusion and increase of the Sph+ concentration, decoupling of both monolayers, and external monolayer rolling up on the DNA molecules, (c) Topological transformation of the external lipid monolayers and encapsulation of DNA within a cylindrical inverted micellar structure. Membrane asymmetry is created (S xt < SJ. (d) Membrane invaginates at a scale of a few micrometers, (e) Formation of the endosome. Reprinted with kind permission of Springer Sciences-Business Media...
In contrast to surfactants, lipids adsorbed on hydrophilic surfaces can be expected to form planar bilayers, due to their large spontaneous radius of curvature. A double chain amphiphile forming a bilayer on silica was already discussed in chapter 3.1.2 in the context of 2H NMR investigations of water soluble amphiphiles. Bilayers from water insoluble lipid amphiphiles have been adsorbed to large spherical silica particles by condensation of unilamellar vesicles from aqueous solution, and a series of studies explored different NMR methods suitable for the measurement of lateral diffusion coefficients in such supported bilayers . [Pg.315]

FRAP was used initially to smdy the dynamics of lateral diffusion of lipids and proteins on cell surfaces [159-163]. It was combined with total internal reflection (Box 3.2 and Sect. 5.10) to study interactions of immunoglobulin fragments with planar bUayer membranes supported on glass surfaces [164] and binding of ligands to immobilized receptors [165]. Its applications expanded rapidly with the development of confocal microscopy (Sect. 5.10) and GFP tags [147,166-168], and now include components of the nucleus [169, 170], mitochonrial matrix [171], endoplasmic reticulum [172], and Golgi apparatus [173]. [Pg.261]

Molecules added to biological systems may find their way into membranes and associate in the lipid system. Several compounds of fungal or bacterial origin (such as alamethacin, which forms micelles in aqueous systems [350]) readily incorporate into planar lipid bilayers and form channels by self-association, channels which allow the translocation of ions and other hydrophilic species through the otherwise hydrophobic membranes. Based on calculations made of the rates of association possible for channel formation by lateral diffusion of... [Pg.217]

Figure 44 The concentration distribution in case of constant planar source diffusion, where concentration at X = 0 remains constant at all later times (a) the x-dependence of concentration at three time instants and (b) the time dependence of concentration at four streamwise locations. Here, Lq is the length scale and Tq = C /D is the time scale... Figure 44 The concentration distribution in case of constant planar source diffusion, where concentration at X = 0 remains constant at all later times (a) the x-dependence of concentration at three time instants and (b) the time dependence of concentration at four streamwise locations. Here, Lq is the length scale and Tq = C /D is the time scale...
The attractive feature of LADM Is that once the fluid structure Is known (e.g., by solution of the YBG equations given In the previous section or by a computer simulation) then theoretical or empirical formulas for the transport coefficients of homogeneous fluids can be used to predict flow and transport In Inhomogeneous fluid. For diffusion and Couette flow In planar pores LADM turns out to be a surprisingly good approximation, as will be shown In a later section. [Pg.262]

Diffusion of electroactive species to the surface of conventional disk (macro-) electrodes is mainly planar. When the electrode diameter is decreased the edge effects of hemi-spherical diffusion become significant. In 1964 Lingane derived the corrective term bearing in mind the edge effects for the Cotrell equation [129, 130], confirmed later on analytically and by numerical calculation [131,132], In the case of ultramicroelectrodes this term becomes dominant, which makes steady-state current proportional to the electrode radius [133-135], Since capacitive and other diffusion-unrelated currents are proportional to the square of electrode radius, the signal-to-noise ratio is increased as the electrode radius is decreased. [Pg.446]

In the years 1910-1917 Gouy2 and Chapman3 went a step further. They took into account a thermal motion of the ions. Thermal fluctuations tend to drive the counterions away form the surface. They lead to the formation of a diffuse layer, which is more extended than a molecular layer. For the simple case of a planar, negatively charged plane this is illustrated in Fig. 4.1. Gouy and Chapman applied their theory on the electric double layer to planar surfaces [54-56], Later, Debye and Hiickel calculated the potential and ion distribution around spherical surfaces [57],... [Pg.42]

In single step voltammetry, the existence of chemical reactions coupled to the charge transfer can affect the half-wave potential Ey2 and the limiting current l. For an in-depth characterization of these processes, we will study them more extensively under planar diffusion and, then, under spherical diffusion and so their characteristic steady state current potential curves. These are applicable to any electrochemical technique as previously discussed (see Sect. 2.7). In order to distinguish the different behavior of catalytic, CE, and EC mechanisms (the ECE process will be analyzed later), the boundary conditions of the three processes will be given first in a comparative way to facilitate the understanding of their similarities and differences, and then they will be analyzed and solved one by one. The first-order catalytic mechanism will be described first, because its particular reaction scheme makes it easier to study. [Pg.191]

The ratios given in Eq. (4.66) are only dependent on the electrode shape and size but not on parameters related to the electrode reaction, like the number of transferred electrons, the initial concentration of oxidized species, or the diffusion coefficient D. For fixed time and size, the values of f or Qf2 are characteristic for a simple charge transfer (see Fig. 4.4 for the plot of Qf2 calculated at time (ti + T2) for planar, spherical, and disc electrodes) and, as a consequence, deviations from this value are indicative of the presence of lateral processes (chemical instabilities, adsorption, non-idealities, etc.) [4, 32]. Additionally, for nonplanar electrodes, these values allow to the estimation of the electrode radius when simple electrode processes are considered. [Pg.247]

In this section, the current-potential curves of multi-electron transfer electrode reactions (with special emphasis on the case of a two-electron transfer process or EE mechanism) are analyzed for CSCV and CV. As in the case of single and double pulse potential techniques (discussed in Sects. 3.3 and 4.4, respectively), the equidiffusivity of all electro-active species is assumed, which avoids the consideration of the influence of comproportionation/disproportionation kinetics on the current corresponding to reversible electron transfers. A general treatment is presented and particular situations corresponding to planar and nonplanar diffusion and microelectrodes are discussed later. [Pg.376]

Interestingly, no and Do can also be obtained from a single CA experiment if an ultramicroelectrode is used [6]. Following the potential step, planar diffusion will initially dominate, as the diffusion layer is smaller than the radius of the electrode. The current thus follows the usual Cottrel equation (Eq. 39). Later in the CA experiment, however, the diffusion layer grows larger than the radius of the ultramicroelectrode, and spherical diffusion will now dominate. This results in a time-independent current, given by Eq. 48 if the electrode is disc shaped, or Eq. 49 for microsphere geometry. [Pg.513]

If the electrode material is assumed to be homogeneous, then the concentration gradient of lithium through the electrode is the only factor that drives lithium transport. Hence, lithium will enter/leave the planar electrode only at the electrode/ electrolyte interface, and cannot penetrate into the back of the electrode. Under such an impermeable (impenetrable) constraint, the electric current (I) can be expressed by Equation (5.18) during the initial stage of diffusion, and by Equation (5.19) during the later stage [45] ... [Pg.150]


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See also in sourсe #XX -- [ Pg.589 ]




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Diffusion planar

Lateral diffusion

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