Electrical double layer retardation effect

For large particles with thin electric double layers, meaning particles for which Ka> 100. This theory takes account of the opposite effect of the applied electric field on the ions in the electric double layer, an effect called electrophoretic retardation which acts to reduce particle velocity,... 

The effect of the phospholipids on the rate of ion transfer has been controversial over the last years. While the early studies found a retardation effect [6-8], more recent ones reported that the rate of ion transfer is either not retarded [9,10] or even enhanced due to the presence of the monolayer [11 14]. Furthermore, the theoretical efforts to explain this effect were unsatisfactory. The retardation observed in the early studies was explained in terms of the blocking of the interfacial area by the phospholipids, and therefore was related to the size of the transferring ion and the state of the monolayer [8,15]. The enhancement observed in the following years was attributed to electrical double layer effects, but a Frumkin-type correction to the Butler Volmer (BV) equation was found unsuitable to explain the observations [11,16]. Recently, Manzanares et al. showed that the enhancement can be described by an electrical double layer correction provided that an accurate picture of the electrical double layer structure is used [17]. This theoretical approach will be the subject of Section III.C. 

Flow movement also has a relationship with the electrokinetic phenomenon, which can promote or retard the motion of the fluid constituents. Electrokinetic effects can be described as when an electrical double layer exists at an interface between a mobile phase and a stationary phase. A relative movement of the two phases can be induced by applying an electric field and, conversely, an induced relative movement of the two will give rise to a measurable potential difference.33... 

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]

Since this book is dedicated to the dynamic properties of surfactant adsorption layers it would be useful to give a overview of their typical properties. Subsequent chapters will give a more detailed description of the structure of a surfactant adsorption layer and its formation, models and experiments of adsorption kinetics, the composition of the electrical double layer, and the effect of dynamic adsorption layers on different flow processes. We will show that the kinetics of adsorption/desorption is not only determined by the diffusion law, but in selected cases also by other mechanisms, electrostatic repulsion for example. This mechanism has been studied intensively by Dukhin (1980). Moreover, electrostatic retardation can effect hydrodynamic retardation of systems with moving bubbles and droplets carrying adsorption layers (Dukhin 1993). Before starting with the theoretical foundation of the complicated relationships of nonequilibrium adsorption layers, this introduction presents only the basic principles of the chemistry of surfactants and their actions on the properties of adsorption layers. 

Because of the limited magnitude of surface tension gradients and absence of electric double-layer effects, the stabilization of foams in nonpolar liquids requires other ways of retarding the thinning of foam lamellae. These include the high liquid-phase viscosity that has been discussed earlier and increased surface viscosity because of presence of highly viscous or even rigid liquid-crystal films. 

In the above argument we have ignored the efl eet of the ionic atmosphere, or electrical double layer, around the charged particle. Its presence has two consequences (Figure G.7). First, the counter-ions in the double layer tend to move in a direction opposite to that of the particle, which in effect has to drag its double layer with it. Secondly, it does not entirely succeed in taking all of its double layer, but fresh ions become attached to it as some an- left behind this reconstruction of the double layer does not take plaee instantaneously. Both effects lead to a retardation of the movement of the particle. More complete theories lead to the introduction of an additional term J(Ka) into equation (6.45) ... 

If the gas diffusion between bubbles is reduced, the collapse of the bubbles is delayed by retarding the bubble size changes and the resulting mechanical stresses. Therefore single films can persist longer than the corresponding foams. However, this effect is of minor importance in practical situations. Electric effects, such as double layers, form opposite surfaces of importance only for extremely thin films (less than 10 nm). In particular, they occur with ionic surfactants. 

The numerical results show that the polarization effect of the double layer impedes particle s migration because an opposite electric field is induced in the distorted ion cloud, which acts against the motion of the particle. For a given ica, the electrophoretic mobility increases first, reaches a maximum value and then decreases as the absolute zeta potential is increased. This maximum mobility arises because the electrophoretic retarding forces increase at a faster rate with zeta potential than does the driving force. 

With electrochemical processes, as mentioned above, it is necessary to take account of the effect of the double-layer electric field and adsorption of ions, solvent molecules, hydrogen, and oxygen on the electrode surface. Thus, the absence of electrochemical oxidation of methane at appreciable rates on rhodium, iridium, ruthenium, and osmium is explained by previous adsorption of oxygen and stronger adsorption of anions on the surface of these metals compared with platinum, and this retards the process [100], Further research into adsorption and electrochemical characteristics of metals and alloys is therefore required for determining the reasons for differing catalytic activities. 

With a 4—1 electrolyte the charge of the double layer is mainly an excess of cations, with a 1—4 electrolyte it is a defect of anions, whereas in the case of a symmetrical electrolyte excess of cations and defect of anions are equal. By the applied electric field the anions are transported in the same direction as the particle, the cations in the reverse direction. It is easy to see that the excess of cations so gives rise to a retarding field and the defect of anions to an accelerating field, which explains the valency rule of Fig. 6. The small retardation by symmetrical electrolytes is due to a somewhat more complicated mechanism. The small effect for small >i a is analogous to the ideal behaviour of very dilute electrolytes. As to the small effect for large /c a see Ovefbsek, ref. 3b p. 210. 

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