Strong electrostatic retardation

From a comparison of Eq. (7A.6) with (7.51) the most interesting case of strong electrostatic retardation exists at... 

The second term on the left of Eq. (7A.17) is associated with the decrease of surfactant concentration at the outer boundary of the DL. This effect decreases with increasing r(0). Thus the applicability of the adsorption kinetics given by Eq. (7A.15) is restricted both at the initial stage and aroimd equilibrium. Nevertheless, the approximation Eq. (7A.15) is a useful solution to the problem of the DL effect on adsorption kinetics because Eq. (7A.8) holds at strong electrostatic retardation. 

Depending on the strength of specific retardation, three type of kinetics curves exist. At very strong specific retardation, the effects of both mechanisms multiply each other. At smaller adsorption barriers a two-step process results. First, both retardations act, and later the electrostatic effect outweighs the specific barrier. Finally, at very low adsorption barriers, only the electrostatic effect controls the retardation. 

The aim of this section is to consider the dynamic adsorption layer structure of ionic surfactant on the surface of rising bubbles. Results obtained in the previous section cannot be transferred directly to this case. The theory describing dynamic adsorption layers of ionic surfactant in general should take into accoimt the effect of electrostatic retardation of the adsorption kinetics of surfactant ions (Chapter 7). The structure of the dynamic adsorption layer of nonionic surfactants was analysed in the precedings section in the case when the adsorption process is kinetic controlled. In this case, it was assumed that the kinetic coefficients of adsorption and desorption do not depend on the surface coverage. On the other hand, the electrostatic barrier strongly depends on F , and therefore, the results of Section 9.1. cannot be used for the present case.. 

An estimate of the total desorption flow from the surface of a strongly retarded region in the neighbourhood of the rear pole of the bubble is derived as follows. When electrostatic retardation of adsorption-desorption kinetics does not exists, the results of Chapter 8 [Eq. (8.145)] can be applied. For ionic surfactant, the equation for surface tension variation somewhat differs from that for non-ionic surfactant. With regard to these differences, the following estimate of desorption flow results. 

The theory of the transport stage of elementary microflotation at strong surface retardation is confirmed by works of Reay Ratcliff (1975), Collins Jameson (1977) and Anfruns Kitchener (1976, 1977). Numerous confirmations of the possibility of contactless and collectorless microflotation, of the importance of overcoming or removing electrostatic barrier at microflotation and of the possibility of flotation even of hydrophilic particles through adsorption of cationic surfactant on bubble surface are presented in Section 10.5. 

An evaluation of the possibility of overcoming the electrostatic barrier by hydrodynamic pressing forces in the transient hydrodynamic regime has not been carried out, but it is clear that the situation will be much better at strong surface retardation than in the Stokes regime (cf. Section 10.5.2). 

For the QPh-x-MV2+ system, the methylviologen cation radical (MV K) generated by laser photolysis decayed with a rate constant of kb = 3.2 x 108 M-1 s-1. This relatively strong retardation of the back ET is due to the electrostatic repulsion of MV + by the polycation [76]. 

The strong retardation by NaPSt is due to the simultaneous contribution of the hydrophobic and electrostatic interactions between the polyanions and dye cations (deceleration factor 10 ). The OH is repelled by the electrostatic repulsion by the polyanions. Other polyanions, which lack the hydrophobic groups, decelerate the reactions much more moderately than NaPSt, because only the electrostatic interactions are operating. The strength of the hydrophobic interactions depends on the hydrophobicity of the dye cations. Thus, the ratio of the highest rate in the presence of CTABr to the lowest one in the presence of NaPSt amounts to 10 . The ratio for MG, which is least hydrophobic among the dyes studied, is about 10 . 

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