Salt concentration, interfacial potential

Normally electrode reactions take place in solutions, or sometimes in molten salts (e.g. aluminium extraction). In order to minimize the phenomenon of migration of the electroactive ions caused by the electric field (Chapter 2) and to confine the interfacial potential difference to the distance of closest approach of solvated ions to the electrode (Chapter 3), the addition of a solution containing a high concentration of inert electrolyte, called supporting electrolyte, is necessary. This has a concentration at least 100 times that of the electroactive species and is the principal source of electrically conducting ionic species. The concentration of supporting electrolyte varies normally between 0.01m and 1.0 m, the concentration of electroactive species being 5 mM or less. The... 

An example of differential capacitance curves is given In fig. 3.49. As with silver iodide there are minima close to the e.c.m. in low concentrations of electrolyte, but beyond that the inner layer capacitances Cj and C dominate. Here the curves show much detail as a function of potential and salt concentration. Numerous attempts have been made to interpret these curves in terms of Interfacial polarization. We shall not discuss this here, except to mention that relatively simple models of adjacent water (counting only dipole moments and allowing for only a few orientations) already work relatively well. 

Potentiometric Results. As shown earlier, a single salt concentration variation has no effect on the interfacial potential. Thus, to study the effect of the dye cation on the interfacial potential, other ions must be present. Supporting electrolytes, selected in such a way that an ideally polarizable interface is formed when the dye is absent, are conveniently used. 

Dissolved inorganic solids have been shown to inhibit organic recovery eflBciency by freeze concentration from aqueous solution (4). Similarly, increased salt content reduces cationic recovery (Figure 7). As ionic concentration increases at the interface, there is an increase in tendency toward dendritic growth and associated entrapment, changes in surface tension and induced interfacial potential, and impedance of specific ionic migration. 

Figure 6 shows the effect of surfactant concentration on interfacial tension and electrophoretic mobility of oil droplets (14). It is evident that the minimum in interfacial tension corresponds to a maximum in electrophoretic mobility and hence in zeta potential at the oil/brine interface. Similar to the electrocapillary effect observed in mercury/water systems, we believe that the high surface charge density at the oil/brine interface also contributes to lowering of the interfacial tension. This correlation was also observed for the effect of caustic concentration on the interfacial tension of several crude oils (Figure 7). Here also, the minimum interfacial tension and the maximum electrophoretic mobility occurred in the same range of caustic concentration (17). Similar correlation for the effect of salt concentration on the interfacial tension and electrophoretic mobility of a crude oil was also observed (18). Thus, we believe that surface charge density at the oil/brine interface is an important component of the ultralow interfacial tension.

Also known as surface or interfacial potential (designated by symbol V), it is opposite to that observed with the diffusion potential. For this interfacial potential to appear, a difference in salt concentrations across the BLM is not required it depends upon the sorbed species and/or the dipoles. With cationic interface-active compounds, such as hexadecyltrimethy-lammonium bromide (HDTAB) andphos-phatidylethanolamine (PE) at low pH, a negative potential results and vice versa with anionic interface-active compounds, such as dodecyl acid phosphate (DAP) and phosphatidylserine (PS). The magnitude of V decreases with increasing of the salt concentration. Two other caveats should be noted about this potential (1) it may have only transient existence however, the time for its decay can take a long time and (2) the law of electroneutrality at the interfaces is not obeyed [1]. 

As illustration, we consider the interpretation of experimental isotherms by Tajima et al. [40,42,43] for the surface tension o versus SDS concentrations at 11 fixed concentrations of NaCl, see Figure 4.2. Processing the set of data for the interfacial tension o = o(Ci , C2J as a function of the bulk concentrations of surfactant (DS ) ions and Na" counterions, and C2 , we can determine the surfactant adsorption, Fi(ci , C2J, the counterion adsorption, F2(Ci , C2J), the surface potential, Vs(Ci< . Czo=)> and the Gibbs elasticity gCcioo, C2J for every desirable surfactant and salt concentrations. 

In experiments on nonionic surfactants, namely Triton X-405 Geeraerts at al. (1993) performed simultaneously dynamic surface tension and potential measurements in order to discuss peculiarities of nonionic surfactants containing oxethylene chains of different lengths as hydrophilic part. Deviations from a diffusion controlled adsorption were explained by dipole relaxations. In recent papers by Fainerman et al. (1994b, c, d) and Fainerman Miller (1994a, b) developed a new model to explain the adsorption kinetics of a series of Triton X molecules with 4 to 40 oxethylene groups. This model assumes two different orientations of the nonionic molecule and explains the observed deviations of the experimental data from a pure diffusion controlled adsorption very well. Measurements in a wide temperature interval and in presence of salts known as structure breaker were performed which supported the new idea of different molecular interfacial orientations. At small concentration and short adsorption times the kinetics can be described by a usual diffusion model. Experiments of Liggieri et al. (1994) on Triton X-100 at the hexane/water interface show the same results. 

This analysis leads us to the conclusion that ionic surfactants in salt-free solutions undergo kinetically limited adsorption. Indeed, dynamic surface tension curves of such solutions do not exhibit the diffusive asymptotic time dependence of non-ionic surfactants, depicted in Fig. 1. The scheme of Section 2, focusing on the diffusive transport inside the solution, is no longer valid. Instead, the diffusive relaxation in the bulk solution is practically immediate and we should concentrate on the interfacial kinetics, Eq. (21). In this case the subsurface volume fraction, t, obeys the Boltzmann distribution, not the Davies adsorption isotherm (15), and the electric potential is given by the Poisson-Boltzmann theory. By these observations Eq. (21) can be expressed as a function of the surface... 

An increase in the concentration of the supporting electrolyte in the presence of an organic substance results in a decrease in the interfacial tension y. But in the case of a capillary inactive electrolyte, Na2S04, this decrease in y is caused by an increase of the chemical potential of the organic substance ( salting out effect), rather than by adsorption of the inorganic ions [3]. 

The non-equilibrium effects on potentiometric responses can be described using the concept of mixed ion-transfer potential (47). When ion transfers at a sample solution/mem-brane interface are fast enough, a local equilibrium at the interface is always achieved. The salt-extraction and ion-exchange processes, however, induce concentration polarization of the ions near the interface so that the potential is determined by the interfacial ion concentration as... 

Valinomycin dissolved in heptane interacts with aqueous solutions of potassium and sodium picrates. This does not change the interfacial tension as compared to the systems without salt [115]. On the other hand, the study of valinomycin monolayers at the water-air interface proved KCl and NaCl to similarly interact with monolayers only up to a concentration of 0.5 M. At a KCl concentration of 0.7 M the surface potential amounts to 1050 mV, while it is only 550 mV at the same concentration of... 

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