Surface active ion

Specific adsorption of ions changes the value of E, hence, one distinguishes the notion of a point of zero charge, in solutions of surface-inactive electrolytes, which depends on the metal, from that of a point of zero charge, in solutions of surface-active ions, which in addition depends on the nature and concentration of these ions. The difference between these quantities. 

FIGURE 14.6 Influence of surface-active ions [N(C4H9)4]+ (curve 2) and I (curve 3) on the polarization curve for hydrogen evolution at a mercury electrode in acidic solutions (curve 1 is for the base electrolyte). 

Figure 4 illustrates the dependence of on Aq for the case when r = 1 at several different values of [Fig. 4(a)] and when = 0.5 and at several different values of r [Fig. 4(b)]. From Fig. 4(a), one can see that takes a maximum around y = 0, i.e., Aq The volume ratio affects strongly the value of as shown in Fig. 4(b), which is ascribed to the dependence of the equilibrium concentration on r through Eq. (25). This simple example illustrates the necessity of taking into account the variation of the phase-boundary potential, and hence the adsorption of i, when one tries to measure the adsorption properties of a certain ionic species in the oil-water two-phase systems by changing the concentration of i in one of the phases. A similar situation exists also in voltammetric measurements of the transfer of surface-active ions across the polarized O/W interface. In this case, the time-varying thickness of the diffusion layers plays the role of the fixed volume in the above partition example. The adsorption of surface-active ions is hence expected to reach a maximum around the half-wave potential of the ion transfer. 

Kong, R.C., Sachok, B., and Deming, S.N. (1980), Combined Effects of pH and Surface-Active-Ion Concentration in Reversed-Phase Liquid Chromatography, J. Chromatogr., 199, 307-316. 

The dispersion interaction between the surface active ions and the water-air interface was recently considered in the modeling of the equilibrium adsorption [62]. The molecular dynamic simulations are used in the recent years to describe the surfactant adsorption at the air-water interface [63-65],... 

CftNBr-CTFNa (cationic-anionic) system. The common cat ionic-anionic mixture of HC surfactants is highly surface active (17), showing the strong interactions between the two oppositely charged surface active ions. Similar re.-sults have been observed in the case of CeNBr-CrFNa system. The "y-log C plot in Fig.5 illustrates such an interaction, lile can see that the 1i1 CeNBr-C-rFNa mixture is much more surface active than CsNBr or CjFNa, The cmc value of surfactants mixture is much more smaller than that of CaNBr or CyFNa, and the-yeme the mixed solution is very low (< 15 rnNrn- ) mixtures with various raolal... 

The physical properties of surface active agents differ from those of smaller or nonamphipathic molecules in one major aspect, namely, the abrupt changes in their properties above a critical concentration. This is illustrated in Fig. 1, in which a number of physical properties (surface tension, osmotic pressure, turbidity, solubilization, magnetic resonance, conductivity, and self-diffusion) are plotted as a function of concentration. All these properties (interfacial and bulk) show an abrupt change at a particular concentration, which is consistent with the fact that above this concentration, surface active ions or molecules in solution associate to form larger units. These association units are called micelles and the concentration at which this association phenomenon occurs is known as the critical micelle concentration (cmc). 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

It is very doubtful, however, if these solutions can be treated as two component systems they contain water and at least two ions, of which the adsorption is not the same and soaps, unless rather strongly alkaline, contain free fatty acid which is much more strongly adsorbed than the anion of the fatty acid. The course of a surface tension-concentration curve in a solution with several components may be complicated it seems possible that a rise of tension with increasing concentration may be due to a moderately surface active ion, present in large amount, displacing a more surface active component, present in smaller amount, from the surface. 

The concept of the electrochemical instability has been illustrated using a simple model for the coupling of the adsorption and partition of surface-active ions. Three main features of the electrochemical instability, that is, (1) the location of the instability in the values of the phase-boundary potential, (2) the existence of a window-like instability... 

Surface Charge at Interfaces. The interface in a crude-oil-water system usually carries a net charge, which can be caused by the adsorption of surface-active ions. These surfactants may be carboxylic acids that... 

Fig. 18. (a) The lines of electrical force radiate in all directions from a single spherical charge in bulk solution, and the electrical potential falls off rapidly with distance (b) The lines of electrical force from surface-active ions are all confined to the aqueous side of the interface. At a small distance below the surface they have become parallel. The electrical potential is high and remains important at relatively great distances away from the interface (Davies, 33). 

A surface charge may be established by adsorption of a hydrophobic species or a surfactant ion. Preferential adsorption of a surface-active ion can arise from so-called hydrophobic bonding or from bonding via hydrogen bonds or from London-van der Waals interactions. The mechanism of sorption of some ions (e.g., fulvates or humates) is not certain. Ionic species carrying a hydrophobic moiety may bind inner-spherically or outer-spherically depending on whether the surface-coordinative or the hydrophobic interaction prevails. 

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