Potential phase boundary

Potentiometric responses of liquid membrane ISEs depend on a change in the phase boundary potential at the membrane/sample solution interface, which is controlled by bulk equilibrium (7). When an ion, i, with charge Zj is transferred across the interface between the sample and membrane phases, the ion-transfer reaction is defined as 

Under equilibrium conditions between the two phases, the electrochemical potentials of the ion in the sample and membrane phases, and /if , respectively, is equal 

To obtain a Nemstian response to the ion, its membrane activity must be constant and independent of the sample activity so that equation (7.2.4) can be simplified to 

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It is this potential difference that is discussed in Chapter IV in connection with monomolecular films. Since it is developed in the space between the phases, none of the uncertainties of phase boundary potentials is involved. 

Such phase boundary potentials were already described in 1902 by Nernst and Riesenfeld 20) who investigated oil cells saturated with a common compound. A classical review of this research dealing with membrane potentials was published in 1922 by Michaelis 21). 

Finally some assumptions could not be verified as, e.g., the complete co-ion exclusion necessary for the treatment of the phase boundary potential as a Donnan potential, or the constant ion mobility through glass membranes with hydrated layers76). 

Pungor E (1998) The theory of Ion Selective Electrodes. Anal Sci 14 249-256 Bakker E, Buhlmann P, Pretsch E (2004) The phase-boundary potential model. Talanta 63 3-20... 

The availability of thermodynamically reliable quantities at liquid interfaces is advantageous as a reference in examining data obtained by other surface specific techniques. The model-independent solid information about thermodynamics of adsorption can be used as a norm in microscopic interpretation and understanding of currently available surface specific experimental techniques and theoretical approaches such as molecular dynamics simulations. This chapter will focus on the adsorption at the polarized liquid-liquid interfaces, which enable us to externally control the phase-boundary potential, providing an additional degree of freedom in studying the adsorption of electrified interfaces. A main emphasis will be on some aspects that have not been fully dealt with in previous reviews and monographs [8-21]. 

Every interface is more or less electrically charged, unless special care is exercised experimentally [26]. The energy of the system containing the interface hence depends on its electrical state. The thermodynamics of interfaces that explicitly takes account of the contribution of the phase-boundary potential is called the thermodynamics of electrocapillarity [27]. Thermodynamic treatments of the electrocapillary phenomena at the electrode solution interface have been generalized to the polarized as well as nonpolarized liquid liquid interface by Kakiuchi [28] and further by Markin and Volkov [29]. We summarize the essential idea of the electrocapillary equation, so far as it will be required in the following. The electrocapillary equation for a polarized liquid-liquid interface has the form... 

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

In the case of the adsorption at a liquid-liquid interface, the adsorption is possible from both sides of the interface and hence the adsorption of an adsorbate at the interface is not independent of its partitioning between the two phases. This link between the adsorption and partition is unique to liquid-liquid interfaces and is of particular importance in the case of the adsorption of ionic components, since both adsorption and partition of ionic components strongly depend on the phase-boundary potential [23]. 

FIG. 2 A schematic representation of the relationship between adsorption free energy for the adsorption from the phase a (a = O or W), and the phase and phase-boundary potential,... 

The other is AG g, at the potential of zero charge (PZC), where no direct electrostatic effect is expected. The former reflects the affinity to the interfacial region when the driving forces toward the interface from W and from O are balanced, notwithstanding that the surface activity at Aq f is usually different from the PZC. AG g, values at the PZC are, however, useful in comparing the intrinsic or chemical surface activities of ionic compounds. 

Effect of Phase-Boundary Potential on Adsorption Free Energy... 

FIG. 3 An illustration of the change in adsorption free energy with phase-boundary potential. 

We consider an oil-water two-phase system, which contains an ionic surfactant i. If we vary the phase-boundary potential either by externally applying the voltage using two electrodes or by adjusting the solution composition of potential determining ions, the concentration of i in each phase varies accordingly, keeping the total amount of i in the system, m, constant. The condition of the latter is... 

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. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

Comparison of the calculated and observed changes in the EMF is shown in Table 1. It can be seen that the calculated changes in the phase boundary potential of membranes with 1.0 mM 1-3 in contact with 0.1 and 0.01 M aqueous KCl or RbCl were in good agreement with the corresponding observed values. Such an agreement indicates that it is reasonable to apply the present surface model to explain that the phase boundary potential is, in fact, determined by the amount of the primary cation permeated into or released out of the membrane side of the interface. 

Karpfen, F. M., and J. E. B. Randles, Ionic equilibria and phase-boundary potentials in oil-water systems, Trans. Faraday Soc.f 49, 823 (1953). 

The theory of ion-selective electrode response is well developed, due to the works of Eisenman, Buck and others [23], Three models used for the description of the ISE response through the years, namely kinetic, membrane surface (or space charge) and phase boundary potential (PBP) models, although being seemingly contradictory, give similar results in most cases [7], The first two sophisticated models are out of the scope of the present chapter, as the PBP model, despite its simplicity, satisfactorily explains most of the experimental results and thus has become widely applicable. The... 

PBP model considers the membrane potential as a sum of the potentials formed at the membrane-solution interfaces (phase boundary potentials), and generally neglects any diffusion potential within the membrane ... 

In comparison to Eq. (11) if the activity of sodium ions is constant for a current pulse of fixed duration and magnitude the phase boundary potential is a function of protamine concentration in the sample. 

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