Water-nitrobenzene interface

FIG. 8 Complex impedance plot associated with the heterogeneous oxidation of Fc by ferri/ferro-cyanide at the water-nitrobenzene interface. The responses only in the presence of 0.1 M ferrocene ( ) are contrasted with ( ) those obtained upon addition of ImM K3Fe(CN)g and 0.1 mM K4Fe(CN)g. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

FIG. 3 Power spectra for capillary waves excited at the water-nitrobenzene interface (a) without CTAB molecules and (b) 10s after injection of a CTAB solution (0.5mL, lOmM) into the water phase. 

The equilibrium concentration of CfiH50Na above which the adsorption was saturated depends on the TBAB concentration. The relationship between the TBAB concentration and the CgH50Na concentration is shown in Fig. 9. These equilibrium concentrations were used to analyze the reaction between the two reactants, assuming that at these concentrations the reaction proceeds without residue and deficiency. The ratio of the TBAB concentration to the C6H50Na concentration deviates from the line for 1 1 below 50 mM. When the TBAB concentration is above 50 mM, the ratio of the TBAB concentration to the C6H50Na concentration at the water-nitrobenzene interface is unity. On the other hand, when the TBAB concentration is below 50 mM, the ratio of the CgHsONa concentration to the TBAB concentration is more than unity. 

Gross et al. [3] and Reid et al. [30] measured surface tension of the water-nitrobenzene interface in the presence of bromides of sodium and tetra-alkylammonium ions in water and tetra-alkylammonium tetraphenylborates in nitrobenzene, i.e., tetra-alkylammonium served as the potential-determining ion, cf. the scheme (13). The surface tension vs. the potential difference A p plot (electrocapillary curve), cf. Eq. (15), was constructed by varying the concentration of tetra-alkylammonium bromide in water, while holding... 

Recently, the newly developed time-resolved quasielastic laser scattering (QELS) has been applied to follow the changes in the surface tension of the nonpolarized water nitrobenzene interface upon the injection of cetyltrimethylammonium bromide [34] and sodium dodecyl sulfate [35] around or beyond their critical micelle concentrations. As a matter of fact, the method is based on the determination of the frequency of the thermally excited capillary waves at liquid-liquid interfaces. Since the capillary wave frequency is a function of the surface tension, and the change in the surface tension reflects the ion surface concentration, the QELS method allows us to observe the dynamic changes of the ITIES, such as the formation of monolayers of various surfactants [34]. 

Kakiuchi and Senda [36] measured the electrocapillary curves of the ideally polarized water nitrobenzene interface by the drop time method using the electrolyte dropping electrode [37] at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetraphenylborate) electrolytes. An example of the electrocapillary curve for this system is shown in Fig. 2. The surface excess charge density Q, and the relative surface excess concentrations T " and rppg of the Li cation and the tetraphenylborate anion respectively, were evaluated from the surface tension data by using Eq. (21). The relative surface excess concentrations and of the d anion and the... 

Girault and Schiffrin [6] and Samec et al. [39] used the pendant drop video-image method to measure the surface tension of the ideally polarized water-1,2-dichloroethane interface in the presence of KCl [6] or LiCl [39] in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane. Electrocapillary curves of a shape resembling that for the water-nitrobenzene interface were obtained, but a detailed analysis of the surface tension data was not undertaken. An independent measurement of the zero-charge potential difference by the streaming-jet electrode technique [40] in the same system provided the value identical with the potential of the electrocapillary maximum. On the basis of the standard potential difference of —0.225 V for the tetrabutylammonium ion transfer, the zero-charge potential difference was estimated as equal to 8 10 mV [41]. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

Samec et al. [15] used the AC polarographic method to study the potential dependence of the differential capacity of the ideally polarized water-nitrobenzene interface at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetra-phenylborate) electrolytes. The capacity showed a single minimum at an interfacial potential difference, which is close to that for the electrocapillary maximum. The experimental capacity was found to agree well with the capacity calculated from Eq. (28) for 1 /C,- = 0 and for the capacities of the space charge regions calculated using the GC theory,... 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

Kakiuehi et al. [84] studied the adsorption properties of two types of nonionic surfactants, sorbitan fatty acid esters and sucrose alkanoate, at the water-nitrobenzene interface. These surfactants lower the interfacial capacity in the range of the applied potential with no sign of desorption. On the other hand, the remarkable adsorption-desorption capacity peak analogous to the adsorption peak seen for organic molecules at the mercury-electrolyte interface can be observed in the presence of ionic surfactants, such as triazine dye ligands for proteins [85]. 

In some cases, an acceleration of the rate of ion transfer was observed [12]. Kakiuchi et al. [11,16] studied by AC impedance perchlorate, tetramethylammoniun (TMA+), and tetraethylammonium (TEA ) ion transfer across water-nitrobenzene interfaces covered by different PCs in the temperature range 5°C < t < 30°C. Short-tail lipids, such as DLPC and DPPC, showed a clear enhancement of the rate of ion transfer (Fig. 5). 

FIG. 5 Enhancement factor observed in the forward rate constant for TMA ( ) and TEA ( ) ion transfer at the water-nitrobenzene interface due to the presence of different PCs. (Experimental data are taken from Ref. 11 and correspond to 30°C.)... 

In both cases, the half-wave potential shifts by RT/ ziF)vaN per pH unit, and a typical example of such a behavior is given in Fig. 9 for the transfer of two acidic fi-diketones at the water-nitrobenzene interface. These results were unexpected, since a current wave is measured at a pH where the compound of interest is by a very large majority neutral, but they in fact represent the typical behavior of ionizable compounds at the ITIES and prove that the interfacial potential and the transfer of protons plays a key role for the distribution in biphasic systems. 

Normally, it is difficult to transfer metal ions across a water/organic solvent interface, which would be a convenient method for extraction of heavy metals. To perform this extraction despite this fact, Katano and Senda successfully tested the transfer of Pb2+ across a water/nitrobenzene interface by 18S6 <1996ANS683>. 

Fig. 16. Observed changes in (A) the apparent standard rate constant k, and (B) the apparent charge transfer coefficient Oq with the concentration of the aqueous base electrolyte (LiCl) for the transfer of (V) Mc4N+, ( ) Et4N, (T) Pr4N, and (O) PF across the water-nitrobenzene interface at 298 K. Composition of the nitrobenzene phase 0.1 M Pn4NPh4B. (After [145]).

Fig. 17. Apparent standard rate constant k% vs. the limiting ionic conductance in water for the transfer of (1) Pr4N +, (2) EtPrsN, (3) Et3PrN+, (4) Et4N4, (5) MejBuN+, (6) EtjMeN-", (7) MejPrN", (8) choline, (9) EtjMejN, (10) EtMe3N, (11) Me4N, and (12) Me3NH across the water-nitrobenzene interface. Vertical bars indicate the 95% confidence intervals. (After [42]).

The effect of temperature on ion transfer across the water-nitrobenzene interface was studied for a series of six quaternary ammonium and phosphonium cations and two anions using cyclic voltammetry and equilibrium impedance measurements [115]. Standard entropies (A S ) and enthalpies (A iT ) of ion transfer have been evaluated from the experimentally accessible reversible half-wave potential ( "572 and standard Gibbs energy of transfer (A G ),... 

Table 2. Thermodynamic functions, apparent activation energies and parameters of the stochastic theory for various ion transfer reactions at the water-nitrobenzene interface at 293 K [115].

In the pioneering study, Koryta et al. [94] found that the rate of the facilitated Na" ion transfer across the water-nitrobenzene interface in the presence of dibenzo-18-crown-6 is decelerated by adsorbed egg lecithin molecules, but only when the temperature is lower than 5 °C. Since the change in the ion transfer rate has been thought to be due to the phase transition of phospholipid, which normally occurs at a higher... 

A most advanced kinetic study was carried out by Senda and coworkers [166], who investigated the transfer of Na across the water-nitrobenzene interface in the presence of dibenzo-18-crown-6. This ligand is known to form a stable 1 1 complex with the transferred ion. It was shown that the ac impedance technique makes it possible to distinguish between three basic mechanisms, which can be described as (1) transfer of the ion followed by its complexation in the organic phase (the EC mechanism) (2) complexation of the ion in the aqueous phase followed by transfer of the complex ion (the CE mechanism) or (3) complexation of the ion at the interface with simultaneous transfer of the ioti from the aqueous to the organic phase (the E mechanism). For the system studied, the E mechanism described by Eq. (58) has turned out to be most probable. The apparent rate constant of the assisted ion transfer... 

As to results for the water-oil interface, fig. 4.22 gives a charge-potential curve for the water-nitrobenzene interface. These results were obtained by Samec et al. who solved the polarization problem by adding LiCl to the water and tetrabufylam-monlum tetraphenylborate (TBATPB) to the nitrobenzene. The former electrol5de... 

Figure 4.22. Surface charge at the aqueous side of the water-nitrobenzene interface. The aqueous concentration of LiCl is indicated. The nitrobenzene contains tribuiylainmonium tetraphenylborate. The potential is referred to the common intersection point, taken as a zero point. Temperature 22°C. (Redrawn from Samec et al., loc. cit.)...

Estimate the Galvani potential difference for KI at the water nitrobenzene interface using the data for the standard Gibbs energy of transfer given in table 8.11. 

---