Water and nitrobenzene

For interfaces between liquid electrolytes, we can distinguish three cases (1) interfaces between similar electrolytes, (2) interfaces between dissimilar but miscible electrolytes, and (3) interfaces between immiscible electrolytes. In the first case the two electrolytes have the same solvent (medium), but they differ in the nature and/or concentration of solutes. In the second case the interface separates dissimilar media (e.g., solutions in water and ethanol). An example for the third case is a system consisting of salt solutions in water and nitrobenzene. The interface between immiscible dissimilar liquid electrolytes is discussed in more detail in Chapter 32. 

TABLE 32.2 Standard Gibbs Energy of Transfer and Standard Ion Transfer Potentials for Ion Transfer Between Water and Nitrobenzene Derived from Partition Measurements... 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Davies has given values for the distribution coefficients of several simple salts between water and nitrobenzene. These figures were substituted in eqn. (6) and compared with the e.m.f. measurements obtained from the cells of the following type ... 

An ITIES is formed between water and nitrobenzene (NB) using electrolytes with the picrate (Pic ) anion at the same activity (0.1 M). The electrolyte in water is Li Pic and that in NB, tetraphenylarsonium (TA" ") picrate. Given that the standard Gibbs energy of transfer of Pic is -4.6 kJmoU [18], estimate the Galvani potential difference at the interface. Use the data in table 8.11 to estimate the activity of Li" " in NB. 

Figure 6.8.1 Schematic diagram for the apparatus for cyclic voltammetry at the ITIES between water and nitrobenzene. Ref a and Ref p are reference electrodes, Wk a and Wk p are metal working electrodes.

If an ion with a smaller Gibbs energy of transfer than that of the other ions is introduced into one of the phases, it will transfer at smaller values of within the potential window governed by the other ions. For example, when tetramethylammonium ion (TMA" ) is added to the aqueous phase, its value is such that it transfers more readily than TBuA" between the water and nitrobenzene (Figure 6.8.2Z ). When the concentration of this ion is small (typically 0.1 to 1 mM), the rate of transfer across the interface is usually governed by the rate of mass transfer of the ion to the interface. Under these conditions, a scan of current va. potential drop across the interface resembles a typical cyclic voltammogram (Figure 6.8.35). 

The membrane or interfacial potential, particularly in biological applications, is often determined from the change in fluorescence of added carbocyanine dyes (12, 13). The fluorescence intensity of the dyes depends on the solvent in which the dyes are present. When the dyes are used as potential-sensitive probes, their fluorescent intensity is a function of the interfacial potential across the membrane. We studied the behavior of dye transport on a phase boundary between water and nitrobenzene to better understand the principles of the potential dye partitioning as a function of interfacial potential (141... 

Table I. Standard Potentials of Transfer between Water and Nitrobenzene for Ions Used in the Potentiometry Measurements...

Small LL interfaces have been used by Girault and co-workers (33-38) and by Senda et al. (39, 40). We have used a small hole formed in a thin glass wall (41-43). Figure 16 shows the voltammetric response of lauryl sulfate anion transport between water and nitrobenzene. Recent analytical applications of these microinterfaces have resulted in construction of gel-solidified probes. The advantage of such a modification is ease of handling (44-47). The immobilization can be extended further to studies of frozen interfaces, or even to solid electrolytes. Significantly, ITIES theory also applies to interfaces that are encountered in ion-doped, conductive, polymer-coated electrodes. 

From distribution experiments at 25°C [150] water and nitrobenzene are mutually saturated. Calculated from the modified Born expression for single-ion transfer [Eq. (17)] with = 34.8 (Table 5) and A, = 0.080 nm. 

Fig. 1.2.4 Cyclic voltammogram of the transfer of tetramethylammonium ions between water and nitrobenzene. c(TmeA ) = 4.7 x 10 mol L the supporting electrolyte is in the aqueous phase 0.1 mol L LiCl, and in nitrobenzene 0.1 mol L tetrabutylammonium tetraphenylborate the scan rate is 20 mV s . (Adapted from [11], with permission)...

The interface between two immiscible solutions (e.g. water and nitrobenzene) containing dissolved species is a site of an electric potential. By measuring this potential difference at the aqueous elec-trolyte/solid electrolyte phase boundary, the phenomena taking place at the interface between two immiscible solutions or the membranes of ion-selective electrode have been studied. Changing the composition of the solutions in contact can alter this potential or applied current can alter the composition of the solutions. Thus, judicious choice of applied potential or current can be used to study the structure of the interface. Since the interface is ul-trathin (< cl nm), it cannot be observed directly. It can be, however, investigated by electrochemical or optical methods [14,... 

Example 1 At a pressure of 101.3 kNm a mixture of water and nitrobenzene distills over at 372 K. The vapour pressure of water at 372K is 97.70 kNmr. Estimate the proportion by weight of nitrobenzene in the distillate. 

At the zero-charge potential the ionic component is equal to zero, thus the zero-charge potential itself is equal to the dipole component. It can be presumed on the basis of the water and nitrobenzene surface potentials (Sect. 5) that the value of A g(dipole) is negligible, and likely to be close to zero [155] (Sect. 5). This is also supported by the previously determined values of zero-charge potentials [61, 137, 145, 149, 150, 160-162]. Dispersion of these data (from 0 to about 60 mV - Table 6) is not only... 

It has been estabUshed that the values of AJ qX and A x are equal to 0.24+0.01 V [119] and 0.10+0.02 V [163], respectively. The above data suggest that the presence of water in nitrobenzene and, in particular, of nitrobenzene in water strongly alters their surface structure. It can be supposed that once mutually saturated water and nitrobenzene achieve a direct contact, their surface structures become even more similar. Such a case would correspond to a lower difference of surface potentials, A g(dipole) (Sect. 4). Hence the experimentally justified inequahty can be written as follows [155] ... 

The pioneering electrochemical investigation of the structure of the ITIES was carried out in 1977 by Gavach and coworkers. They studied first the interface between two solutions of tetraalkylammonium bromide partitioned between water and nitrobenzene. By varying the concentration of the salt and by... 

They analyzed their results using a thermodynamic approach based on the Gibbs adsorption equation and the main conclusion of their work was that relative surface excesses of the ionic species were well described by the Gouy-Chapman theory. They adopted the MVN model of the ideally polarized interface stating that the compact layer is an ion-free layer consisting of laminated layers of water and nitrobenzene sandwiched between two diffuse layers. The potential difference across this inner layer was estimated to be about 20 mV at the PZC but was found to vary with the surface charge density. 

Evnochides and Thodos [9] evaporated water and nitrobenzene into air from celite spheres and obtained the following correlation for the mass transfer coefficient ... 

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