Dichloroethane /water systems interface

The ionic potentials can be experimentally determined either with the use of galvanic cells containing interfaces of the type in Scheme 7 or electroanalytically, using for instance, polarography, voltammetry, or chronopotentiometry. The values of and Aj f, obtained with the use of electrochemical methods for the water-1,2-dichloroethane, water-dichloromethane, water-acetophenone, water-methyl-isobutyl ketone, o-nitrotol-uene, and chloroform systems, and recently for 2-heptanone and 2-octanone [43] systems, have been published. These data are listed in many papers [1-10,14,37]. The most probable values for a few ions in water-nitrobenzene and water-1,2-dichloroethane systems are presented in Table 1. 

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]. 

Monte Carlo and molecular dynamics calculations of the density profile of model system of benzene-water [70], 1,2-dichloroethane-water [71], and decane-water [72] interfaces show that the thickness of the transition region at the interface is molecu-larly sharp, typically within 0.5 nm, rather than diffuse (Fig. 4). A similar sharp density profile has been reported also at several liquid-vapor interfaces [73, 74]. The sharpness of interfaces thus seems to be a general characteristic of the boundary between two stable phases and it is likely that the presence of supporting electrolytes would not significantly alter the thickness of the transition region at an ITIES. The interfacial mixed solvent layer [54, 55], if any, would probably have a thickness comparable with this thin inner layer. 

Benjamin recently found for a 1,2-dichloroethane-water model system that, although the interface was molecularly sharp on time-average over hundreds of picoseconds, thermal fluctuations superimposed capillary waves as long as 0.8 nm on the sharp interface and generated a rough surface on the timescale of tens of... 

Very little has been done regarding the kinetic study of assisted ion transfer reactions. Senda et studied the transfer of sodium at the water-nitrobenzene interface facilitated by dibenzo-18 Crown-6 in order to elucidate the mechanism of the transfer, and concluded that the transfer occured by a TIC mechanism. Recently, Shao revisited this system at the water-1,2-dichloroethane interface. The results obtained for the following charge transfer reactions are illustrated in Fig. 16. 

Geblewicz and Schiffrin have studied the system [Fe(CN)6] / in water-Lutetium biphthalocyanine in 1,2-dichloroethane, and very recently Cheng and Schiffrin investigated the systems [Fe(CN)6] / in water-bis(pyridine) mejo-tetraphenylporphyrinato iron(II) and ruthenium(III) in 1,2-dichloroethane. These systems have the advantage that none of the products of the reaction would cross the interface, thereby impeding the measurements. 

Every liquid interface is usually electrified by ion separation, dipole orientation, or both (Section II). It is convenient to distinguish two groups of immiscible liquid-liquid interfaces water-polar solvent, such as nitrobenzene and 1,2-dichloroethane, and water-nonpolar solvent, e.g., octane or decane interfaces. For the second group it is impossible to investigate the interphase electrochemical equilibria and the Galvani potentials, whereas it is normal practice for the first group (Section III). On the other hand, these systems are very important as parts of the voltaic cells. They make it possible to measure the surface potential differences and the adsorption potentials (Section IV). 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

Most of the liquid-liquid interfaces that have been studied involve water and an organic solvent such as nitrobenzene or 1,2-dichloroethane (1,2-DCE). Although these systems form stable interfaces, the solubility of one solvent in the other is usually quite high. For example, the solubility of water in 1,2-DCE is 0.11 M, and that of 1,2-DCE in water is 0.09 M. So each of the two liquid components is a fairly concentrated solution of one solvent in the other. It is therefore unlikely that the interface is sharp on a molecular level. We rather expect an extended region with a thickness of the order of a few solvent diameters, over which the concentrations of the two solvents change rapidly (see Fig. 12.1). The lower the solubility of one solvent in the other, the thinner this interfacial region should be. These expectations are supported by the indication that the dipole potentials at these interfaces seem to be small, at least near the pzc, but spectroscopic information is lacking at present. 

The MEMED technique has been used to study the hydrolysis of aliphatic acid chlorides in a water/l,2-dichloroethane (DCE) solvent system [3]. It was shown unambiguously that the reaction proceeds via an interfacial process and shows saturation kinetics as the concentration of acid chloride in the DCE increases the data were well fitted to a model based on a pre-equilibrium involving Langmuir adsorption at the interface. First-order rate constants for interfacial solvolysis of CH3(CH2) COCl were 300 150(n = 2), 200 100(n = 3) and 120 60 s-1( = 8). 

Fig. 4. Average density of water and 1,2-dichloroethane (DCE) at 300 K. Solid lines, density calculated relative to the system s center of mass dotted lines, densities calculated relative to the location of the interface. (Reprinted from [71] with permission. Copyright American Institute of Physics).

By using cyclic voltammetry, Schiffrin and coworkers [26, 186, 187, 189] studied electron transfer across the water-1,2-dichloroethane interface between the redox couple FefCNls /Fe(CN)6 in water, and lutetium(III) [186] and tin(IV) [26, 187] diphthalocyanines and bis(pyridine)-me50-tetraphenylporphyrinato-iron(II) or ru-thenium(III) [189] in the organic solvent. An essential advantage of these systems is that none of the reactants or products can cross the interface and interfere with the electron transfer reaction, which could be clearly demonstrated. Owing to a much higher concentration of the aqueous redox couple, the pseudo-first order electron transfer reactions could be analyzed with the help of the Nicholson-Shain theory. However, though they have all appeared to be quasireversible, kinetic analysis was restricted to an evaluation of the apparent standard rate constant o. which was found to be of the order of 10 cm s [186, 189]. Marcus [199] has derived a relationship between the pseudo-first-order rate constant for the reaction (8) and the rate... 

Figure 10. Electronic absorption line shape of N,N -diethyl-p-nitroaniline in several bulk and interfacial systems, calculated by molecular dynamics computer simulation at 300K. (a) The spectrum in bulk water (solid line) and at the water liquid/vapor interface (dashed line), (b) The spectrum in bulk 1,2-dichloroethane (solid line) and at the water/1,2-dichloroethane interface.

We now return to the example introduced in Section 20.1.1, where two electrolyte solutions a and P were placed in contact and a hypothetical barrier was invoked that prevented the transfer of one of the ions between the phases. Although this example may have seemed obscure, such a system may readily be constructed for the interface between two immiscible electrolyte solutions (ITIES). Such a system can be formed by contacting water with an immiscible organic solvent such as nitrobenzene or 1,2-dichloroethane, as discussed in Section 17.3 in Chapter 17 of this handbook. The two solvents possess a slight mutual solubility. Once the two phases are equilibrated, the system is composed of an organic saturated aqueous phase in contact with a water saturated organic phase. For this reason, measmement of any transport property should always be performed on mutually pre-saturated solutions. 

Using the above cells, the potentials of the reference systems have been determined, e.g. for water-nitrobenzene and water-1,2 dichloroethane interfaces [65, 66, 125] ... 

At the present time the electrochemistry of immiscible electrolyte solutions is developing mainly on the basis of the studies of the water-nitrobenzene and water-1,2-dichloroethane interfaces. The polarizability ranges of these interfaces in the presence of typical electrolytes (system XV) are about 0.30 V. Extension of these ranges towards negative currents has been achieved by substitution of TBA" ions by tetraphenyl-arsonium ions [145], and by crystal violet cations [133,134]. But the substitution of TPhB ions by dicarboUyl cobaltate (III) anions leads to an extension of the above range towards positive currents [127]. 

The classical electrochemical methodologies have been applied outside the classical water-nitrobenzene (NB) or water-l,2-dichloroethane (DCE) interface new solvent systems have been investigated and, in particular, Kakiuchi et al. have demonstrated that organic electrolyte solutions can be replaced by ionic liquids, also called Room Temperature Molten Salts (RTMS). 

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