Electrolyte interfaces immiscible, applications

APPLICATIONS OF IMMISCIBLE ELECTROLYTE INTERFACES IN ANALYTICAL CHEMISTRY... 

The structure of the interface between two immiscible electrolyte solutions (ITIES) has been the matter of considerable interest since the beginning of the last century [1], Typically, such a system consists of water (w) and an organic solvent (o) immiscible with it, each containing an electrolyte. Much information about the ITIES has been gained by application of techniques that involve measurements of the macroscopic properties, such as surface tension or differential capacity. The analysis of these properties in terms of various microscopic models has allowed us to draw some conclusions about the distribution and orientation of ions and neutral molecules at the ITIES. The purpose of the present chapter is to summarize the key results in this field. 

Nevertheless, an important application of electrostatic models is to the interface between two immiscible electrolyte solutions. This can be viewed as two electrolyte double layers arranged back to back. In reality, however, total immiscibility never occurs and the degree of miscibility increases with the presence of electrolyte, so that corrections to the models need to be introduced. 

In Section 2.3.6 we considered ion transfer at the interface between two immiscible electrolyte solutions (ITIES), where we found that a potential difference can arise because of differential transfer of ions. Ion movement across the interface can also be driven by the application of an external potential, and the rate of ion transfer can be detected as a current flow. This response allows one to examine the ITIES via voltammetric methods in the same way that electron transfer can be monitored at electrode surfaces (29-32). 

Interfaces between two immiscible solutions with dissolved electrolytes, which are most interesting to workers in several disciplines, cover theoretical physical electrochemistry and analytical applications for sensor design. These interfaces are used in interpretation of processes that occur in biological membranes and in biological systems. The interface between two immiscible electrolyte solutions was studied for the first time at least 100 years ago by Nemst (I), who performed the experiments that provide the theoretical basis for current potentiometric and voltammetric studies of interfaces. In 1963, Blank and Feig (2) suggested that an interface between two immiscible liquids could be used as a model (at least as a crude approximation) for... 

Before closing this section, the case of polarized interfaces has to be introduced since SSHG at interfaces between two immiscible electrolyte solutions (ITIES) constitues one of the main trends of nonlinear optical applications at liquid/liquid interfaces. It has been shown for metals, that upon polarization by an externally applied electric potential, a specific SH response was generated from the coupling between the static dc-field established across the interface and the fundamental electromagnetic wave [20]. The main property of this contribution is that it evolves... 

A future application of PBD techniques would be the study of ion exchange across liquid/hquid interfaces like those found in ITIES (interface between two immiscible electrolyte solutions) systems. Similarly, the effect of potential on the ion transfer across artificial or natural membranes could be studied by PBD. 

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

Application of assumptions (1) and (2) and of cells of type XII allows construction of a common potential scale for various interfaces of immiscible electrolyte solutions. To achieve this goal in practice it is necessary to determine experimentally the standard distribution potential of TEAPi, or the formal potential of TBA" or any other reference standard in the water-given solvent system (w/s), versus the zero TEAPi water-nitrobenzene system. For this purpose, the cells schematically shown below can be used [64-67, 124-125] ... 

The geometry of real polymer membranes still induces some problems with quantitative application of model calculations, and calibration procedure remains more or less empiric. However, the model systems imitating membranes, the interfaces of two immiscible electrolyte solutions (ITIES), are free from this shortcoming. Various types of LJP behavior for ITIES dependent on the ratios of ion partition coefficients are considered in ref. [95] remarks in ref. [96] are also useful. The effect of initial concentration distribution on the temporal LJP behavior is considered in ref. [97] self-consistently for the limiting cases of thick membranes (assumed to operate as ion-selective electrodes) and thin membranes (assumed to imitate biological membranes). 

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