Kinetics, solvent extraction

Catalytic Effect of the Liquid-Liquid Interface in Solvent Extraction Kinetics Hitoshi Watarai... 

A significant advance was made in this field by Watarai and Freiser [58], who developed a high-speed automatic system for solvent extraction kinetic studies. The extraction vessel was a 200 mL Morton flask fitted with a high speed stirrer (0-20,000 rpm) and a teflon phase separator. The mass transport rates generated with this approach were considered to be sufficiently high to effectively outrun the kinetics of the chemical processes of interest. With the aid of the separator, the bulk organic phase was cleanly separated from a fine dispersion of the two phases in the flask, circulated through a spectrophotometric flow cell, and returned to the reaction vessel. 

One of the most attractive roles of liquid liquid interfaces that we found in solvent extraction kinetics of metal ions is a catalytic effect. Shaking or stirring of the solvent extraction system generates a wide interfacial area or a large specific interfacial area defined as the interfacial area divided by a bulk phase volume. Metal extractants have a molecular structure which has both hydrophilic and hydrophobic groups. Therefore, they have a property of interfacial adsorptivity much like surfactant molecules. Adsorption of extractant at the liquid liquid interface can dramatically facilitate the interfacial com-plexation which has been exploited from our research. 

II. HOW TO STUDY THE INTERFACIAL REACTION IN SOLVENT EXTRACTION KINETICS... 

III. SOLVENT EXTRACTION KINETICS AND CATALYTIC INTERFACIAL COMPLEXATION... 

Diffusion is a complex phenomenon. A complete physical description involves conceptual and mathematical difficulties associated with the need to involve theories of molecular interactions and to solve complicated differential equations [3-6]. Here and in sections 5.8 and 5.9, we present only a simplified picture of the diffusional processes, which is valid for hmiting conditions. The objective is to make the reader aware of the importance of this phenomenon in connection with solvent extraction kinetics. 

Equations (5.16) of Table 5.1 refer to series first-order reactions. Of interest for the solvent extraction kinetics is a special case arising when the concentration of the intermediate, [Y], may be considered essentially constant (i.e., d[Y]/dt = 0). This approximation, called the stationary state or steady-state approximation, is particularly good when the intermediate is very reactive and present at very small concentrations. This situation is often met when the intermediate [Y] is an interfacially adsorbed species. One then obtains... 

Section 5.1 describes how, in a stirred system, solvent extraction kinetics can be controlled only by slow chemical reactions or only by diffusion through the interfacial films. An intermediate situation can also occur whereby both the rates... 

When one or more of the chemical reactions is sufficiently slow in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, such that diffusion can be considered instantaneous, the solvent extraction kinetics occur in a kinetic regime. In this case, the extraction rate can be entirely described in terms of chemical reactions. This situation may occur either when the system is very efficiently stirred and when one or more of the chemical reactions proceeds slowly, or when the chemical reactions are moderately fast, but the diffusion coefficients of the transported species are very high and the thickness of the two diffusion films is close to zero. In practice the latter situation never occurs, as diffusion coefficients in liquids generally do not exceed 10 cm s, and the depth of the diffusion films apparently is never less than 10 cm. 

The conventional mechanism and mathematical treatment for solvent extraction kinetics was proposed by Oele et al. in 1951 (11) and has since been accepted and used by others. Oele assumed that the coal will enter the liquid solvent in accordance with a zero-order rate law up to a certain time ... 

What is the specific feature in the reaction at the liquid/liquid interface The catalytic role of the interface is of primary importance in solvent extraction and other two-phase reaction kinetics. In solvent extraction kinetics, the adsorption of the extractant or an intermediate complex at the liquid/liquid interface significantly increased the extraction rate. Secondly, interfacial accumulation or concentration of adsorbed molecules, which very often results in interfacial aggregation, is an important role played by the interface. This is because the interface is available to be saturated by an extractant or mehd complex, even if the concentration of the extractant or metal complex in the bulk phase is very low. Molecular recognition or separation by the interfacial aggregation is the third specific feature of the interfacial reaction and is thought to be closely related to the biological functions of cell membranes. In addition, molecular diffusion of solute and solvent molecules at the liquid/liquid interface has to be elucidated in order to understand the molecular mobility at the interface. In this chapter, some examples of specific... 

P.R. Danesi, Solvent Extraction Kinetics in Principles and Practices of Solvent Extraction, Eds. I. Rydberg, C. Musikas and G.R. Choppin, Marcel Dekker, New York, 1992. p. 157. 

Baiimler, E., Crapiste, G. and CareUi, A. (2010) Solvent extraction kinetic study of major and minor compounds. Journal of the American Oil Chemists Society, 87(12), 1489-1495. 

Kondo,K., Tsuneyuki,T. and F.Nakashio. "Solvent extraction kinetics of copper by benzoylacetone". (Proceedings of Solvent Extraction Conf., 1SEC 80, Belgium, 1980). 

H. Watarai, L. Cunningham, and H. Freiser, Automated system for solvent extraction kinetic studies. Anal. Chem., 54, 2390—2392 (1982). 

The theoretical description of the kinetics of transmembrane transport through a liquid membrane should be based on the principles of solvent extraction kinetics. It should be determined by the processes at both water/membrane interphases and should also involve the intermediate step of diffusion in the membrane. Thus the existence of all these three steps makes the membrane system and its description much more complicated than the relatively simple water/organic phase. However, even the kinetics mechanism in simpler extraction systems is often based on the models dealing only with some limiting situations. As it was pointed out in the beginning of this paper, the kinetics of transmembrane transport is a fimction both of the kinetics of various chemical reactions occurring in the system and of diffusion of various species that participate in the process. The problem is that the system is not homogeneous, and concentrations of the substances at any point of the system depend on the distance from the membrane surface and are determined by both diffusion and reactions. The solution of a system of differential equations in this case can be a serious problem. 

---