Interface, aqueous/organic

The interfacial transfer kinetics were then investigated by perturbing the equilibrium, through the depletion of Cu + in the aqueous phase, by reduction to Cu at an UME located in close proximity to the aqueous-organic interface. This process promoted the transfer of Cu into the aqueous phase, via the transport and decomplexation of the cupric ion-oxime complex, resulting in an enhanced steady-state current at the UME. Approach curve measurements of i/i oo) vs. d allowed the kinetics of the transfer process to be determined unambiguously [9,18]. 

The electrical oscillations at the aqueous-organic interface or at membranes in the absence of any substances relative to the channel or gate were introduced. These oscillations might give some fundamental information on the electrical excitability in living organisms. Since the ion transfer at the aqueous-organic or aqueous-membrane interface and the interfacial adsorption are deeply concerned in the oscillation, it has been stressed that the voltammetry for the ion transfer at an interface of two immiscible electrolyte solutions is... 

FIG. 6 Typical cyclic voltammogram obtained for the transfer of pilocarpine hydrochloride at the water-DCE interface. The organic phase contains 0.01 M tetrabutylammonium tetrakis(4-chloro-phenyl)borate, the aqueous solution is 0.01 M HCl + 0.2 mM pilocarpine hydrochloride, and the sweep rate is fixed at 10, 25, 75, 100, and 150mV/s. (Reprinted from Ref. 229.)... 

This type of sensor often does not have a membrane it instead utilizes the properties of a water-oil interface, a boundary between an aqueous and a non-aqueous (organic) phase. Traditionally, sensors based on non-equilibrium ion-selective transport phenomena were distinguished as a separate group and considered as the electrochemistry of the ion transfer between two immiscible electrolyte solutions (IT1ES). Here, we will not distinguish polymeric membrane electrodes and ITIES-based electrodes due to the similarity in the theoretical consideration. 

The foregoing discussion of micellar charge effects has implicitly assumed that differences in water activity or substrate location in cationic and anionic micelles are not of major importance. If such differences were all important it would be difficult to explain the differences in k+/k for carbonyl addition and SN reactions, because increase of water content in an aqueous-organic solvent speeds all these reactions (Johnson, 1967 Ingold, 1969). As to substrate location, there is very extensive evidence that polar organic molecules bind close to the micelle-water interface in both anionic and cationic micelles, although the more hydrophobic the solute the more time it will spend in the less polar part of the micelle. Substrate hydrophobicity has a marked effect on the overall rate effects in both cationic and anionic micelles, but less so on values of k+/k. It seems impossible to explain all these charge effects in terms of differences in the location of substrates in cationic and anionic micelles. 

The prototype reaction was the hydroformylation of oleyl alcohol (water insoluble) with a water-soluble rhodium complex, HRh(C0)[P(m-C6H4S03Na)3]3 (Figure 6.5). Oleyl alcohol was converted to the aldehyde (yield = 97%) using 2 mol % Rh with respect to the substrate and cyclohexane as the solvent, at 50 atmospheres CO/H2, and 100°C. The SAPCs were shown to be stable upon recycling, and extensive work proved that Rh is not leached into the organic phase. Since neither oleyl alcohol nor its products are water soluble, the reaction must take place at the aqueous-organic interface where Rh must be immobilized. Also, if the metal catalyst was supported on various controlled pore glasses with... 

In Eq. (5.26), Tt is the interfacial pressure of the aqueous-organic system, equal to (Yo - Y) e to the difference between the interfacial tensions without the extractant (Yo) and the extractant at concentration c (y)], c is the bulk organic concentration of the extractant, and is the number of adsorbed molecules of the extractant at the interface. The shape of a typical n vs. In c curve is shown in Fig. 5.4 rii can be evaluated from the value of the slopes of the curve at each c. However, great care must be exercised when evaluating interfacial concentrations from the slopes of the curves because Eq. (5.26) is only an ideal law, and many systems do not conform to this ideal behavior, even when the solutions are very dilute. Here, the proportionality constant between dHld In c and is different from kT. Nevertheless, Eq. (5.26) can still be used to derive information on the bulk organic concentration necessary to achieve an interface completely saturated with extractant molecules (i.e., a constant interfacial concentration). According to Eq. (5.26), the occurrence of a constant interfacial concentration is indicated by a constant slope in a 11 vs. In c plot. Therefore, the value of c at which the plot n vs. In c becomes rectilinear can be taken as the bulk concentration of the extractant required to fully saturate the interface. 

The ability to transfer the desired metal selectively across the aqueous-organic interface in both directions... 

As enzymes could be used to carry out synthetic reactions in organic solvents [244-246] only under certain specific conditions, the appHcation ofRMs as enzyme hosts to perform biotransformations has attracted a great deal of research attention in the recent past. The reverse micellar environment represents a medium where the aqueous/organic interface is very large ( 100 m ml ) [247]. 

The previous extension of solvent mixtures involved solvent interfaces. This organic-water interfacial technique has been successfully extended to the synthesis of phenylacetic and phenylenediacetic acids based on the use of surface-active palla-dium-(4-dimethylaminophenyl)diphenylphosphine complex in conjunction with dode-cyl sodium sulfate to effect the carbonylation of benzyl chloride and dichloro-p-xylene in a toluene-aqueous sodium hydroxide mixture. The product yields at 60°C and 1 atm are essentially quantitative based on the substrate conversions, although carbon monoxide also undergoes a slow hydrolysis reaction along with the carbonylation reactions. The side reaction produces formic acid and is catalyzed by aqueous base but not by palladium. The phosphine ligand is stable to the carbonylation reactions and the palladium can be recovered quantitatively as a compact emulsion between the organic and aqueous phases after the reaction, but the catalytic activity of the recovered palladium is about a third of its initial activity due to product inhibition (Zhong et al., 1996). 

Lactobacillus kefir (ADH E.C. 1.1.1.1) for use in organic solvents [11, 12]. Both biocatalysts are characterized by a very low stability in pure organic solvents or standard aqueous-organic two-phase systems [20], though their broad substrate ranges include many hydrophobic compounds [21, 22]. Figure 3.2.2 illustrates the denaturation of native BAL at the interface between a buffered aqueous solution and octanone. 

As previously observed for BAL, entrapment of ADH in PVA and its subsequent application in hexane as a standard organic solvent enabled the conversion of a number of interesting hydrophobic substrates (Table 3.2.3) by stabilizing the delicate biocatalyst against deactivating effects of the aqueous-organic interface. 

With soluble quaternary ammonium salts as catalysts the reaction is thought to take place at the aqueous/organic interface because a) the solubilities of quaternary ammonium hydroxides in organic solvents are too low to account for the observed reaction rates, and b) the most active catalysts are benzyltriethylammonium and... 

Fig. 24.4 Mechanism of enzyme inactivation at an aqueous-organic interface. Step 1 reversible enzyme adsorption to the interface and concomitant enzyme structural rearrangement at the interface. Step 2 unfolding of enzyme molecule at the interface. Step 3 desorption of inactivated/un-folded enzyme molecules from the interface. Step 4 irreversible aggregation and precipitation of inactivated enzyme. (From [34])...

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