Interfacial complexes

The above model can be extended to assisted ion transfer, in which the ion forms a complex with a suitable ionophore. The various mechanisms for such reactions have been classified by Shao et al. [21] and reviewed by Girault [22]. Schmickler [23] has examined the case of transfer by interfacial complexation, which is marked by the following reaction sequence (see Fig. 14) The transferring ion moves from the bulk of solution 1 towards the interface with solution 2, in which it is poorly soluble. At the interface it reacts with an ionophore from solution 2, and then the complexed ion is transferred towards the bulk of solution 2. 

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

The synergistic effect of DPP on the extraction of Ni(II) with dithizone was also studied and confirmed the formation of an interfacial complex [24]. 

From the fundamental knowledge concerning the interfacial complexation mechanism obtained from the kinetic studies on chelate extraction, ion-association extraction, and synergistic extraction, one can design the interfacial catalysis. The main strategy is to raise the concentration of the reactant or intermediate at the interface. 

E. Analysis of Interfacial Complex by a Time-Resolved Fluorescence Spectroscopy... 

In the mechanism of an interfacial catalysis, the structure and reactivity of the interfacial complex is very important, as well as those of the ligand itself. Recently, a powerful technique to measure the dynamic property of the interfacial complex was developed time resolved total reflection fluorometry. This technique was applied for the detection of the interfacial complex of Eu(lII), which was formed at the evanescent region of the interface when bathophenanthroline sulfate (bps) was added to the Eu(lII) with 2-thenoyl-trifuluoroacetone (Htta) extraction system [11]. The experimental observation of the double component luminescence decay profile showed the presence of dinuclear complex at the interface as illustrated in Scheme 5. The lifetime (31 /as) of the dinuclear complex was much shorter than the lifetime (98 /as) for an aqua-Eu(III) ion which has nine co-ordinating water molecules, because of a charge transfer deactivation. 

Interfacial adsorption of extractant increases the interfacial concentration, thus accelerating the interfacial complexation and extraction rate. 

The design of the catalytic complexation system will be developed utilizing the fundamental knowledge of the interfacial complexation. 

The rate of reaction (13) can be evaluated from the difference between values at E. = 0.25 V and = 0.6 V, which represents the total amount of complexed potassium arriving at the collector. Dividing this quantity by collection efficiency one gets an estimate for the total complexation rate in the system. To evaluate homogeneous complexation rate the contribution of interfacial complexation (i.e., ig KOBisce) has to be subtracted ... 

The surface acid or base sites in combination with adsorbed hydrated cations or anions, represented by -0 (a) K or -OH2 (b) A , are called the interfacial ion pair [Tamura-Puruichi, 1991]. In some cases the surface acid or base site is covalent-bonded with dehydrated cations or anions to form an interfacial complex [Stumm, 1992]. 

Case 3 There are two interfacial rate-determining steps, consisting of 1) formation of an interfacial complex between the interfacially adsorbed molecules of the extractant and the metal ion and (2) transfer of the interfacial complex from the interface to the bulk organic phase and simultaneous replacement of the interfacial vacancy with bulk organic molecules of the extractant. For this mechanism, we distinguish two possibilities. The first (case 3.1) describes the reaction with the dissociated anion of the extracting reagent, B"(ad). The second (case 3.2) describes the reation with the undissociated extractant, BH(ad). 

Two seqnential interfacial reactions the first one being the reaction with the anion of the extractant that saturates the interface the second one being the slow desorption of the interfacial complex from the interface [see Eq. (5.56)]. 

Surface shear rheology at the oil-water interface is a sensitive probe of protein-polysaccharide interactions. In particular, there is considerable experimental evidence for a general increase in surface shear viscosity of protein adsorbed layers as a result of interfacial complexation with polysaccharides (Dickinson et al., 1998 Dickinson and Euston, 1991 Dickinson and Galazka, 1992 Semenova et al., 1999a Jourdain et al., 2009). One such example is the case of asi-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = - 334 x 10 cm /mol) the interfacial viscosity after 24 hours was found to be five times larger in the presence of pectin (i.e., values of 820 80 and 160 20 mN m 1 with and without pectin, respectively) (Semenova et al., 1999a). 

For SDS, the reaction proceeded to a reproducible end point rapidly —viz., 1 to 2 minutes—when nonionic surface active impurities such as parent dodecyl alcohol, DOH, were removed by ethyl ether extractions. This impurity effect was verified by adding traces of alkyl alcohol—viz., 1 X 10 9 mole per liter—to purified SDS, whereupon the penetration reaction rate was halved. A possible explanation for this behavior is that formation of an SDS-DOH interfacial complex reduced the SDS activity in the interface and consequently its rate of reaction with the protein monolayer. The reasons for the somewhat slower rate of reaction of Cetab with the protein film are more obscure. The reaction rate did not increase after extracting the detergent repeatedly. Two possible reasons for the time dependence in this case may have been that (1) the ether extraction method was not effective in removing surface active impurities, or (2) because of the greater bulk of the Cetab hydrocarbon chain, Ci6 vs. Ci2 for SDS, more time was required for diffusion and appropriate orientation before complex formation. 

Fig. 7. 3D CLM/Raman spectra depicting the formation of interfacial complex of PdLCI.

Most attractive features of the liquid-liquid interfacial reaction are the catalytic effect in the solvent extraction or interfacial complexation and the aggregation of hydrophobic dyes and metal complexes. 

In summary, the catalytic role of the liquid-liquid interface is realized through the adsorption of the extractant or interfacial complex at the interface. Therefore, the enlargement of the specific interfacial area is necessary for the increase of the contribution of the interfacial reaction in the overall extraction rate. 

Facilitated transfer (at ITIES) — Figure 1. Various reaction pathways for the transfer of an ion Mz+ in the presence of a neutral ligand L, with highlighted transfer by interfacial complexation (TIC). Reprinted with permission from [ii]. 2000 Elsevier B.V. 

The addition of TBP in the dodecane increases the free energy of adsorption of PNP in a manner consistent with the formation of an interfacial complex between TBP and... 

The complexation proceeded almost completely at the interface. The values of the interfacial complexation rate constants are listed in Table 10.3. The rate constant, k = 5.3 X 10 M- -s", was determined in the aqueous solution using stopped-flow spectrometry in the region where the formation rate was independent of pH. The conditional interfacial rate constants represented by k, = k ki y L i/ k2 -I- ) were... 

Figure 10.10 shows typical spectra depicting the spectral change that occurs upon interfacial complex formation of PdLCl. Raman intensities at 1599,1408 and 1303 cm ... 

Tliese results revealed that the liquid/liquid interface produced by agitation or stirring could catalyse the extraction rate by increasing the interfacial concentration of extractant and facilitating the interfacial complexation rate, similar to gas/solid or liquid/solid catalysis. 

The results of surfactant-dependency on protein trahsfer indicate that protein extraction reverse micelles not only provide a hydrophilic droplet in a non-aqueous solvent to facilitate protein partition, but also make proteins sufficiently hydrophobic to solubilize into an organic solvent by coating the protein surface. Consequently, we suggest that proteins in the aqueous phase are extracted through the formation of an interfacial complex, a surfactant-coated protein and that the hydrophobic property dominates the extraction efficiency of the proteins, as seen in Figure 14.4. The unsaturated or branched alkyl chain may contribute to the formation of a soluble protein-surfactant complex into a non-aqueous solvent. 

Hallworth and Carless (1 ) discuss several possibilities for the effect of light liquid paraffin on the stability of emulsions with light petroleum or chlorobenzene as the main components. They seem to prefer an explanation previously advanced by them and several other authors for the effect of fatty alcohol, namely that the increased stability is due to the formation of an interfacial complex between the additive and sodium hexadecyl sulphate. The condenced mixed film will resist coalescence primarily by virtue of its rheological properties. With mixed films of the present type, the importance of the film viscoelasticity lies in its ability to maintain electrical repulsion between approaching droplets by preventing lateral displacement of the adsorbed ions. The effective paraffinic oil has chains at least as long as those of the alkyl sulphate and will be associated by van der Waals forces with the hydrocarbon chain of the alkyl sulphate at the interface. 

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