Interface/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. 

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. 

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. 

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)]. 

While the bulk behavior of polyampholytes has been investigated for some time now, studies of interfacial performance of polyampholytes are still in their infancy. There are several reasons for the limited amount of experimental work the major one being the rather complex behavior of polyampholytes at interfaces. This complexity stems from a large array of system parameters governing the interaction between the polymer and the substrate. Nearly all interfacial studies on polyampholytes reported to-date involved their adsorption on solid interfaces. For example, Jerome and Stamm and coworkers studied the adsorption of poly(methacryhc acid)-block-poly(dimethyl aminoethyl methacrylate) (PMAA-fc-PDMAEMA) from aqueous solution on sihcon substrates [102,103]. The researchers found that the amount of PMAA-fo-PDMAEMA adsorbed at the solution/substrate interface depended on the solution pH. Specifically, the adsorption increased... 

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. 

In microscale channels, the viscous forces dominate the inertial effect resulting in a low Reynolds numbers. Hence, laminar flow behavior is dominant and mixing occurs via diffusion. However, in a liquid-liquid system, the interfacial forces acting on the interface add complexity to the laminar flow as the relationship between interfacial forces and other forces of inertia and viscous results in a variety of interface and flow patterns. Gunther and Jensen [202] illustrated this relationship as a function of the channel dimension and velocity as shown in Figure 4.12. The most regularly shaped flow pattern is achieved when interfacial forces dominate over inertia and viscous forces at low Reynolds numbers, as represented in Figure 4.12 by the area below the yellow plane [202,203]. 

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. 

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... 

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. 

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. 

The presence of a long-chain alcohol at the oil-water interface decreases the electrostatic repulsion force between the charged emulsifier molecules, which enhances the density of the droplet surface layer and probably promotes the formation of an interfacial complex film. The existence of the hydrophobic tail of coemulsifier retards the molecular diffusion of coemulsifier into the aqueous phase, but promotes the interaction between the hydrophobic tail of coemulsifi-... 

The most specific role of liquid-liquid interfaces that we found is a catalytic effect in the solvent extraction of metal ions and interfacial complexation kinetics. Shaking or stirring of a solvent-extraction system generates a wide interfacial area or a high specific interfacial area defined as the interfacial area divided by the bulk phase volume. Almost all extractants, and an auxiliary ligand in some cases, are more or less interfacially active, since they have both hydrophilic and hydrophobic groups. Interfacial adsorption of the extractant or an intermediate complex at the liquid liquid interface can very effectively facilitate the extraction rate. In this chapter, the catalytic role of the interface in metal complexation will be discussed. 

TABLE 3 Interfacial Complexation Rate Constants of Ni(II) and Zn(II) with 5-Br-PADAP in Heptane/Water and Toluene/Water Interfaces... 

The most important step in the interfacial catalysis in complex formation is the adsorption of extractant, which increases the interfacial concentration thus, the interfacial complexation and extraction rate are accelerated. The kinetic solvent effect of the liquid-liquid interface is very sensitive to the location where the ligand molecule is adsorbing. The interfacial solvent effect in the nanometer region has to be studied more extensively. 

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