Complex formation, interfacial interface

Modern attempts to formulate a quantitative theory of emulsions and emulsion stability have looked most closely at the nature of the interfacial region separating the two immiscible phases, especially the chemical and physical nature of the adsorbed film, the role of mixed films and complex formation, interfacial rheology, and steric and electronic factors at the interface. The theoretical foundations for current ideas concerning emulsion formation and stability are presented in several of the references cited in the Bibliography. A few of the most basic ideas, however, are presented below. 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

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). 

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. 

Coacervation. If an oil phase is emulsified in a polymer water solution, and the polymer is precipitated (for example) by changing the pH, the polymer precipitate (coacervate) has a tendency to accumulate at the interface. This is the coacervation process called simple if one polymer is involved and complex if two polymers are involved. If the coacervation is obtained by dropping one polymer solution into a polymer solution of opposite charge, this is termed interfacial coacervation, or polyelectrolyte complex formation . 

The combination of resonance Raman microscope spectrometry and the CLM method allowed us to directly observe the Raman spectra of the liquid-liquid interface and the bulk phases by shifting the focal point of an objective lens. A schematic diagram of the measurement system is shown in Fig. 6. CLM/ Raman microscope spectrometry was applied in order to measure the rate of complex formation between Pd(II) and 5-Br-PADAP (HL) at the heptane-water interface and it was demonstrated that this method was highly useful for the kinetic measurement of the interfacial reaction [37],... 

The complex formation proceeded almost completely at the interface. The rate constant of k=5.3xl02M 1 s 1 was determined by a stopped-flow spectrometry in the region where the formation rate was independent of pH. The conditional interfacial rate constants represented by k[ = k k2 [HL] / (k2 + k i[H + ]) were larger in the heptane-water interface than the toluene-water interface, regardless of metal ions. The molecular dynamics simulation of the adsorptivities of 5-Br-PADAP in heptane-water and toluene-water interfaces suggested that 5-Br-PADAP could be absorbed at the interfacial region more closely to the aqueous phase, but 5-Br-PADAP in the toluene-water... 

The fast reaction rate between Zn(II) ion with Hocqn at the 1-butanol-water interface was measured by two-phase sheath flow method [33]. The formation of a fluorescence complex at the interface was measured in the period less than 5 ms after the two-phase contact as shown in Fig. 18. By the help of digital simulation, the initial process of the interfacial reaction between Zn(II) and Hqn was analyzed (Fig. 18). This approach is promising for the measurement of rapid interfacial reactions such as protein folding and luminescence lifetime as well [36]. 

One more factor, the contact, interaction, and transfer of chemical species on the hquid-frquid interface of two immiscible phases have to be mentioned in the general consideration of chemical kinetics. Little direct information is available on physicochemical properties (interfacial tension, dielectric constant, viscosity, density, charge distribution, etc.) of the interface. The physical depth of the interfacial region can be estimated in the distance in which molecular and ionic forces have their influence. On the aqueous side (monolayers of charged or polar groups) this is several nanometers, on the organic side is the influence of Van der Waals forces. These interfacial zone interactions may slower exchange and complex formation... 

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. 

The lanthanide complexes lack distinct ultraviolet-visible spectra and, hence, kinetic information on their complex formation and dissociation reactions was obtained indirectly by the metallochromic indicator method. These studies indicate that in the case of the Cyanex 272 complexes, the CPC efficiencies are mainly limited by the slow dissociation of the M(HL2)HL complex at the heptane-H20 interface. In the case of the complexes of the acylpyrazolones, the CPC efficiencies are again limited by the dissociation of the lanthanide-pyrazolone complexes at the organic-aqueous interface with the rate-limiting step being the dissociation of the ML complex. It was also shown that because the dissociation reactions are interfacial separations, efficiencies can be dramatically improved by the addition of surfactants like Triton X-100 to the organic phase and by... 

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