Interfacial aggregation

Another unique feature of the interfacial reaction for metal complexes is the formation of the aggregate of the complex. As observed in many cases, the liquid/liquid interface can be saturated by any active surface molecules at the interfacial concentration of the order of 10 mol cm, which can be attained even at adiluted bulk concentration as in the case of highly hydrophobic solutes. Thus, the interface is prepared to become a two-dimensionally concentrated state for the solute. This situation very often results in the formation of the metal complex aggregates at the liquid/liquid interface. During the solvent extraction procedure for metal ions, the precipitate at the interface was observed, which was called crud or scum in the field of hydrometallurgy. This crud or scum must 

A typical example of the interfacial aggregation was observed in the interfacial protonation of tetraphenylpoiphyrin (TPP). The aggregation rate of H2tpp + in the dodecane/trichloroacetic acid solution was measured using a two-phase stopped-flow method [7] and a CLM spectrophotometric method [10]. 

FIGURE 10.13. Unit structures of ASl (a) and ASl (b) formed under the low and high total interfacial tpyp concentrations, respectively. Aggregation at the interface is illustrated in (c). 

The palladiumCn) ion forms a 1 1 complex with 5-Br-PADAP, leaving one site for the coordination of another ligand. The Pd(II)-5-Br-PADAP (PdL) complex cation has specific properties such as an extremely high molar absorptivity ( 554 = 4.33 x 10 M - cm in toluene), a high adsorptivity to the liquid/liquid interface and a soft Lewis acid easy to be bound to a soft Lewis base. Therefore, PdL is expected to function as an interfacial molecular recognition reagent of Lewis bases. 

A linear relationship was obtained between the logarithmic interfacial formation constant (log P ) for PdL+-diazine derivative complexes and the logarithmic ratio of 

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Adachi K, Watarai H (2005) Interfacial aggregation of thioether-substituted phthalocyaninatomagnesium(II)-palladium(II) complexes in the toluene/water system. J Mater Chem 15(44) 4701 1710... 

The study of interfacial aggregation also has been developed by this technique, since this method is very advantageous for measuring the formation of interfacial aggregates of dyes and metal complexes, which are formed only at the interface and usually exhibit unique spectral changes. 

Another unique and specific feature of the interfacial reaction is the formation of aggregate of dye molecules, metal complexes, and other solvophobic molecules. As reported in many interfacial adsorption systems, the saturated interfacial concentration of usual molecules is of the order of 10 10mol/cm2, which can be attained even under an extremely low bulk phase concentration. This means that the liquid-liquid interface is ready to be saturated to form a two-dimensionally condensed state for the adsorbate. In solvent extraction process of metal ions, we used to find formation of some precipitate at the interface, which is called crud. The study of the interfacial aggregate is therefore important to know the real interfacial reaction as met in the industrial solvent extraction where rather concentrated solutes have to be treated. 

Fig. 20. Absorption spectra of PdLCl and the interfacial aggregates of ternary PdL-Dz complexes measured by the CLM method. [PdLCl]in t = 5.6 x 10 5M, [pyridazine] = 2.0 x 10 4M, [pyrimidine] = 8.0 x 10 3M, [pyrazine] = 8.0 x 10 5M, [C104 ] = 0.1 M, [Cl ] = 0M, pH 2.0. The left structure shows a probable unit of the membrane-like aggregate formed in pyrazine complexes.

Fig. 21. Critical aggregation concentration in the interfacial aggregation of Ni(II) 5-Br-PADAP.

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... 

In spite of the fact that pyrazine has the lowest stability among Dz isomers in the formation of PdL complex at the toluene/water interface, the stability for the formation of the interfacial aggregate was the highest for the pyrazine complex. This result indicates that the formation of the interfacial aggregate of PdL-Dz isomers was governed by the geometric structure of Dz, not by its basicity. 

The resonance Raman spectra for the interfacial aggregates were measured by means of CLM Raman microspectroscopy, and the Raman intensity ratio of the imine and azo forms (/azo/fimine) was determined as shown in Table 10.5. The ratio was smaller in more polar solutions such as at the interface. This observation indicated that the nanoenvironment of the azo-group in the membrane-like aggregate of cationic PdL-l,4-Dz was more polar than that of the crystal-like PdL-l,2-Dz cation, although both were in more polar environment than the toluene solutions. 

The interfacial aggregation of PdL with pyridazine, pyrazine, adenine and guanine occurred at very low concentrations of Dzs and the purine bases, not observed in the bulk phases. This indicates that the interfacial aggregation could be amplified by the presence of a very minute amount of the base. 

Similar interfacial aggregation phenomena were observed in the complexation of Cu(II) with octadecylthiazolylazophenol (CigTAR, HR) at the heptane/water interface [54], The CuR+ complex was formed at the interface with the absorption maximum at 560 nm. The increase in pH promoted the formation of an aggregate of Cu2R shifting the absorption maximum from 560 to 510 nm. 

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