Two-phase stopped flow method

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion beeomes as high as 700 em and the interfacial reaction in the dispersed system can be measured by photodiode array speetrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After approximately 200 ms, the dispersion system starts to separate, even in a rather 

FIGURE 2.21 Schematic drawing of the centrifugal liquid membrane (CLM) cell. 

By the total internal reflection condition at the liquid-liquid interface, one can observe interfacial reaction in the evanescent layer, a very thin layer of approximately 100 nm in thickness. Fluorometry is an effective method for a sensitive detection of interfacial species and its dynamics [24]. Time-resolved laser spectrofluorometry is a powerful tool for the elucidation of rapid dynamic phenomena at the interface [25]. Time-resolved total reflection fluorometry can be used for the evaluation of rotational relaxation time and the viscosity of the interface [26], Laser excitation can produce excited states of adsorbed compound. Thus, the triplet-triplet absorption of interfacial species was observed at the interface [27]. 

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From the beginning of the 1980s, some effective experimental approaches based on new principles have been invented for the study of interfacial reactions in solvent extraction chemistry. Recently, some methods were developed from our laboratory, the highspeed stirring (HSS) method [4,5], the two-phase stopped flow method [6], the capillary plate method [7], the reflection spectrometry [8], and the centrifugal liquid membrane (CLM) method [9]. 

A typical example is the protonation of tetraphenylporphirin (TPP) at the dodecane-acid solution interface. The interfacial protonation rate was measured by a two-phase stop flow method [6] and a CLM method [9]. In the former method, the stagnant layer of 1.4 jxm still existed under the highly dispersed system. In the CLM method, the liquid membrane phase of 50-100 /am thickness behaved as a stagnant layer where the TPP molecule has to migrate according to its self-diffusion rate. 

The measurement of fast chemical reactions at the liquid-liquid interface is very difficult, since the reaction which starts just after the contact of two phases has to be measured. The dead time of the HSS method was several seconds and that of the two-phase stopped flow method was from few tenth millisecond to several hundred milliseconds. The micro-two-phase sheath flow method is the only method that can measure such fast interfacial reactions, which finish within 100 ps. An inner organic microflow was generated in an outer aqueous phase... 

Interfacial protonation of tetraphenylporphin (TPP) was investigated by means of a two-phase stopped-flow method [7] and a CLM method [10]. The protonation of TPP at the interface, which can be detected from a definite shift in its absorption maximum from 416 to 438 nm, is very fast. The rate of interfacial protonation was spectrophoto-metrically determined from the rate of formation of the protonated TPP at the interface of dodecane/aqueous trichloroacetic acid. The rate of protonation was so fast that the observed rate was governed by the TPP diffusion rate in the organic phase. In the two-phase stopped-flow study, the rate of protonation was observed less than 200 milliseconds after two-phase mixing. The observed first-order rate constant of 19 s , which was independent of both acid and TPP concentrations, was analysed on the basis of the simple diffusion layer model [3] ... 

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]. 

A typical example is the protonation of tetraphenylporphirin (TPP) at the dodec-ane/acid solution interface. The interfacial protonation rate was measured by a two-phase stopped flow method [6] and a CLM method [9]. 

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