Some Reactions In Microemulsions And Critical Solutions

Some reactions have been investigated in microemulsions and critical solutions by UV spectrophotometry and microcalorimetry, which include:1. The reactions of Ca(NO3)2+ Na3PO4 in water/AOT/alkane microemulsions saturated by CO2 with various molar ratios of water to AOT (R), oil phases, and surfactant concentrations have been investigated by microcalorimetry. The product of the reaction was confirmed to be sodium-and-carbonate-substituted hydroxyapatite by Fourier transform infrared spectra, energy dispersive spectrometry, and X-ray diffraction. By measurements of the enthalpy change of dissolving water in AOT/n-dodecane micelle and the molar enthalpy of the reaction in the microemulsion, it was found that there was no free water in water/AOT/n-dodecane when R< 7. The reaction rate constant decreased with increasing R, but was almost unaffected by the surfactant concentration and the nature of the hydrocarbon.2. The catalytic activities of Candida rugosa lipase using castor oil as substrate in water/(AOT+PEG400D)/cyclohexane mixed-surfactant microemulsions have been studied by UV spectrophotometry. The significant increase of the activity of Candida rugosa lipase with the presence of PEG400D has been observed. The effects of mole fraction of PEG400D in the total surfactants, the reaction temperature, pH, R and the concentration of castor oil on enzyme activity were studied at the constant overall concentration of the surfactants.3. The reactions of the alkaline fading of crystal violet have been studied by UV spectrophotometry and microcalorimetry in the critical solution of 2-butoxyethanol+ water at various temperatures. The results from the two methods were in good agreement. It was found that the reaction rate kobs well described by the Arrhenius equation in the temperature range far from the critical point. The critical slowing down of the reaction was detected by both methods near the critical point. A simple crossover model was proposed and used to analyze the experimental kinetic data to obtain the critical exponent p, which controlled the slowing down effects from both the thermodynamic and dynamic sources. It was found that the value of p from UV spectrophotometry was 0.158±0.013 which was in good agreement with the theoretical prediction of 0.151, while the value of p from microcalorimetry was 0.133 ±0.012. The slightly lower value from microcalorimetry may be attributed to the stirring in the microcalorimeter that weakened the critical reduction of the diffusion coefficient.4. The reaction of the alkaline fading of phenolphthalein in the critical solution of 2-butoxyethanol+water has been studied by UV spectrophotometry. The critical slowing down of the reaction and the critical singularity of the chemical equilibrium were detected near the critical point. The value of the critical exponent β was obtained to be 0.156±0.014 which agreed with the theoretical prediction. The experimental results supported the theoretical prediction of 0.11 for the critical exponent characterizing the weak divergence of the singularity of the chemical equilibrium.5. The reaction of I- and S2O82- in the critical water/AOT/n-decane microemulsions has been investigated by microcalorimetry. The relation between the reaction rate constant and the equilibrium constant of intermedium formation was deduced, and was used in analyzing the kinetic data at various temperatures to obtain the value of the critical exponent for the microemulsions with R= 35.0 and 40.8, which were 0.187±0.023 and 0.193±0.032 respectively and agreed with the theoretical prediction 0.207. It indicated that the partial derivative of the equilibrium constant of intermedium formation with respect to temperature was strongly divergent.