Chemical Reactions In Microemulsions And Micellar Solutions

The spectrum of phenolphthalein (PN) have been studied in solutions by UV-vis spectrophotometer, and the equilibrium constants and molar absorption coefficient of the color ion The have been obtained with the relationship between the absorbance and the concentration of H+. The molar absorption coefficient of the colorless ion LOH3-was obtained according to the Lambert-Beer law.The reactions of the alkaline fading of PN have been studied in water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane microemulsions by monitoring the absorbance changes of PN in the system with the time and the results compared with those found for the same reactions in aqueous solutions. It was found that the values of the equilibrium constants and the forward reaction rate constants in the microemulsions were significantly larger than that in aqueous solutions and decreased with increasing the molar ratio of water to AOTω except for that with low ω. The temperature dependence of the reaction rate constant was analyzed to obtain the values of free energy, enthalpy and entropy of activation, which suggests the existence of an isokinetic relationship and a common mechanism for the reactions occurring in the microemulsions with different co. It was also observed the competition between the reactions of the alkaline fading of PN and the hydrolyzation of AOT in water/AOT/isooctane microemulsions when the reaction time was sufficiently long.The kinetics of the alkaline hydrolysis of AOT in water/AOT/isooctane microemulsions has been studied by monitoring the absorbance changes of the phenolphthalein in the system with time. The apparent first-order rate constant kobs was obtained and found to be dependent on both ω and the temperature. The dependence ofbs on ω was analyzed by a pseudophase model to give the true rate constant ki of the AOT-hydrolysis reaction on the interface and the partition coefficient Kwi for the distribution of OH between aqueous and interface pseudophases at various temperatures; the latter was found to be almost independent on the temperature and ω. The temperature dependence of the reaction-rate constants kobs and ki were analyzed to obtain the values of enthalpy△H≠, entropy△S≠, and energy Ea of activation, which indicated that the distribution of OH-between the aqueous and interface pseudophases increased the value of△S≠for the interface reaction, while it made no contributions to Ea and△H≠. The influence of the overall concentration of AOT in the system on the reaction constants was examined and found to be negligible. It contradicted with what reported by Garcia-Rio et al., but confirmed the first-order reaction of the AOT hydrolysis taking place on the surfactant interface. The study of the influence of the AOT hydrolysis on the kinetics of the alkaline fading of crystal violet or phenolphthalein in the water/AOT/isooctane microemulsions suggested that the corrections for the AOT hydrolysis in these reactions are required.The electron transfer reaction between chloropentaamminecobalt (Ⅲ) and hexacyanoferrate (Ⅱ) has been studied in the mixed surfactants of sodium dodecyl sulfate (SDS) and octylphenol (ethylene oxide)9.5ether (TX100). The observed rate constants kobs have been determined for various micellar media with different concentrations. A pseudophase model together with an outer-sphere electron transfer reaction mechanism has been used to analyze the experimental results to obtain the precursor complex formation equilibrium constants KIP, the electron transfer rate constants ket, and the pseudo phase equilibrium constant K. The dependence of those parameters on the micellar concentration has been discussed.