Spectrometric Study of AOT-Hydrolysis Reaction in Water/AOT/Isooctane Microemulsions Using Phenolphthalein as a Chemical Probe

The kinetics of the alkaline hydrolysis of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in water/AOT/isooctane microemulsions has been studied by monitoring the absorbance change of the phenolphthalein in the system with time. The apparent first-order rate constant <i>k</i>_obs has been obtained and found to be dependent on both the molar ratio of water to AOT ω and the temperature. The dependences of <i>k</i>_obs on ω have been analyzed by a pseudophase model which gives the true rate constants <i>k</i>_i of the AOT-hydrolysis reaction on the interface and the partition coefficients <i>K</i>_wi for the distribution of OH^– between aqueous and interface pseudophases at various temperatures; the latter is almost independent of the temperature and ω. The temperature dependences of the reaction rate constants <i>k</i>_obs and <i>k</i>_i have been analyzed to obtain enthalpy Δ<i>H</i>^≠, entropy Δ<i>S</i>^≠, and energy <i>E</i>_a of activation, which indicate that the distribution of OH^– between aqueous and interface pseudophases increases Δ<i>S</i>^≠ but makes no contribution to <i>E</i>_a and Δ<i>H</i>^≠. The influence of the overall concentration of AOT in the system on the rate constant has been examined and found to be negligible. It contradicts with what was reported by García-Río et al. but confirms that the first-order reaction of the AOT-hydrolysis takes place on the surfactant interface. The study of the influence of AOT-hydrolysis on the kinetics of the alkaline fading of crystal violet or phenolphthalein in the water/AOT/isooctane microemulsions suggests that corrections for the AOT-hydrolysis in these reactions are required