Study of the Alkaline Fading of Phenolphthalein in Microemulsions

Abstract

The reactions of the alkaline fading of phenolphthalein (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 AOT (ω), 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 ω. It was also observed that 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.

Critical anomalies of alkaline fading of phenolphthalein in the critical solution of 2-butoxyethanol + water

“…The values of k and K at various temperatures with their estimated uncertainties are listed inTable I. It was found that the observed rate constant of the alkaline fading of PN in (2BE + PN) aqueous solution was about 2.0 × 10 −3 mol l −1 s −1 at 298.15 K, two times smaller than that in the aqueous medium determined in our previous work 26. …”

“…The reaction of alkaline fading of PN may be de- 26 where L 2− is red-pink quinonoid and LOH 3− is colorless carbinol. The reaction becomes a pseudo-first-order reaction with respect to L 2− when the concentration of OH − was much higher than that of L 2− .…”

“…From our previous experimental investigations of both the reaction rates and the chemical equilibrium constants of the alkaline fading of phenolphthalein (PN) at various conditions, 26 we found that the values of chemical equilibrium constant are appropriate for study the critical influence on the chemical equilibrium. It has urged us to study the kinetics of the alkaline fading of PN in the critical solution (2BE + water) with a lower critical solution temperature by using three-wavelength UV-spectrophotometry at a series of temperatures both far away and near the critical point.…”

Effect of microheterogeneous environments of CTAB, Triton X‐100, and Tween 20 on the oxidative degradation of d‐fructose by nanoparticles of MnO2

“…There is growing interest in recent times on the influence of different surfactants on the kinetics and mechanism of different reactions . Reaction rates are often modified owing to hydrophobic and/or electrostatic interactions between the reactant species and surfactant monomers or micelles.…”

Study of Fuchsin Acid Fading in Micellar Media

“…Chemical reactions rates are changed in the presence of self‐organized assemblies such as micelles . Effects of surfactant molecules and their micelles on these reactions can be attributed to their electrostatic and hydrophobic interactions with reactants.…”