Partition of bromophenol blue in toluene/water/sodium bis(2‐ethylhexyl)‐sulfosuccinate water‐in‐oil microemulsions

Abstract

The partition of bromophenol blue between oil-water interfaces and water droplets in water-in-oil microemulsions of toluene/water/sodium bis(2-ethylhexyl)sulfosuccinate was studied by a spectroscopic method at different water fractions and pH. The partition equilibrium constant, K p , between the two domains decreased considerably with an increase in the water fraction and pH. The decrease in K p with an increase in the water fraction suggests the retention of more dye molecules in water. The decrease in K p with an increase in pH has been attributed to a lower tendency of the base form to associate with the anionic oil-water interface compared with that of the acid form and to an increase in the negative charge density at the oil-water interface at a higher pH.Paper no. S1484 in JSD 8, 277-281 (July 2005). KEY WORDS:Aerosol OT, AOT, bromophenol blue, partition equilibrium, sulfonephthalein dye, toluene, W/O microemulsion.Knowledge of the microstructure of microemulsions is important for their applications in physicochemical and technological processes (1,2). The partition of solutes between the polar droplet core, the apolar bulk solvent, and the interface may affect the microstructure and determine the physicochemical properties of microemulsions. It is also important for determining the possible localization/distribution of reagents and the reaction positions in microemulsions (3). The pseudophase ion-exchange model has been widely used for describing the partition behavior and reaction rates in micellar solutions (4). The method also has been applied to study partition equilibria of solutes between different pseudophases of microemulsions (5-8). Although several reports can be found in the literature on partition equilibria and the acid-base behavior of solutes or indicator dyes in microemulsion systems, most of them involve the partition between all three pseudophases, viz., oil, water, and the oil-water interface (3,9-13).We have developed an equilibrium method that allows us to determine equilibrium constants of the partition of dyes between aqueous and micellar pseudophases. The method has been applied to predict apparent pK a of the dyes in micellar solutions of surfactants of different charge types, where the dye is partially incorporated into the micelles (14-16). Recently, this partition equilibrium approach has been extended to o/w (oil-in-water) microemulsions to gain information on microemulsion microstructure and behavior (17). We have also applied the partition equilibrium approach to study the effects of alcohols as cosurfactants and the change in pH on partition equilibrium of a dye, viz., phenol red in o/w microemulsions (18,19). The partition equilibrium approach is a simple and time-saving approach, and the studies add to the lack of direct information on microemulsion microstructure, with some assumptions widely present in the literature. The studies carried out so far have involved only o/w microemulsions stabilized by anionic surfactants, viz., sodium dodecyl sulfate and sodium dodecyl be...

β-Blockers and benzodiazepines location in SDS and bile salt micellar systems

“…It is important to refer that the pK a values are pK a1 = 2.98 ± 0.01 and pK a2 = 10.55 ± 0.03 [35,43], and thus only two forms of the drug exist in significant amounts in solution: the neutral (HB) and the deprotonated (B − ). The observations above mentioned are in agreement with the behaviour of these pharmaceutical compounds in anionic surfactant solutions observed in previous studies [28,43].…”

“…Generally, the molecular probes are chosen for their spectroscopic handles and the ability to interpret the observations made. Experiments utilizing molecular probes of micro and nanoscopic structures rely on the location of the probe molecule, frequently assuming the location based on a particular feature [26][27][28].…”

Molecular Probe Location in Reverse Micelles Determined by NMR Dipolar Interactions

“…Thus, if Coulomb interactions dominate then the negatively charged sulfonate headgroup of the AOT molecules should repel the [VO 2 dipic] - complexes, forcing the vanadium complex into the water pool. Indeed, reports in the literature, ,,,, including our own, echo this view. These reports confirm that small, highly polar and charged solutes should be preferentially solvated in the water pool, while less polar solutes partition into the interfacial region.…”

“…Generally, the molecular probes are chosen for their spectroscopic handles and the ability to interpret the observations made. Experiments utilizing molecular probes of micro- and nanoscopic structures rely on the location of the probe molecule, frequently assuming the location based on a particular feature. − A comprehensive review documents the interpretation that charged molecules reside in the most polar environment available, such as an aqueous phase, while less polar molecules may reside in the micellar interface …”