Acidity Function of Water‐in‐Oil Microemulsions

Abstract

Three compartments are available for the localization of small solutes in water-in-oil (w/o) microemulsions: [1][2][3] 1) the internal aqueous core or water pool, 2) the micellar interface formed by a monolayer of surfactant molecules with their polar head groups oriented toward the water pool, and 3) the external organic phase. The existence of these microenvironments is the basis of their use as reaction media, [4] due to their capacity to solubilize and compartmentalize substrates with different properties. Likewise, these systems are a promising reaction medium for the deactivation of mustard and nerve agents [5] and an alternative to phase-transfer catalysis. [6] One of the problems arising from their use as reaction media is their destabilization by additives needed for the reactions to take place. The amount of solubilized water is affected by the addition of electrolytes. [7] The maximum amount of water solubilized in NaOT-based microemulsions (NaOT = sodium salt of bis(2-ethylhexyl)sulfosuccinate) in the presence of electrolytes increases up to the optimal ionic strength, and then decreases with increasing ionic strength. These results are interpreted in terms of salting-in and salting-out phenomena, as well as the counteracting effects of attractive intermicellar interaction and interfacial bending stress.[8] This problem can be overcome by using surfactants with reactive counterions, where the surfactant itself can act as a source of H + to give a strongly acidic medium that can be used for deactivation reactions.Here we report the results obtained for a w/o microemulsion in which the Na + surfactant counterion is replaced by H + to give a system with a strongly acidic character. We show that the acidic properties of this new microemulsion is controlled by simply modifying the water-to-surfactant molar ratioThe use of the H 0 acidity function of the dispersed phase of the microemulsion is of crucial importance in understanding the properties of this system as a reaction medium. HOT [9] was prepared from NaOT by ion exchange using Amberlite IR 120 (plus) resin. We checked the extent of Na + /H + exchange by two methods: atomic absorption spectroscopy showed the absence of Na + in the HOT sample (the residual Na + content is consistent with a degree of Na + /H + exchange of greater than 99 %), and acid/base titration, which also indicated that the extent of Na + /H + exchange is greater than 99 %. All other reagents used were supplied by Aldrich at the highest level of purity available and were used without further purification. Absorption spectra were recorded with a Varian Cary 50 spectrophotometer fitted with thermostatically controlled cell holders at 25.0 8C. To determine the distribution constant of 2-nitroaniline between water and isooctane (K The compositions of the w/o microemulsions for this study were selected to cover a wide range: the NaOT content varied between 6 and 30 wt %, water between 0.5 and 43 wt %, and that of isooctane between 29 and 93 wt %. These composition intervals allowe...

The Effect of Changing the Microstructure of a Microemulsion on Chemical Reactivity

“…Influence of the Counterion on Chemical Reactivity. The results obtained in this work, as well as other results previously obtained in our laboratory, 32c,, allow us to analyze the influence of the nature of the counterion on solvolysis reactions. Figure shows the influence of the microemulsion composition on the solvolysis rate constant of 4-MeO in water/surfactant/isooctane microemulsions of HOT, NaOT, and NH 4 OT.…”

Different Kinetic Behaviors for Unimolecular and Bimolecular Ester Hydrolysis Reactions in Strongly Acidic Microemulsions

“…Comparison of the results obtained for similar surfactant concentrations and different W values shows that the rate constant increases with W . Because the acidity of HOT-based microemulsions ranges from H 0 = 0.6 to −1.4, we can conclude that both NPP and DNPP are fully protonated. In this way, the rate-limiting step for the reaction is the water attack on the protonated substrate.…”

“…The acidity function, H 0 , of the aqueous core of these microemulsions decreases with W ( W = [H 2 O]/[HOT]) from H 0 ≈ 0.6 at W > 20 to H 0 = −1.4 at W = 2 . For smaller nanodroplet sizes, the acid character is similar to the one that could be obtained in aqueous solutions of HCl, HNO 3 , or CH 3 SO 3 H at concentrations ranging from 3 to 7 M. Subsequently, the degree of counterion binding (β) has been determined starting from values of H 0 .…”

Evaluation of proton activity in microemulsions by a kinetic probe

“…Recently a functionalized ionic surfactant attained by replacing the Na + ions by H + ions has been shown to be an efficient tool to conduct chemical reactions that require high acidity levels to evolve measurably; the surfactant itself may act as a source of protons in the range of Hammett Acidity Function levels, thus yielding strongly enough HOT-based acid microemulsions to monitor the kinetic experiments [10]. The same authors have applied this technique to clarify the mechanism of ester hydrolysis for reactions that require such high acid concentrations to evolve that would be impossible to achieve by other means [11].…”