Inhibited Phenol Ionization in Reverse Micelles: Confinement Effect at the Nanometer Scale

Abstract

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.

Differences in Interactions of Benzoic Acid and Benzoate with Interfaces

“…22–29 The reverse micelles allow for placement of the drug by spectroscopic characterization of the micellar interfaces by changing the ratio of water to surfactant ( w 0 ). 21,22,27,30–33 Although this system is powerful in providing the molecular detail, Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) are frequently used models of eukaryotic cell membranes. Langmuir monolayers have been used previously for a variety of studies, detailing interactions of lipids with cholesterol, 34 phospholipids, fatty acids, 35 proteins, 36,37 and drugs.…”

Kinetic Study on the Formation of Bimetallic Core-Shell Nanoparticles via Microemulsions

“…So there will be more encounters between reactants, but they will be together a shorter time. Micelles were previously described as “supercages” [56], and many studies proved a cage-effect in chemical reactions in micelles [25,57,58]. The hypothesis is that this approach can be used to compare the chemical kinetics in micelles and in a solution, instead of in liquid and in gas phases.…”

The impact of the confinement of reactants on the metal distribution in bimetallic nanoparticles synthesized in reverse micelles

“…The rate of intermicellar exchange is the same for both metals, so the reason for this different behavior is not obvious. Previous studies suggest that micelles may act as a cage [27,48]. This assumption is based on the well-known cage effect in chemical kinetics.…”

“…It is understood that reverse micelles are reaction vessels in which classical assumptions cannot always be used. For example, it was demonstrated that the concept as simple as the classical definition of pH cannot be applied in the interior of reverse micelles [27–28]. Previous simulation studies concluded that the structure of nanoparticles obtained in reverse micelles is determined by the difference in the reduction rates only if both reductions occur at the same rate or if reductions have very different rates [23,29].…”