Mass Exchange and Equilibration Processes in AOT Reverse Micelles

Green AOT reverse micelles as nanoreactors for alkaline phosphatase. The hydrogen bond "dances" between water and the enzyme, the reaction product, and the reverse micelles interface

In this work, we present an investigation of the influence of water encapsulated in 1,4-bis-2-ethylhexylsulfosuccinate/methyl laurate and 1,4-bis-2-ethylhexylsulfosuccinate/isopropyl myristate reverse micelles on the enzymatic hydrolysis of 1-naphthyl phosphate by alkaline phosphatase. Our results show that the enzyme is active in the biocompatible reverse micelles studied and that the Michaelis-Menten kinetic model is valid in all systems. We found that both micellar systems studied have a particular behavior toward pH and that the penetration of external solvents into the interfaces is crucial to understanding the effect. Methyl laurate does not disrupt the interface and is not necessary to control the pH value since alkaline phosphatase in the center of the micelles is always solvated similarly. In contrast, isopropyl myristate disrupts the interfaces so that the water and 1-naphthol molecules cannot form hydrogen bond interactions with the polar head of the surfactant. Then, when the water is at pH = 7, the 1-naphthol moves away to the interfaces inhibiting alkaline phosphatase which is not observable when the water is at pH = 10. Our study shows that the concept of pH cannot be used directly in a confined environment. In addition, our research is of great importance in the field of reactions that occur in reverse micelles, catalyzed by enzymes.

Non-Equilibrium Mass Exchange in AOT Reverse Micelles

Reverse micelles (RMs) composed of water and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane have a remarkably narrow size distribution around a mean value determined by the water loading ratio of the system. It has been proposed that RMs establish this equilibrium size distribution either by the diffusion of individual components through the isooctane phase or by cycles of fusion and fission. To examine these mechanisms, a 24 μs all-atom molecular dynamics simulation of a system containing one small RM and one large RM was performed. Results show that the net movement of water from the small RM to the large RM occurred in a direction that made the small RM smaller and the large RM larger-according to water loading ratios that would have been appropriate for their size. Changes in AOT number that would bring the water loading ratio of each RM closer to that of the overall system only occurred via cycles of RM fusion and fission. These behaviors are most likely driven by the electrostatics of sodium AOT and the dielectric effects of water.