Aqueous reverse micelles are composed of a water core held together by surfactant molecules in a hydrophobic solvent. We simulated such reverse micelles composed of surfactant dioctyl sodium sulfosuccinate (AOT), water represented by TIP3P model and isooctane solvent. The reverse micelles were created using Packmol, and constant pressure-temperature simulations were conducted using GROMACS molecular dynamics package. These fully atomistic simulations were carried out using the CHARMM 36 force field. The reverse micelle was unconstrained and allowed to move freely during the simulation. The simulated reverse micelles had w0 values of 10, 15, and 20. The w0 number is the ratio of water to surfactant molecules. For each w0 value, we simulated reverse micelles with KCl, MgCl2, and CaCl2 at concentration values of 0.25 M, 0.50 M, 0.80 M, and 1.0 M. The results indicate that the ions tend to form well-defined layers in the interfacial region. Specifically, the cations reside near the anionic surfactant head groups and even replace the Na+ counter-ion of the surfactant while the Cl- anions prefer the water core. Density calculations suggest that the interfacial region of the reverse micelles are structured as follows: R-SO3- &gt; Na+ ≥ K+ &gt; Cl- (Core). The radial distribution functions, g(r), for SO3--K+ and SO3--Na+ of all reverse micelles were calculated and indicate that majority of both ions are localized near the head groups of the AOT. Other measurements included mass distribution in the simulation box and the distance of ions from the center of the water core. We also analyzed the impact of ionic concentration on shape and size of reverse micelles. A general trend that emerges is that as w0 increases the reverse micelles tend to stay spherical. The connection between ionic concentration and shape of reverse micelle is more nuanced and depends on the composition of the reverse micelle.

Simulations of AOT reverse micelles with chloride salts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.