Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles

Abstract

Aqueous reverse micelles, which are surfactant aggregates in nonpolar solvents that enclose packets of aqueous solution, have been widely studied experimentally and theoretically, but much remains unknown about the properties of water in the interior. The few previous molecular dynamics simulations of reverse micelles have not examined how the micelle size affects these properties. We have modeled the interior of an aqueous reverse micelle as a rigid spherical cavity, treating only the surfactant headgroups and water at a molecular level. Interactions between the interior molecules and the cavity are represented by a simple continuum potential. The basic parameters of the modelsmicelle size, surface ion density, and water contentsare based on experimental measurements of Aerosol OT reverse micelles but could be chosen to match other surfactant systems as well. The surfactant head is modeled as a pair of atomic ions: a large headgroup ion fixed at the cavity surface and a mobile counterion. The SPC/E model is used for water. The simulations indicate that water near the cavity interface is immobilized by the high ion concentration. Three structural regions of water can be identified: water trapped in the ionic layer, water bound to the ionic layer, and water in the bulklike core. The basic properties of bulk water reemerge within a few molecular layers. Both the structure and dynamics of water near the interface vary with micelle size because of the changing surface ion density. The mobility of water in the interfacial layers is greatly restricted for both translational and rotational motions, in agreement with a wide range of experiments.

Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump−Probe Studies of Reverse Micelles

“…Furthermore, simulations show that the Na ions are associated with the headgroups. 28 Consequently the ionic strength of the nanoscopic water is much lower than 5.6 M, and the influence of free ions in the water nanopools on the vibrational echo decay will be much less than that measured for the 6 M NaCl solution.…”

Time-dependent fluorescence in nanoconfined solvents: Linear-response approximations and Gaussian statistics

“…30,31,35,36 The potential depends only on the radial distance of the Lennard-Jones site on the molecule from the center of the cavity. We consider a single solvent density, ρ = 0.7 g/cm 3 (the bulk density of CH 3 CN is 0.786 g/cm 3 ).…”

Simulations of time-dependent fluorescence in nano-confined solvents

“…32,33 The interactions of the solute and solvent molecules with the cavity walls involve only Lennard-Jones interactions. 32,33 The potential depends only on the radial distance of the Lennard-Jones site on the molecule from the center of the cavity. We consider two solvent densities, ϭ1.4 g/cm 3 and 2.0 g/cm 3 ͑the bulk density of CH 3 I is 2.279 g/cm 3 ͒.…”

“…Their hydrophobic cavity model, 33 developed by Linse and Halle, 32 is the same as that used here; the reverse micelle model consists of the same cavity framework with fixed anionic headgroups and mobile cationic counterions added. 33 The solvation dynamics were studied using an anionic diatomic probe molecule with symmetrically ͑ground state, ϭ0͒ or asymmetrically ͑excited state, ϭ7.76 D͒ distributed charge. They simulated the solvation dynamics for the first 2 ps after excitation and obtained results that were relatively independent of the size of the reverse micelle.…”