Molecular Dynamics of Space-Confined Water inside Span 80 Reverse Micelles with All-Atom and Coarse-Grained Solvent

Abstract

We investigate the structural and dynamical properties of confined water in reverse micelles (RMs) by means of molecular dynamics simulations and we assess the effect of coarse-grained (CG) modeling of the solvent on their reproducibility. In particular, simulations of three differently-sized RMs constituted by Span 80 (sorbitan monooleate) surfactant molecules dispersed in cyclohexane solvent (‘oil phase’) were set up by varying the number of surfactant molecules and the water-to-surfactant ratio. The resulting RMs have number of Span 80 molecules equal to 51, 246, and 475, respectively, and average inner-core water radii comprised in the range 10–60 Å. For the oil phase, three different levels of description of the cyclohexane molecules are considered: an all-atom (AA) description and two different CG versions constituted by three and two beads, respectively, where each bead interacts with other beads and atoms via a standard Lennard-Jones 12-6 potential. All simulations were performed with the NAMD package. Our results highlight the presence of two distinct regions of inner-core and interfacial water, and show that the structural and dynamical properties of these are not significantly affected by a CG modeling of cyclohexane, confirming that the simulation time can be considerably reduced by blurring the solvent details while keeping an accurate AA description of the real part of interest.