Molecular dynamics of space-confined water inside Span 80 reverse micelles with an all-atom and coarse-grained solvent

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 a number of Span 80 molecules equal to 51, 246, and 475, respectively, and average inner-core water radii in the range of 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 part of the system of real interest.