Dynamic Susceptibility and Structural Heterogeneity of Large Reverse Micellar Water: An Examination of the Core-Shell Model via Probing the Layer-wise Features.

In this work, we explored, via molecular dynamics simulations, layer-wise structural and spatio-temporal heterogeneity features of confined water inside rigid spherical reverse micelles of 55 Å inner diameter. These confined aqueous pools were divided into four fictitious concentric layers of 5 Å thickness and a central core layer. Reverse micellar confinements were constructed using model potentials mimicking AOT (charged) and IGEPAL (neutral) surfactant molecules for encapsulating SPC/E water. Density profiles for confined water were obtained and compared to validate the present simulations. The simulated layer-wise structural features were: dipole orientation distributions, tetrahedral angle distributions, tetrahedral order parameter, and the average number of H-bonds per water molecule and the relevant population distributions. Simulated dynamical features included mean-square displacements, velocity autocorrelation functions, non-Gaussian parameters, single-particle displacement distributions, dynamic susceptibilities, and the collective single-particle reorientational relaxations of first and second ranks. Analyses of simulation results revealed a strong impact of the confinement on bulk water structure and dynamics. The chemical nature of the confinement was found to influence both structure and dynamics. Interfacial water molecules were found to be the most severely affected ones, and the successive progression toward the center revealed a tendency for restoration of the bulk limit, although the bulk values were never fully recovered. A close inspection of the simulated results revealed an overlap among the layer-wise structural and dynamical features. These observations suggest a breakdown of the two-state core-shell model even for large reverse micelles (RMs) where an ample amount of "free" water is available. The simulated collective reorientational relaxations of reverse micellar water agree well with the existing time-resolved two-dimensional infrared (2D-IR) measurements.