Structure and dynamics of water confined in nanoporous carbon

Abstract

We study the structure and dynamics of water confined in a nanoporous graphitic carbon structure using molecular-dynamics (MD) simulations. The carbon structure is generated by a reactive MD simulation of oxidation of a silicon carbide nanoparticle. We embed water molecules in the nanopores and study structural and dynamical properties of nanoconfined water as a function of temperature. MD simulation results indicate the presence of high-density water (HDW) and low-density water. Radial distribution functions and spatial density functions indicate that the second solvation shell of the HDW is broken. We calculate the self-diffusion coefficient of confined water molecules as a function of temperature and find a significant decrease in the diffusion of water molecules around $T=190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The cage correlation function $c(t)$ of confined water molecules at $T=200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ exhibits stretched exponential decay, $c(t)=\mathrm{exp}[\ensuremath{-}{(\mathrm{t}/\ensuremath{\tau})}^{\ensuremath{\beta}}]$, with $\ensuremath{\beta}=0.43$, which matches exactly with the theoretical prediction $\ensuremath{\beta}=3/7$. Furthermore, the self-intermediate scattering function at $T=200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ indicates differences in small-scale and large-scale dynamics of water molecules.