(591g) Functionalized Nanoporous Graphene As a Candidate for Selective Water Evaporation from Ionic Solutions

Using molecular dynamics simulations, we report enhancement in evaporation flux from various ionic solutions (LiCl, NaCl, and KCl) using functionalized nanoporous graphene (FNPG) placed at the liquid-vapor interface. The pore sizes examined in our study range from 0.454 nm – 1.303 nm with edges functionalized with hydroxyl functional groups. The enhancement in evaporation flux with respect to the bare liquid-vapor interface (ξ) depends upon the salt type, concentration, and pore size of the FNPG. Due to cation-Ï€ interactions, we found an enhanced density of cations at the surface of FNPG leaving the contact water layer depleted of ions. The propensity of ions to adsorb on the FNPG surface decreases in the order K⁺ > Na⁺ > Li⁺ > Cl^-, resulting in enhancement factors, 7.5 < ξ < 11.5 for the smallest nanopores. The highest value of ξ of about 11 is observed for the 0.6 M KCl solution for smallest nanopores with pore edges functionalized with hydroxyl functional groups. The interaction between edge functional groups at the nanopores and water molecules in the vicinity of the nanopore results in a reduced number and short-lived hydrogen bonds between water molecules. Umbrella sampling computations reveal a decrease in the free energy barrier for evaporation and reduced surface tension at the water-vapor interface further assists in enhancing the escape of water molecules through the nanopores. The increased propensity of water molecules to escape from the liquid-vapor interface is also evident from the higher normal diffusion coefficient of water molecules. This study illustrates the potential for using nanoporous graphene for desalination applications as well as separation processes with low thermal energy input.