Water self-diffusivity confined in graphene nanogap using molecular dynamics simulations

Click on the DOI link to access the article (may not be free).Fundamental understanding of water confined in graphene is crucial to optimally design and operate sustainable energy, water desalination, and bio-medical systems. However, the current understanding predominantly remains in the static properties near the graphene surfaces. In this paper, a key water transport property, i. e., self-diffusivity, is examined under confinement by various graphene nanogap sizes (L-z = 0.7-4.17nm), using molecular dynamics simulations with various graphene-water interatomic potentials (Simple Point Charge (SPC/E) and TIP3P water models). It is found that the water self-diffusivity nearly linearly decreases as the graphene-water interatomic potential energy increases at a given nanogap size. It also decreases as the graphene nanogap size decreases down to L-z = 1.34 nm; however, it shows the peak water self-diffusivity at L-z = 0.8 nm and then continues to decrease. The peak water self-diffusivity is related to the significant change of the overlapping surface force, and associated, nonlinear local water density distribution. The in-plane water self-diffusivity is higher up to nearly an order of magnitude than that of the out-of-plane due to the geometrical confinement effect by the graphene nanogap. The obtained results provide a roadmap to fundamentally understand the water transport properties in the graphene geometries and surface interactionsNational Science Foundation under Award No. EPS-0903806 and matching support from the State of Kansas through the Kansas Board of Regents. This work was also partially supported by the start-up fund from the College of Engineering, Wichita State University. This work also used the Extreme Science and Engineering Discovery Environment (XSEDE), which was supported by National Science Foundation Grant No. ACI-1053575