  Researchers are currently exploring nanoporous two-dimensional materials,
such as hexagonal boron nitride (hBN), as potential desalination membranes.
Here, we study the effect of grain boundaries (GBs) and interfacial
electrostatic interactions on the desalination performance of bicrystalline
nanoporous hBN, using classical molecular dynamics simulations supported by
quantum-mechanical density functional theory (DFT) calculations. We investigate
three different nanoporous bicrystalline hBN configurations, with symmetric
tilt GBs having misorientation angles of 13.2{\deg}, 21.8{\deg}, and
32.2{\deg}. We find that grain boundaries alter the areas and shapes of
nanopores in bicrystalline hBN, as compared to the nanopores in monocrystalline
hBN. We observe that, although bicrystalline nanoporous hBN with a
misorientation angle of 13.2{\deg} shows improved water flow rate by ~30%, it
demonstrates reduced Na$^\mathrm{+}$ ion rejection by ~6%, as compared to
monocrystalline hBN. We also uncover the role of the nanopore shape in water
desalination, finding that more elongated pores with smaller sizes (in
21.8{\deg}- and 32.2{\deg}-misoriented bicrystalline hBN) can match the water
permeation through less elongated pores of slightly larger sizes, with a
concomitant ~3-4% drop in Na$^\mathrm{+}$ rejection. Simulations also predict
that the water flow rate is significantly affected by interfacial electrostatic
interactions. Indeed, the water flow rate is the highest when altered partial
charges on B and N atoms were determined using DFT calculations, as compared to
when no partial charges or bulk partial charges (i.e., charged hBN) were
considered. Overall, our work on water/ion transport through nanopores in
bicrystalline hBN indicates that the presence of GBs and surface charge can
lead, respectively, to a drop in the ion rejection and water permeation
performance of hBN membranes.
