Molecular dynamics has been used to study the separation of supercritical solutions using zeolite membranes. In the simulations, two thin membranes cut from a cubic cell of ZK-4 zeolite were used as the semipermeable membranes to separate water from aqueous NaCl solutions. The results indicate that molecular dynamics is a viable technique for studying such separation processes at the fundamental molecular level. The study also showed that ZK-4 zeolite membranes show promise for use in membrane-based separation of aqueous electrolyte solutions, as well as other similar systems. Our simulations have also shown the important role external electric fields can play in enhancing the separation rate in such systems. Finally, through our simulations, we found the serious risks in applying principles of macroscopic hydrodynamics to nanoscale systems.