Ions are expected to undergo partial dehydration under confined pore environments, enabling selective passage of monovalent ions through the 6 Å triangular windows of UiO-66-NH2 which is located ~54.7° away from the preferred oriented 200 facet (Figure ). The rejection of the divalent Mg 2+ ion can be further attributed to electrostatic repulsion with the positively charged-NH2 groups. Non-vdW 2D UiO-66 thin films were also prepared following a similar method (Figure -21, Supplementary Note 9) to serve as a charge comparison since UiO-66-NH2 possesses charged amino functional groups, whereas UiO-66 does not. In this case, average K + /Mg 2+ , Li + /Mg 2+ , and Na + /Mg 2+ selectivities were restricted to 11.8, 14.5, and 6.3, respectively. The much higher selectivity observed from the 2D UiO-66-NH2 film can be attributed to a positively charged functional group in UiO-66-NH2.