Dipeptide Crystals as Reverse Osmosis Membranes for Water Desalination: Atomistic Simulation

An atomistic simulation study is reported to investigate the capability of dipeptide crystals as reverse osmosis (RO) membranes for water desalination. Eight dipeptides are considered, namely, Ala-Val (AV), Val-Ala (VA), Ala-Ile (AI), Ile-Ala (IA), Val-Ile (VI), Ile-Val (IV), Val-Val (VV), and Leu-Ser (LS). It is revealed that water flux is governed by both pore size and helicity. With a relatively larger pore size, AV, AI, VV, and LS exhibit a higher water flux than VA, IA, VI, and IV. Despite similar pore size in AI and VA, a higher flux is observed in AI because of a lower helicity. On the other hand, VI, LS, IV, and IA possess higher salt rejection (>90%, and 100% for VI) than the rest (<70%). The salt rejection is determined by the electric potential difference across the membrane, induced by the staggered arrangement of −NH3+ and −COO– groups in the dipeptides. This unique arrangement of charge groups is not observed in other types of RO membranes. A higher electric potential difference allows more ions to pass through the membrane, leading to lower salt rejection. The lifetime of hydrogen bonding of water in LS membrane is shorter than those in VI and IV, which follows the decreasing trend of the water flux. An Arrhenius relationship is found between the water flux and temperature in LS and VI, and the activation energies are predicted to be 27.72 and 33.42 kJ/mol, respectively. Furthermore, the membrane flexibility is examined; a correlation between the water flux and the position restraint force constant is obtained. This simulation study provides microscopic insights into the important structural and dynamic properties of water in dipeptide membranes and suggests their potential use as RO membranes for water desalination.