Amorphous Porous Organic Cage Membranes for Water Desalination

Emerged as a new class of nanoporous materials, porous organic cages (POCs) possess salient features of solvent processability and water stability; thus, they are envisioned as promising membrane materials for water desalination. In this study, we propose a simulation protocol to construct atomic models of amorphous POC membranes and examine their desalination performance. Five membranes (AC1, AC2, AC3, AC16, and AC17) with similar cage structure but different periphery groups are considered. All the five membranes exhibit 100% salt rejection. In contrast to crystalline CC1 membrane, which is impermeable to water, AC1 has a water permeability <i>P</i>_w of 3.6 × 10^–8 kg·m/(m²·h·bar). With increasing interconnected pores in AC2, AC3, AC16, and AC17, <i>P</i>_w increases. Due to the existence of hydroxyl groups in CC17 cages, AC17 exhibits the highest <i>P</i>_w of 3.17 × 10^–7 kg·m/(m²·h·bar), which is higher than in commercial reverse osmosis membranes. Significantly, <i>P</i>_w is found to enhance in mixed AC3/AC17 and AC16/AC17 membranes with up to one-fold enhancement. The enhanced <i>P</i>_w is attributed to the counterbalance between water sorption and diffusion. This simulation study provides the bottom-up insights into the dynamics and structure of water in amorphous POC membranes, highlights their potential use for water desalination, and suggests a unique strategy to enhance desalination performance by tuning the composition of mixed POC membranes

Spreading of a Unilamellar Liposome on Charged Substrates: A Coarse-Grained Molecular Simulation

Supported lipid bilayers (SLBs) are able to accommodate membrane proteins useful for diverse biomimetic applications. Although liposome spreading represents a common procedure for preparation of SLBs, the underlying mechanism is not yet fully understood, particularly from a molecular perspective. The present study examines the effects of the substrate charge on unilamellar liposome spreading on the basis of molecular dynamics simulations for a coarse-grained model of the solvent and lipid molecules. Liposome transformation into a lipid bilayer of different microscopic structures suggests three types of kinetic pathways depending on the substrate charge density, that is, top-receding, parachute, and parachute with wormholes. Each pathway leads to a unique distribution of the lipid molecules and thereby distinctive properties of SLBs. An increase of the substrate charge density results in a magnified asymmetry of the SLBs in terms of the ratio of charged lipids, parallel surface movements, and the distribution of lipid molecules. While the lipid mobility in the proximal layer is strongly correlated with the substrate potential, the dynamics of lipid molecules in the distal monolayer is similar to that of a freestanding lipid bilayer. For liposome spreading on a highly charged surface, wormhole formation promotes lipid exchange between the SLB monolayers thus reduces the asymmetry on the number density of lipid molecules, the lipid order parameter, and the monolayer thickness. The simulation results reveal the important regulatory role of electrostatic interactions on liposome spreading and the properties of SLBs

Molecular Theory for Electrokinetic Transport in pH-Regulated Nanochannels

Ion transport through nanochannels depends on various external driving forces as well as the structural and hydrodynamic inhomogeneity of the confined fluid inside of the pore. Conventional models of electrokinetic transport neglect the discrete nature of ionic species and electrostatic correlations important at the boundary and often lead to inconsistent predictions of the surface potential and the surface charge density. Here, we demonstrate that the electrokinetic phenomena can be successfully described by the classical density functional theory in conjunction with the Navier–Stokes equation for the fluid flow. The new theoretical procedure predicts ion conductivity in various pH-regulated nanochannels under different driving forces, in excellent agreement with experimental data