Water Flow inside Polamide Reverse Osmosis Membranes: A Non-Equilibrium Molecular Dynamics Study

Water flow inside polyamide (PA) reverse osmosis (RO) membranes is studied by steady state nonequilibrium molecular dynamics (NEMD) simulations in this work. The PA RO membrane is constructed with the all-atom model, and the density and average pore size obtained thereby are consistent with the latest experimental results. To obtain the time-independent water flux, a steady state NEMD method is used under various pressure drops. The water flux in our simulations, which is calculated under higher pressure drops, is in a linear relation with the pressure drops. Hence, the water flux in lower pressure drops can be reliably estimated, which could be compared with the experimental results. The molecular details of water flowing inside the membrane are considered. The radial distribution function and residence time of water around various groups of polyamide are introduced to analyze the water velocities around these groups, and we find that water molecules flow faster around benzene rings than around carboxyl or amino groups in the membrane, which implies that the main resistance of mass transport of water molecules comes from the carboxyl or amino groups inside the membranes. This finding is in good consistency with experimental results and suggests that less free carboxyl or amino groups should be generated inside RO membranes to enhance water permeance

Homoporous Membranes with Tailored Pores by Soaking Block Copolymer/Homopolymer Blends in Selective Solvents: Dissolution versus Swelling

Extraction homopolymers premixed in aligned films of block copolymers by rinsing with selective solvents has long been used for the preparation of membranes with uniform straight pores (homoporous membranes). It is frequently assumed that only the dissolution of homopolymers contributes to the pore formation. However, in this work, we demonstrate that the effect of swelling plays a significant role in determining the pore sizes. We prepare blended films of block copolymers of polystyrene-<i>block</i>-poly­(2-vinyl­pyridine) (PS-<i>b</i>-P2VP) and P2VP homopolymers with low molecular weight and anneal the films to perpendicularly align the P2VP microdomains. Rinsing the aligned films in ethanol results in homoporous membranes, and the pore sizes can be tuned by the dosages of P2VP homopolymers. Interestingly, the pore sizes can also be effectively tailored by changing the rinsing temperatures and/or durations because of the significant contribution of the selective swelling of P2VP blocks under strong rinsing conditions in addition to the contribution of the dissolution of P2VP homopolymers. We identify the portion of the contribution from dissolution and from swelling and demonstrate that the pore sizes can be flexibly tuned within a wider range at no expense of pore ordering and uniformity by balancing the effect of dissolution and swelling

An Alternative Scalable Process for the Synthesis of the Key Intermediate of Omarigliptin

An alternative scalable process for the synthesis of the key intermediate of omarigliptin is described. The asymmetric synthesis relies on the initial diastereoselective alkylation and subsequent aluminum-catalyzed substrate-controlled Meerwein–Ponndorf–Verley reduction. A highly regioselective 5-exo-dig iodocyclization followed to afford <b>11b</b>, which was then subjected to ring-opening cycloetherification to give product <b>1</b> with >99:1 dr and >99% ee in 31.2% overall yield in nine steps. This synthetic strategy has been successfully applied for multikilogram scale production