Knowledge Bank

Reverse osmosis (RO) is a critical technology for the desalination of water that will become increasingly important in the future to maintain our supply of clean water. To maximize the efficacy of this RO process, a critical factor is the material of the nanoporous membrane through which water and salt are separated. At the nanoscale, it is expected that the specific shape and three-dimensional geometric structure of the pores within these membrane materials can be a strong predictor of the overall effectiveness of the membrane. Therefore, for the rational design of new RO membrane materials, it is useful to model desalination to predict the properties and performance of membrane materials with varying geometry. In this thesis, a molecular dynamics (MD) model is utilized to examine a variety of RO membrane material geometries in both flat, 2D nanopores, akin to nanoporous graphene, as well as in 3D channel structures, which are representative of a wide variety of nanoporous membrane candidates. Specifically, an ultradense carbon model is developed such that the bulk interactions match those of a more realistic graphene mesh and is subsequently utilized to perform a full desalination simulation through circular, square, and triangular flat pores of varying sizes. The same model is then extended into a 3D channel structure with three variations: a straight channel with constant cross-sectional area (A), a 'cage' channel with regions of smoothly increasing A approaching the channel center from either direction, and an 'hourglass' channel with regions of smoothly increasing A approaching either channel edge. These channel structures are also examined through the course of a full desalination simulation. Additionally, for selected channel structures, especially cage channels where the cage size approximates the size of a hydrated salt ion, a determination of the applied feed pressure needed to fully saturate the channel with water is conducted. The results of this thesis indicate that in the flat nanopores, triangular pores exhibit a much higher flux of water than familiar circular pores, with square pores at an intermediate value. This is attributed to the larger accessible pore area in triangular pores with the same pore-limiting diameter (PLD) as analogous circular pores. The penalty to salt rejection from this larger accessible area is relatively minor due to the inability of hydrated salt ions to traverse the narrow corners of the triangular pore, and so the triangular pore is found to offer an overall increased RO performance. The results similarly indicate an increased performance for hourglass channels over straight ones. The larger entrance diameter appears to encourage water flux, while the equivalent PLD maintains salt rejection. In the saturation pressure study, the scope of these simulations appears insufficient to fully characterize the relationship between cage channel structure and accessibility to water flow, although a promising candidate for future study may be a prolate spheroid cage with major (x, y) diameter ~8 Angstrom and z-direction diameter augmented by ~30%. The geometries identified in this thesis can serve as promising starting points for ongoing development efforts in the improved RO desalination membrane material space.