Physicochemical Simulation on Membrane Transport Phenomena

다국어 초록 (Multilingual Abstract)

Membrane technologies have garnered a lot of attention among several separation techniques and are widely used around the world due to the rapid increase of world’s population, acceleration of industrialization and aggravation of environmental pollution. There are three dominant categories of membrane technologies namely gas separation, water desalination and decontamination. For these dominant categories, conventional polymeric membranes such as poly (vinylidene fluoride), polyamide and polysulfone have been used without any substitution for a long time. Polyamide membrane composites that are typically used in commercial reverse osmosis (RO) systems were developed thirty years ago. The water flux of this material has only increased about two-fold in the past twenty years and the performance of commercial RO desalination membranes has not been improved significantly in terms of selectivity and permeability.
Wide range of nanomaterials have been tested to improve salt ion rejection and water permeation. Nanoporous materials have well-defined pores with diameter of single digit nanometer or less contrary to the conventional polymer-based membranes. Zeolite, carbon nanotubes, graphene and boron nitride composites are the commonly studied nanomaterials for desalination process on account of their improved ion selectivity and water permeability. Carbon nanotube (CNT) membranes have been deeply researched since the disclosure of the fact that CNT permits water molecules to pass through inside bulk water. This equilibrium molecular dynamics (EMD) simulation resulted in numerous molecular dynamics and experimental works. Many researchers conducted non-equilibrium molecular dynamics (NEMD) to examine the probability of CNT as an RO membrane under pressurized conditions. Despite these countless studies, CNT membranes’ applicability as an RO membrane was rated to be low due to its low ion rejection rate. In the same manner, nanoporous graphene was considered a complete inorganic material. Studies have shown that porous single-layer graphene has excellent properties that result in high water flux and salt rejection capabilities. Even though the application of single-layer graphene in the desalination process shows a sensational outcome, the preparation of single-layer graphene with a large surface area is problematic due to the formation of cracks and overlap of graphene sheets.
The use of multilayered graphene can be a good alternative to the above-mentioned issues faced by single-layer graphene. Compared to single-layer graphene membranes, multilayered membranes are easier to prepare, have higher efficiency and are less expensive. Three factors should be taken into consideration to carry out NEMD simulation of multilayered membrane system: The choice of water model, physical performance improvement and chemical performance improvement. Rigid 3-point water models such as SPC, SPC/E, TIP3P-FB, TIP3P-EW and OPC3, are commonly used models for MD simulation. However these five water models were developed under atmospheric pressure condition which means that they are not targeted to be used in NEMD simulation. Owing to the absence of theoretically appropriate water model, the selection of water model should be tested even in a practical way. Large number of simulations have been carried out to enhance the performance of the nanomaterial membranes physically and chemically. Pore morphology and layer orientation are chosen to increase the performance physically. The shape of pore affects the transport of water molecules due to graphene’s honeycomb lattice and this honeycomb lattice causes layer orientation to generate a pore morphology-like effect indirectly. Interfacial atoms, which are located in the first shell of pore, play a key role to characterize mass transport chemically. Pore hydrogenation is one of the diverse functionalization methods and the simplest way to create a charged pore. Boron nitride (BN) nanosheets are the structural analogs of graphene but also at the same time they are diatomic composites which have positive and negative charged atoms simultaneously. Due to unneutrality and graphene-like structure of BN composites, pore consisting of BN is a proper candidate to investigate the atomistic diversity compared to graphene pore system.
This thesis presents NEMD computation of membrane transport phenomena through multilayered membrane systems under various conditions. The main part of this thesis is divided into three parts: (1) Comparison of water models through non-functionalized and functionalized pores, (2) effect of layer orientation and pore morphology and (3) atomistic diversity of pore by adopting BN compared to graphene.