Elucidation of liquid phase permeation phenomena in nanoporous
membranes by non-equilibrium molecular dynamics
Hayato HIGUCHI
Abstract
The objective of this work is to explain the phenomenon of liquid phase permeation in nanopores
separation membranes, which have applications in fields such as water treatment. The results ofinvestigations using the non-equilibrium molecular dynamics method, a molecular simulation
technique, are summarized here. Molecular simulation enables the analysis of diffusion phenomena at
the molecular level in nanostructures. However, the adaptation of this approach to non-equilibriumsystems, such as membrane permeation, has been limited by methodological challenges andcomputational burdens, which have restricted previous research. This thesis focuses on theFluctuating-Wall Molecular Dynamics (FW-MD) approach, which is capable of handling nonequilibrium systems. It applies molecular simulations to liquid-phase membrane separation systems,
where no previous reports have been available. After demonstrating the quantitative aspects ofsimulating liquid phase permeation phenomena, the study applies this methodology to organic solvent
separation and the evaluation of novel water-permeable membranes, revealing the permeation
phenomena within nanopores at the molecular level. The thesis comprises chapters 1 through 7.
Chapter 1 serves as an introduction, elucidating the background, objectives, and research approach of
the study. This text discusses the importance of membrane separation technology in addressing theglobal water scarcity issue. It highlights the role of molecular simulation in membrane engineeringand outlines the principles of molecular dynamics and the methodological characteristics of FW-MD.
Chapter 2 discusses the application of FW-MD to predict the permeation flux of isotropic membranes
with nanopores. The osmotic pressure difference drives the osmotic process and knowledge of theosmotic pressure of the solution allows the prediction of the permeation flux. The permeation flux
obtained through FW-MD closely matches the theoretical values. In addition, this chapter proposes amethod to determine the concentration dependence of osmotic pressure from the solvation structurearound the solute. This method has been validated using simulation results from FW-MD. Although
the membrane model used in this chapter is relatively simple, it effectively demonstrates theapplicability of FW-MD to isotropic membrane research. This demonstration considers more complex
membrane structures and operating conditions.
Chapter 3 discusses the application of FW-MD to organic solvent reverse osmosis (OSRO). This is aseparation technique that is gaining attention as an alternative to distillation. The study predicts thepermeability and separation performance of organic solvents in Y-type zeolite membranes with
nanoporous, visualizing the permeation phenomenon within membrane pores at the molecular level.
The results reveal differences in permeation characteristics compared to vapor-permeation separation.
Selecting suitable membrane materials and separation systems is considered essential for effective

OSRO based on the discussion of results. Therefore, FW-MD is used to separate low-molecular-weightalcohol mixtures, demonstrating the usefulness of Y-type zeolite membranes.
Chapter 4 proposes methods for enhancing permeation flux in OSRO and verifies them using FWMD. The pressure dependence of permeation flux in OSRO is lower than in aqueous systems, soincreasing operating pressure does not yield sufficient permeation flux. Therefore, a less polar highsilica FAU zeolite is selected to increase permeation flux, which is verified using FW-MD. The results
demonstrate improvements in permeation flux and selectivity in ethanol/cyclohexane mixturescompared to Y-type zeolite membranes. Moreover, it has been demonstrated that modifying theadsorption characteristics on the permeate side membrane surface can enhance permeation flux.
Additionally, the modification of the permeate side membrane surface has been shown to improve
permeation flux. The study finds that membrane materials suitable for OSRO are different from thoseused for vapor permeation separation and that modifying the permeate side membrane surface
improves permeation flux.
Chapter 5 evaluates the permeability of water in carbon nanotube with charged pore walls as an
example of a novel membrane structural design using FW-MD. Changing the charge distribution on
the pore walls can change the orientation of the permeating water molecules, thereby affecting thepermeation flux. Molecular-level analysis shows that the ease of hydrogen bond formation between
pore wall charges and permeating water molecules influences this phenomenon. Thus, designing thecharge distribution on the walls of nanoporous can enable the creation of highly permeable waterpermeable membranes.
Chapter 6 proposes a novel membrane material based on self-assembled supramolecular to achieve
high water permeability. The permeability is predicted using FW-MD. The study focuses on the pillarshaped structure of cyclic tetramers of 2-phenyl-1,3,4-oxadiazole and analyzes water permeation
within membranes stacked with these supramolecular in detail. The study reveals that a single-filediffusion of molecules within the pores leads to high water permeability. By altering thesupramolecular structure, multiple membrane models with varying pore sizes were created to identify
the optimal pore size for maximizing permeability. The proposed mechanism of supramolecularmembrane permeation is unique and suggests a novel water permeation mechanism for achieving highpermeability.
Chapter 7 serves as a summary of chapters 1 through 6. Throughout the thesis, simulations of liquid
phase membrane permeation phenomena using FW-MD have been consistently conducted. Using FWMD has allowed analyzing membrane permeation phenomena under a constant pressure gradient. The
knowledge gained provides theoretical design guidelines for the development and improvement of
nanoporous separation membranes for water treatment systems. Selective water-permeable
membranes are crucial for effectively addressing water scarcity issues, making the findings of this
doctoral thesis industrially significant.