Molecular Dynamics Simulation Studies of Two-Dimensional Materials for Water Desalination


Student thesis: Doctoral Thesis

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Awarding Institution
Award date10 Aug 2023


The membrane-based desalination technologies are considered as the most efficient desalination technology to rescue the increasingly severe water scarcity. However, desalination membrane suffers from low water permeability, membrane oxidation, and high energy consumption. Exploring and developing alternative membrane materials for desalination has arisen great interest in the entire research community, with the expectation to improve the desalination performance and alleviate the instinctive drawbacks of traditional desalination membranes.

The booming nanotechnology and nanomaterials pave the way for exploiting new desalination membranes. Among all the newly emerging nanomaterials, two-dimensional (2D) materials refer to nanomaterials with atomic thickness. 2D nanosheets or nanoporous materials have been proven to be qualified with excellent water desalination performance by virtue of their atomic thickness size to regulate the transport of water molecules and ion particles precisely.

Experimental studies have revealed that 2D materials possess exceptional desalination performance. However, we cannot fully understand the interactions between 2D membranes and water molecules or ions using experimental methods. And these interactions are highly responsible for the desalination performance. Fortunately, molecular dynamics (MD) simulations provide us with a “computational microscope” to observe these mechanisms. In this thesis, we adopted MD methods to systematically explore the application potential of 2D materials as desalination membranes. The mechanisms responsible for the exceptional desalination performances are revealed, and we also propose strategies to obtain better desalination performance.

As desalination membranes, 2D materials are often stacked layer by layer to achieve excellent performance. And in this manner, nanochannels and nanoslits are inherently produced. In addition, both axial and transverse pressure can be applied to the desalination membranes. As a result, nanochannels and nanoslits are mainly forms for desalination adopting 2D materials as membranes.

Hydrophilic stacking laminates suffer from swelling when exposed to solution environments, which enlarges the interlayer spacings of nanochannels, thus deteriorating the water ion sieving performance. Intercalations of cations have been proven to effectively improve the anti-swelling and precisely control the interlayer spacing of stacked nanosheets, where the negative surface charge exerts a significant influence on stabilizing the cations and the desalination performance. However, little previous work investigated the surface charge on the desalination performance of stacked laminates. To address this issue, the surface charge of graphene oxide (GO) was included to explore its effects on desalination performance using molecular dynamics simulation methods. Simulation results showed that the increase of surface charge density enhanced the interactions between GO nanochannel and water as well as ions. As a result, the water flux was improved, but the ion rejection rate decreased with increased surface charge density. We further explored the effects of intercalated cations on desalination performance. The simulation results showed that the desalination performance of GO nanochannel was optimized at an appropriate interlayer distance. Among all the investigated cases, GO nanochannel intercalated with Mg2+ possesses the best desalination performance. Surface charge density was found to exert little effect on optimizing the Li+/Mg2+ separation performance. However, the Li+/Mg2+ separation performance could be optimized by narrowing the interlayer distance of the GO nanochannel.

Except for the size exclusion effects, the electrostatic Donnan exclusion effects also play a significant role in the desalination performance of 2D desalination membranes. The electrostatic interactions between nanosheets and water/ions are mainly determined by the surface functionalization and compositions of nanosheets. In this aspect, MXenes have been proven to hold broad application prospects as desalination membranes in experiments considering their richness of composition, manipulatable terminations, and intriguing charge features. Motivated by this, we explored the influences of the termination group, including fluorine (F), oxygen (O), and hydroxyl groups (OH), on the desalination performance of the Ti3C2Tx membrane. The simulation results showed that the surface charge features, as well as the hydrogen bond interactions, significantly influence the interactions between the termination group and water. The water permeability through MXene channel with different surface terminations follows the order F > O > OH. The charge nature of surface terminations also plays a vital role in their interactions with ions. The negative charge of surface terminations traps Na+ ions both near the mouth and inside the channel of MXene membranes. As a result, MXene membranes possess remarkable Na+ ion rejection performance. After comparison, we conclude that the desalination performance of Ti3C2F2 is much better than traditional desalination membranes.

Except for nanochannels, MXene nanoslits are also inherently produced by its stacked laminates. And the MXene nanoslits could sieve water and ions under specific work conditions. Thus, we further assessed the desalination performance of Ti3C2TX nanoslits. The influence factors, including the termination group, the slit width, temperature, pressure, and thickness, were discussed with the expectation of probing the optimal application conditions for MXene nanoslit membranes. Simulation results showed that MXene nanoslit possesses excellent desalination performance thanks to the charge features of the nanoslit channel. Increasing slit width, temperature, and pressure promotes water permeance but deteriorates ion rejection performance. Fortunately, the termination groups can mediate the tradeoff between water permanence and ion rejection rate. The increase of MXene layer number could effectively improve the desalination performance, taking both water flux and ion rejection performance into account.

Taken together, a comprehensive investigation of the desalination performance of stacked nanosheets was performed. The simulation results showed that the interactions between nanosheets and water molecules or ions dominate the desalination performance of 2D membranes. Despite these interactions, the arrangement manners of nanosheets, geometry size, and work conditions significantly influence the desalination performance. The present work might provide some valuable suggestions for designing water desalination membranes with exceptional desalination performance by manipulating the electrostatic properties of membranes.

    Research areas

  • Molecular dynamics simulation, Desalination membranes, Two-dimensional materials, Surface ionization, Graphene oxide, MXene