Continuous Water Filling in a Graphene Nanochannel: A Molecular Dynamics Study

Study on Confined Water in Flexible Graphene/GO Nanochannels

The structural evolution of flexible nanochannels within a 2D material membrane, influenced by the ingress of water molecules, plays a crucial role in the membrane's filtration and structural stability. However, the experimental observation of nanoscale water is challenging, and current studies mostly focus on rigid nanochannels. Further investigation on the nanoconfined water is urgently needed, considering the flexibility and deformation of the channel. In this work, MD simulations and theoretical analyses are conducted to investigate the water structure and thermodynamic properties when confined within both rigid and flexible graphene/graphene oxide (GO) nanochannels. In free rigid graphene nanochannels, the interlayer distance exhibits a quantized increase with the number of water molecules, along with sudden changes in entropy, potential energy, and free energy of the water molecules. Meanwhile, in flexible graphene nanochannels, the average interlayer space increases linearly with the number of water molecules. In free rigid GO nanochannels, with the increase of oxidation concentration, the quantized increase in the interlayer space gradually diminishes, accompanied by a decrease in both potential energy and free energy. This work provides insights into the configurational evolution of flexible nanochannels within water, offering guidance in fields such as desalination and mass transport of 2D material membranes.

Ab Initio Molecular Dynamics Investigation of the Solvation States of Hydrated Ions in Confined Water

Ionic transport in nanoscale channels with a critical size comparable to that of ions and solutes exhibits exceptional performance in water desalination, ion separation, electrocatalysts, and supercapacitors. However, the solvation states (SSs), i.e., the hydration structures and probability distribution, of hydrated ions in nanochannels differ from those in the bulk and the perspective of continuum theory. In this work, we conduct ab initio enhanced-sampling atomistic simulations to investigate the ion-specific SSs of monovalent ions (including Li+, Na+, K+, F-, Cl-, and I-) in the graphene channel with a width of 1 nm. Our findings highlight that the SSs of those ions are primarily determined by ion-water hydration, where ion-wall interactions play a minor role. The distribution of ions in layered confined water is a result of ion-specific hydration, which arises from the synergy of entropy and enthalpy. The free energy barriers for transitions between SSs are on the order of 1kBT, allowing for modulation through applying external fields or modifying surface properties. As the ion-wall interaction strengthens, as observed in vermiculite and carbides and nitrides of transition metal channels, the probability of near-wall SSs increases. These results help to improve the performance of nanofluidic devices and provide crucial insights for developing accurate force fields of molecular simulations or advanced theoretical approaches for ion dynamics in confined channels.

Unexpected Behavior in Thermal Conductivity of Confined Monolayer Water

Monolayer water can be formed under extreme confinement and will present distinctive thermodynamic properties compared with bulk water. In this work, we perform molecular dynamics simulations to study the thermal conductivity of monolayer water confined in graphene channels, finding an unexpected way of thermal conductivity of monolayer water dependent on its number density, which has a close correlation with the structure of water. The monolayer water is in an amorphous state, and its thermal conductivity increases linearly with the area density when the water density is low at first. Then, the thermal conductivity increases as the number density of water rises, which is attributed to the formation of a crystal structure and the reduction of crystal defects as the number of water molecules increases. After reaching the zenith, the thermal conductivity decreases rapidly owing to the formation of a wrinkle structure of monolayer water with excessive water molecules, which weakens the phonon dispersion. Moreover, we further investigate the remarkable effects of the channel height on both the structure and thermal conductivity of monolayer water. In summary, this study demonstrates the close connection between the thermal conductivity of monolayer water and its structure, contributing to not only expanding the understanding of the thermodynamic property of nanoconfined water but also benefiting the engineering applications for nanofluidics.

Computational Strategies for Entropy Modeling in Chemical Processes

Computational simulations of entropy are important in understanding the thermodynamic forces that drive chemical reactions on a molecular scale. In recent years, various algorithms have been developed and applied in conjunction with molecular modeling techniques to evaluate the change of entropy in solvation, hydrophobic interactions, and chemical reactions. The aim of this review is to highlight four specific computational entropy calculation methods: normal mode analysis, free volume theory, two-phase thermodynamics, and configurational entropy modeling. The technical aspects, applications, and limitations of each method will be discussed in detail.

Atomistic Simulations of the Permeability and Dynamic Transportation Characteristics of Diamond Nanochannels

Through atomistic simulations, this work investigated the permeability of hexagonal diamond nanochannels for NaCl solution. Compared with the multilayer graphene nanochannel (with a nominal channel height of 6.8 Å), the diamond nanochannel exhibited better permeability. The whole transportation process can be divided into three stages: the diffusion stage, the transition stage and the flow stage. Increasing the channel height reduced the transition nominal pressure that distinguishes the diffusion and flow stages, and improved water permeability (with increased water flux but reduced ion retention rate). In comparison, channel length and solution concentration exerted ignorable influence on water permeability of the channel. Further simulations revealed that temperature between 300 and 350 K remarkably increased water permeability, accompanied by continuously decreasing transition nominal pressure. Additional investigations showed that the permeability of the nanochannel could be effectively tailored by surface functionalization. This work provides a comprehensive atomic insight into the transportation process of NaCl solution in a diamond nanochannel, and the established understanding could be beneficial for the design of advanced nanofluidic devices.

Reduced Ionic Conductivity but Enhanced Local Ionic Conductivity in Nanochannels

The ionic transport in nanoscale channels with the critical size comparable to ions and solvents shows excellent performance on electrochemical desalination, ion separation, and supercapacitors. However, the key quantity ionic conductivity (σ) in the nanochannel that evaluates how easily the electric current is driven by an external voltage is still unknown because of the challenges in experimental measurement. In this work, we present an atomistic simulation-based study, which shows that how the ion concentration, nanoconfinement, and heterogeneous solvation modify the ionic conductivity in a two-dimensional graphene nanochannel. We find that σ in the confined channel is lower than that in the bulk (σ_b) at the same concentration along with enhanced ion-ion correlation. However, surprisingly, the local σ near the channel wall is more conductive than σ_b and is about 2-3 folds of the inner layer due to the highly concentrated charge carriers. Based on the layered feature of σ along the width of the channel, we propose a model that contains two dead (or depletion) layers, two highly conductive layers, and one inner layer to describe the ionic dynamics in the nanochannels. Our findings may open the way to unique nanofluidic functionalities, such as energy harvesting/storage and controlling transport at single-molecule and ion levels using the liquid layer near the wall.