Theoretical Study Of Gas Diffusion Through Nanoscale Water Layer

The diffusion process of gas through nanoscale water layer has extensive applied foundation and prospect in various fields,including physics,chemistry,biology,environmental science and engineering.The whole diffusion process can be divided into several parts,which are adsorption,dissolution,diffusion and desorption.From general points of view,the diffusion efficiency of gas through nanoscale water layer is mainly affected by the solubility and diffusion velocity of gas molecule in the water layer.Recently,research about the gas diffusion is mainly focusing on improving its diffusion efficiency through nanoscale water layer.Though great progress has been made in this field,there are still large amount of basic scientific questions to be managed urgently.Therefore,research on the gas diffusion through nanoscale water layer and the potential scheme to improve its diffusion efficiency,make it of great importance for the academic research and practical applications.To study the above issues,based on the molecular dynamics simulations,we study the diffusion behavior of gas through nanoscale water layer.Particularly,we point out that the amphipathic patterned Janus particles can self-assemble into uninterrupted hydrophobic channel inside aqueous nano-space.This hydrophobic channel in the Janus particles aggregates can help gas diffuse through nanoscale water layer,resuting to the improved diffusion efficiency of gas.Further,on the basis of prior work,we also consider the effect of salt ions on the diffusion efficiency of gas through nanoscale water layer.The detailed contents are described as following.Existence of nanoscale water layer will cause great damage to the diffusion efficiency of gas molecules.For example,during the exploitation of low permeability gas reservoir,nanoscale water layer always exists in the hydrophilic nanochannel due to the capillary force.This water layer distributes widely and decreases the diffusion ability of gas in the nanochannel greatly.Thus,the question comes that how to improve the diffusion efficiency of gas through nanoscale water layer.Inspired by the ion channel in the protein and the self-assemble process of hydrophobic particles by hydrophobic interaction,we propose that the amphipathic patterned Janus particles may help the gas molecules to break down the nanoscale water layer in nanochannel and improve the diffusion efficiency of gas greatly.Our simulation result reveals that this amphipathic patterned Janus particles can form self-assembly structures with uninterrupted hydrophobic channel in the nanoscale water layer.The physical mechanism is the hydrophobic interaction between the hydrophobic regions on the Janus particles.The hydrophobic interaction leads the Janus particle to approach each other and thus form the uninterrupted hydrophobic channel inside the self-assembly structures.Further simulation result also reveals that those self-assembly Janus particle structures with uninterrupted hydrophobic channel can greatly improve the diffusion efficiency of gas through nanoscale water layer.Moreover,the hydrophilic region on the Janus particle can hold strong interaction between particle and water,guaranteeing that the Janus particle can diffuse quickly with the water fluid.Thus,our paper provides a novel idea to improve the diffusion efficiency of gas through nanoscale water layer by using amphipathic Janus particle and reveals the underlying mechanism detailly.On the basis of our prior work,we then consider the fact that most of the nanoscale water layers in nanochannel are not composed of pure water but contain various kinds of salt ions.For example,nanoscale water layers in the underground rock usually contain carbonate.Nanoscale water layer with salt ions can also be used to separate and purify certain kind of gas from the mixed gas.The question arises whether there is any difference between the gas diffusion process through those salt water layer and that through pure water layer.Based on the molecular dynamics simulations,we compared the diffusion behavior of the gas through nanoscale salt water layer and pure water layer.We find that the diffusion efficiency of gas though salt water layer is much less than that through pure water layer.Firstly,we analyze the solubility and internal diffusion velocity of gas molecules in these two solution layers and find that these factors are not the main causes of the hindered diffusion of gas through nanoscale salt water layer.Then,we analyze the density of gas near the gas-liquid interface and find gas can accumulate at the interface due to the adsorption.Gas accumulation at the interface forms a concentration barrier for gas to diffuse through,resulting to a negative flux of gas.Moreover,according to our calculation,the diffusion efficiency of gas through nanoscale water layer has a negative linear correlation with the gas accumulation barrier at gas-liquid interface.This conclusion is helpful for us to have better understanding of the diffusion process of gas through nanoscale water layer and provides feasible scheme to improve the diffusion efficiency.This work is the indispernsible extension and complement to our precious reserarch.