| Water is a wonderful material with simple structure and complex nature.It participates in many physical and chemical processes and provides basic protection for life activities.The diverse properties of water are closely related to its form of existence,such as bulk water,interfacial surface water,confined water,water clusters,etc.,which exhibit many different behaviors.Among them,confined water is widely present in granular porous materials,as well as around and within cells,macromolecules,supramolecular structures and gels.Recent studies have pointed out that the confined water has the characteristics of high melting point,high flow rate,low dielectric and amorphous ice.Even in the small space,the hydrogen atoms are highly delocalized and tunnelling to make the water molecules appear "quantum state".At the same time,in the process of rotation and transport in a one-dimensional channel,adsorption in a two-dimensional plane,the interaction between adjacent water molecules,water molecules and confined materials can cause complex and diverse microscopic morphology of water.These special properties are strongly related to the size of the confinement,the nature of the molecules on the surface of the confinement cavity,and the temperature and pressure.The change of water hydrogen network structure in the confined environment is an important factor causing the complex behaviors,which is often achieved by the rotation of water molecules and proton transfer.Under certain conditions such as low temperature,the H-bond rearrangement process is dominated by quantum tunneling effects.The study of water in a confined environment occupies an important position in various subject areas.For example,the blocking and degradation of organic pollutants,the filtration of water molecules in functional two-dimensional materials during water purification are all carried out in the environment in which water molecules participate.The water purification process is the result of the interaction of confined materials,water molecules and organic pollutants.The study of confined materials and confined water under these conditions contributes to resolve the problem about filtration membrane fouling and increase the efficienty about organic pollutant degradation to achieve a better wastewater treatment scheme.Therefore,understanding the existence form,nature and behavior of water in the confined environment from the atomic level is of great significance to understand the basic problems of biological cell water channels,seawater desalination membranes,sewage filters and other related fields,as well as the design and application of functional devices.Based on the first-principles calculations,the following four aspects of research content were carried out: 1)H-bond rearrangement by water molecules in a one-dimensional confined channel: 2)Abnormal behavior of dipole turnover of the water molecules in tight confinement;3)Concerted proton transfer of water-ring in free and confined spaces;4)Adsorption properties of functional two-dimensional confined material surface in water environment.Firstly,selected the water dimer(H2O)2,the basic H-bond model,and used carbon nanotubes of different sizes to construct different degrees of confined environments.Based on the first-principles method,we searched the transition state structure in which water dimers can achieve H-bond flipping in different confinement environments,traced the corresponding reaction path,then calculated the tunneling probability related to the rotation process and compared it with the thermal disturbance flip probability.At the same time,the first-principles dynamics simulation of the confined system is carried out,and its structural stability and turnover properties are verified.We compared the energy barrier,the shape of the reaction path,the rotational frequency and the vibration mode,and statistically averaged the H-bond length of the water dimer,the position in the one-dimensional confined tube,and the distance between the two oxygen atoms extracted from the simulation trajectory.In addition,the impact of dispersion correction in the calculation is also discussed.The results indicate that there are two rotation types of water dimer in confinement to achieve H-bond turnover,which is more difficult to rotate than those in free space.Moreover,there are certain discrepancies in the difficulty of flipping in different confined environments.The calculation of quantum tunneling gives its probability of achieving rotation through tunneling behavior.Compared with the thermal disturbance,the tunneling dominates the rotation below 100 K.Especially,when a small amount of energy is excited to the vibration mode along the rotation direction,it can significantly increase the quantum tunneling rotation effect,which provides a new strategy for the adjustment of the H-bond network in the confined water.Secondly,when the confinement size is reduced to a certain extent,the van der Waals ranges between the confined material and the confined molecules intersect with each other,so that the rejection becomes dominant.This situation is often referred to as tight confinement,or ultra-confinement,and the previous situation can be called the general confinement.In this way,two situations are distinguished.In view of the recent research that the tight confinement enables qualitative changes in confined molecules,we extended the study of H-bond turnover of water dimers to tight confinement.Through the transition-state-structure search and reaction-path tracing,we find that the water dimer in the tight confined environment has special rotation properties completely different from that in the general confined environment,which has a phenomenon that the energy barrier is low and the reaction path is bifurcated.It supports the assumption that tight confinement tends to qualitatively change certain properties of confined molecules.Compared with the general confinement,the tight confinement greatly reduces the hindrance to the rotation of its internal confined molecules.The statistics of the number of the dipole flips in the first-principles dynamics simulation trajectory also indicate that the rotation of water molecules in the tight confinement is freer than that in the general confinement.This gives an important explanation for the role of water molecules in biological aquaporins in the perspective of confined effect.Thirdly,to continue the above research on H-bond flipping,we extend quantum tunneling from rotation to proton transfer systems.We studied the concerted proton transfer process of the ternary water ring in free space,two-dimensional confinement and zero-dimensional confinement.The corresponding transition state structure and reaction path were obtained by the same method as above.We calculated the probability that the proton transfer process is achieved by the tunneling effect.According to the comparison of the height of the energy barrier and the results of the tunneling probability analysis,it is known that the confined environment has no hindrance to the concerted proton tunneling process of the water ring.On the contrary,it will reduce the energy barrier and increase the probability of tunneling of the process to a certain extent,which will enhance the occurrence of this process.This highlights the different effects of the confined environment on the two mechanisms of H-bond rearrangement.Fourthly,to expand the the understanding of the properties of the confined water and the mastering of the relevant computational analysis methods,we worked with experiment and studied the adsorption nature of two-dimensional confinement materials in water environment.In the field of sewage treatment,functional two-dimensional materials are often used in the process of water molecular filtration and promoting the degradation of organic pollutants,but still suffer from many problems such as membrane fouling and low degradation efficiency of pollutants.Our research has found that the above problems can be greatly alleviated by modifying the two-dimensional materials through functional functional groups and structural defects to control their adsorption properties.And it is found that the control of the adsorption properties of two-dimensional materials is the result of the interaction of material surface,water molecules,and organic pollution.This research provides important support and guidance for the application and improvement of wastewater treatment materials.In summary,the study of two tunneling mechanisms for H-bonding rearrangement of water in a confined environment reveals the law of the complexity of the confinement effect.At the atomic level,we further explain the tunneling rotational transport of water in the one-dimensional channel,the proton tunneling of the water ring in different confinements,and the adsorption properties of the two-dimensional material in the water environment.At the same time,we have also developed the tunneling probability calculation approach based on the potential energy curve and the corresponding analysis method for molecular dynamic simulation.It is hoped that the results and processing methods of this work will play an important role in understanding the basic issues in the relevant fields at the atomic level and promote their application in physics,chemistry,materials,electronics,biology and so on. |
