Study On The Structure Of Water Molecules In LiCl Solution Confined In The Graphene Oxide And Its Dynamics Under High Pressure

Confined water is ubiquitous in nature,exhibits intriguing dynamical and structural behavior,and plays an important role in protein folding,bionic architecture,nanofluids and many other fields.In nature and living organisms,confined water is mostly rich in a variety of ions,such as K⁺,Na⁺,Li⁺,etc.,which are widely present in cells and minerals.Under the confinement conditions,the introduction of salt ions will dramatically alters the hydrogen bond interaction between water molecules.However,the experimental study of salt solution in the confined space is not yet widely carried out,and the effect of salt ions on the structure and properties of the confined water is not yet clearly understood.Therefore,it is urgent to implement experimental research on the hydrogen bond structure and dynamic behavior of water molecules in salt solution under confinement,which is of great significance for people to further comprehend important biological processes and develop their applications in seawater desalination,biosensors and supercapacitors.Graphene oxide?GO?is one of the derivatives of graphene,which has good hydrophilicity.Lithium chloride?LiCl?is highly hygroscopic and has excellent solubility in water.In addition,Li⁺ion radius is the smallest in alkali metal elements,which is easy to fill the GO layer.Consequently,we chose graphene oxide?GO?as the confined template and LiCl solution as the representative of salt solution,to investigate the structure of water molecules in LiCl solution confined in the graphene oxide and its dynamic behavior under high pressure.The main conclusions of the experiment are as follows:1.Atmospheric pressure studies have been carried out on the confined LiCl solution in graphene oxide.X-ray diffraction studies indicate that when LiCl solution?0.25M?enters GO,its interlayer distance can be maintained in a high hydration state?d=13.3??under ambient conditions or even low vacuum conditions,without rapid water desorption.By comparing the changes of the interlayer spacing after the entry of different concentrations of LiCl solutions and different types of saline solutions?KCl,NaCl and LiBr?into GO,it can be found that the size of the GO interlayer spacing and the maintenance of its high hydration state are mainly related to the ion concentration,ion type and the solubility of salt in water.Infrared absorption spectroscopy studies demonstrate that water molecules in the GO confined LiCl solution exist in four different hydrogen bond structures,including interface water(3029 cm^-1),ice-like water(3201 cm^-1),liquid-like water(3394 cm^-1)and oligomeric water(3611 cm^-1).Compared with the GO confined pure water solution,the water molecules in the GO confined LiCl solution are still dominated by a four-coordinate ice-like structure,but the proportion has decreased,while the proportion of oligomer water has increased by 10 times.This result indicates that the local electric field generated after LiCl dissolves in water destroys the ordered structure in confined water,and forms more oligomer water around Li⁺and Cl^-,which increased the surface activity of water molecules in the confined space.According to high resolution transmission electron microscopy,the ordered ice structure formed by water molecules in GO confined LiCl solution?0.25M?is a square-like ice structure with lattice constants a=2.47±0.03?and b=2.39±0.03?.Synchrotron X-ray diffraction studies indicate that a new diffraction peak was generated at d=2.39?in GO confined LiCl solution?0.25M?,which is consistent with the lattice parameters observed in HRTEM and support the existence of ordered ice structure in GO confined LiCl solution?0.25M?.2.High pressure studies have been performed on the dynamic behavior of water molecules in LiCl solution confined in the graphene oxide.High-pressure in-situ infrared spectroscopy studies have found that when pressurized to about 2.48 GPa,GO confined LiCl solution will show obvious C-H bond stretching vibration peaks near 3047cm^-1 and 3124cm^-1,and it disappear again when the pressure was unloaded to about 1GPa.This phenomenon indicates that water molecules and GO have chemically reacted to form metastable C-H bond under pressure.Compared with anhydrous GO and GO confined pure water solution,the formation of metastable C-H bond only occurs in GO confined salt solution?LiCl,KCl,NaCl?.The reason for this phenomenon is that the local electric field around the salt ions promotes the ionization of oligomeric water in the nanoscale space.The H⁺generated by ionization of water molecules is adsorbed near the negatively charged carbon vacancies in the GO layer,and forms a metastable C-H bond under pressure.The ionization of water molecules in GO confined space can be further demonstrated by the chemical reaction between free OH^-and carboxyl groups.The results of this part prove that the introduction of salt ions in nanoscale space promote the ionization of water molecules,increase the surface activity of water molecules,and enable water molecules to participate in chemical reactions.