Water Structure And Phase Transition Characteristics Between The Walls Of Coaxial Carbon Tubes

Experiments show that carbon nanotube can be used as to transport molecular materials through cell membranes,which may bring new drug delivery routes for cancer and tumor treatment.Therefore,the research on the physical properties of "CNT / water " system is of great practical significance,and also helpful to the preparation of other related nano functional devices.In this paper,molecular dynamics simulation is used to research the water structure and solid-liquid phase transition in confined space of self-assembled coaxial single-walled carbon nanotubes.The results show that:(1)The size of the confined space has a strong influence on the water structure inside the tube wall.With the increase of the space,the water structure between the tube walls presents a stratified aggregation effect,which can be divided into single,double,and three-layer cylindrical water film and ice tube,respectively.For the single layer water film,its location tends to be closer to the outer carbon tube under the ambient conditions to reduce the stress energy caused by curvature.In the case of multi-layer water film,the water film close to the out tube generally has the highest density,the water film near to the inner tube has the second highest density.(2)Based on the fact that the liquid density remains continuous at the nanoscale,it is inferred that the water repulsive thickness of the carbon wall is 1.96 (?) according to our simulation results,i.e.,the water molecule will not appear within a distance less than 1.96 (?) from the carbon wall.(3)In response to pressure changes,the water structure has three strain modes:(a)adjusting the ice tube radius;(b)adjusting the ice tube helicity;(c)dislocations are generated between the chains of water molecules.These three modes may occur separately or coexist simultaneously.(4)By analyzing the process of oxygen atom arrangement in the microstructure of water film changing with temperature,it is found that the water molecules between the tube walls is planar triangular packed.This is a kind of water or ice structure that never watched before.Although the structure violates the usual ice rule,it may appear under extremely confined space.(5)The structure of the ice tube is usually represented by a curly flat square grid.However,it is impossible to characterize the planar triangular packed ice tube by curling planar square grid.We set up new representation formula for this new phase of ice tube,derived the radius and chiral angle of the ice tube under the new characterization.(6)The solid-liquid phase transition of these new phases is first-order transition at atmospheric pressure.The continuous transition only occurs at high pressure.The water chains within the ice film have the same chiral angle.With the pressure increasing,the chirality of the water chains grows.Pressure increasing lead transition temperature up but hardly affect the triangular packed structure of water molecules.