Molecular Dynamics Study On Wetting Behaviour And Energy Harvesting Of Nano-Confined Imidazolium Ionic Liquids

With the rise of nanoscience and nanotechnology,the miniaturization of devices has become an inevitable trend in the future scientific development.It is of great theoretical significance and potential application value to study the properties and applications of materials at the nanoscale.However,due to the cost of the experimental equipment and the complexity of the technical means,it is difficult to observe the details of the microscopic properties of the substance at the nanoscale.Molecular dynamics?MD?is an effective tool for detailed observation of the microscopic properties of the substance,which can reveal the nature of the microscopic properties of the substance from the molecular or atomic level.Imidazolium ionic liquids?ILs?are a new type of soft functional materials that are widely applied in various fields due to their excellent physical and chemical properties?such as wide liquid range,non-volatile,low vapor pressure,high thermal stability,non-toxicity and environmental friendly?.Therefore,this thesis takes MD simulation as the main research method to probe the wetting behaviour and energy harvesting of nano-confined imidazolium ILs.In this paper,the dynamic wetting behaviour of the bulk imidazolium ILs confined to the surface of the nanoscale silicon substrate is probed at room temperature at first.The results show that when the bulk imidazolium ILs are confined to the surface of the silicon substrate,the van der Waals force?vdW?between the imidazolium ILs and the silicon substrate compels the imidazolium ILs to form a strong adsorption layer at the solid-liquid interface.The plane of the imidazolium ring of the cation in the adsorption layer is approximately parallel to the surface of the silicon substrate,and the anions and cations have certain orientation structure and the density is about twice than the density of the bulk phase.When the imidazolium ILs droplets are confined to the surface of the silicon substrate,the anions and cations spread along the surface of the silicon substrate and wet the surface of the silicon substrate.When the droplets of the imidazolium ILs completely wet the surface of the silicon substrate,the equilibrium contact angle is anisotropic,which is caused by the difference in the vdW force between the imidazolium ring and the alkyl chain in the cation and the silicon substrate.At the same time,the results show that when the droplet radius increases from 1.0 nm to 3.0 nm,the equilibrium contact angle gradually increases and tends to be saturated.This saturation phenomenon can be attributed to the numbers of ions in the adsorption layer at the solid-liquid interface increase and tends to be saturated with the droplet radius increases.In addition,the equilibrium contact angle of the droplets increases as the viscosity of the imidazolium ILs increases,and decreases as the temperature increases.When the temperature increases to a threshold of 400 K,the droplet equilibrium contact angle tends to be saturated,and the hemispherical droplet turns into a liquid film.Further,the bulk imidazolium ILs are confined to a channel composed of two layers of parallel graphene to investigate the energy harvesting at room temperature in this thesis.The results show that the imidazolium ILs also form a strong adsorption layer at the solid-liquid contact surface.When the imidazolium ILs flow through the graphene channel driven by external acceleration,the Coulomb field around the imidazolium ILs drags the free carriers on the surface of the graphene channel to move in the direction of the liquid flow,thus generating the flow-induced voltage?FIV?.Considering the combined effect of anions and cations on free carriers on the graphene channel surface and the characteristics of Coulomb interaction,an advanced equation for accurately and efficiently calculating the FIV generated by imidazolium ILs and graphene channel system is proposed.Based on advanced calculation equation,it is found that a².1?V FIV is generated when imidazolium ILs with a volume of 5×5×5 nm³ flow through a graphene channel with a size of 5 nm driven by an acceleration of 0.15 nm?ps^-2 at room temperature.The FIV increases to saturation as the average flow velocity of the anion and cation increases,and the main reason for this phenomenon is the balance between the external driving force and the internal viscous drag of the imidazolium ILs.In addition,the results also show that when the temperature increases from 300 K to 375 K,the maximum value of the FIV increases by 11.9%from 2.1?V to 2.35?V;when the channel size increases from 1nm to 5 nm,the maximum value of the FIV increases by 10.5%from 1.9?V to 2.1?V;when the single-layer graphene area increases from 1 nm² to 25 nm²,the maximum value of the FIV decreases by 8.7%from 2.3?V to 2.1?V.Finally,the energy harvesting of bulk imidazolium ILs in a single-walled carbon nanotube?SWCNT?is probed at room temperature in this thesis.Similarly,when the bulk imidazolium ILs are confined to the SWCNTs,the imidazolium ILs also form a strong adsorption layer at the solid-liquid interface and it is also known as the first shell layer.As the diameter of the SWCNTs increases,the second shell layer also appears at the solid-liquid interface.When the imidazolium ILs move along the radial direction of the SWCNTs driven by external acceleration,the Coulomb field around the imidazolium ILs drags the free carriers moving along the inner surface of the SWCNTs which results in a FIV.Considering the behavior of free carriers on the surface of the SWCNTs dragged by anions and cations and the characteristics of Coulomb interaction,an advanced and accurate equation is proposed to calculate the FIV generated by imidazolium ILs confined by SWCNTs.According to the developed FIV calculation equation,the results show that the FIV of about 2.22?V can be generated when the imidazolium ILs flow through the?25,25?SWCNTs under the acceleration of 0.15 nm?ps^-2 at room temperature.The FIV tends to be saturated with the increase of the average flow velocity of the anion and cation.This saturation phenomenon can be attributed to the balance between the external driving force,the internal viscous drag and the friction.Additionally,the results show that when the diameter of the SWCNTs increases from 1.220 nm to 4.068 nm,the maximum value of the FIV increases by 22.5%from 2.91?V to 2.34?V;when the temperature increases from 300 K to 375 K,the maximum value of the FIV increases by 15.8%from 2.22?V to 2.57?V;when the anion volume increases?[Cl]^-<[BF₄]^-<[PF₆]^-?,the maximum value of the FIV is 2.71?V,2.57?V and 2.28?V.In summary,the study of the dynamic wetting behavior of imidazolium ILs on nanoscale silicon substrates can provide a solid theoretical basis for the application of imidazolium ILs-based nanowetting systems.The study of imidazolium ILs confined to nanoscale graphene channels and SWCNTs for energy harvesting not only reveals the mechanism of FIV generation in energy harvesting systems based on imidazolium ILs and low-dimensional carbon materials at nanoscale,but also provides theoretical guarantees for its practical application in various fields.