Experimental Research And Molecular Dynamics Simulation Of Optical Constants Of Liquids

Liquids are widely found in nature and industrial production,for instance,liquid water and other inorganic liquids,vegetable oils,mineral oils,biodiesel,liquid alcohol and other organic liquids,and mixed liquids such as salt solution and seawater.Temperature-dependent optical constants and other radiation properties of liquids have important application backgrounds in remote sensing,aerospace,energy and power,chemical and biological fields.However,due to the flow characteristics and selective absorptivity of liquids,the measurement wavebands and application conditions of various experimental methods for measuring the optical constants of liquids are limited.Furthermore,experimental measurements at high temperatures and pressures are difficult to achive.On the other hand,theoretical calculation methods,such as molecular dynamics(MD)method and Ab initio molecular dynamics(AIMD)method,etc.,are not well used to obtain temperature-dependent optical constants of liquids.The optical constants of liquids,especially the data at different temperatures,are in urgent need.This has severely limited the application of liquids in remote sensing,energy and power fields.Therefore,it is of great theoretical significance and application value to carry out the systematic research on “experimental measurement and molecular dynamics simulation of the optical constants of liquids”.In this paper,experimental measurements,AIMD,MD,and the combined allatom/coarse-grained molecular dynamics(AA-CGMD)simulation are carried out to accurately obtain the temperature-dependent optical constants of liquids,which provide data support for related research on liquids.In terms of experimental measurements,the combined “Liquid/Wafer” ellipsometry-transmission method for the measurements of optical constants of liquids in the visible and near-infrared regions,and the combined “Prism/Liquid” ellipsometry-transmission method for measuring optical constants of liquids in the mid-infrared regions are proposed in this paper.The mathematical models of reflection coefficients and complex ellipsometry parameters are built considering the multiple reflections among interfaces.In terms of theoretical simulations,AIMD method is used to calculate temperature and pressure-dependent dielectric functions and absorption spectra of liquids.In addition,the dielectric function calculation scheme under the framework of the combined AA-CGMD method is derived based on the analysis of forces acting on atoms.The details of this study are:The optical constants of the liquid in the visible and near-infrared regions show important applications in the quantitative and qualitative analysis of liquid components,the analysis of the doping of grease,and the detection of warships.In this paper,the combined “Liquid/Wafer” ellipsometry-transmission method is verified using distilled water.The concentration-dependent optical constants of binary salt solutions are measured by the combined “Liquid/Wafer” ellipsometry-transmission method,and the temperature-dependent optical constants of rapeseed oil,corn oil,and biodiesel are measured by the double optical path transmission method.As the measurement results show,the refractive index of binary salt solutions increases linearly with increasing concentration at a certain wavelength.In the studied temperature ranges,the refractive indices of rapeseed oil,corn oil,and biodiesel decrease with increasing temperature.The influence of temperature on the absorption index is not obvious.Furthermore,the refractive index mixing rule and absorption index mixing rule to predict wavelength and concentration-dependent optical constants of multi-component mixed-salt solutions are proposed in this paper.The above mixing rules are verified using the optical constants of multi-component mixed-salt solutions at various concentrations measured by the combined “Liquid/Wafer” ellipsometry-transmission method.The given mixing rule can be used to predict the refractive index and absorption index of the mixed-salt solutions at specified concentration and wavelength.It is a simple method to calculate the refractive index and absorption index of mixed-salt solutions.In order to measure the optical constants of liquids in the mid-infrared regions using the ellipsometry method,a prismatic liquid pool is designed and manufactured to hold the liquid.The mathematical model of reflection coefficient and complex ellipsometry parameter of the “prism-liquid” system is established considering the multiple reflections among interfaces.For less-absorbing wavebands,the ellipsometry method cannot give an accurate absorption index value,thus the transmission method is used for the measurement in this case.The optical constants of some kinds of organic liquids,such as palm oil,palm oil biodiesel,diesel,kerosene,ethanol,isopropanol and n-butanol,are measured by the combined “Prism/Liquid” ellipsometry-transmission method at different temperatures.As the measurement results show,the refractive indices and the absorption indices of the main absorption peaks of the measured organic liquid decreases with increasing temperature in the studied temperature and wavelength regions.We apply a linear fitting analysis to obtain the temperature-dependent refractive indices for certain wavenumbers in this paper,the deviation of which compared to the experimental results is relatively small.To break the limits of experimental research in measuring conditions,theoretical calculations are carried out to obtain the optical constants of liquids under extreme conditions in this paper.Water and heavy water are used to validate the reliability of the Car-Parrinello molecular dynamics(CPMD)method.The temperature-dependent infrared dielectric functions of n-butanol,as well as the temperature and pressure-dependent infrared dielectric functions of methanol are calculated using the secondgeneration CPMD method proposed by Kühne et al.As the numerical analysis indicates,due to the introduction of the fictitious electron mass,the absorption peak calculated by the CPMD method shows a more pronounced red shift than that calculated by the secondgeneration CPMD.With the increase of temperature and the decrease of pressure,the radial distribution functions of liquid methanol become less structured,the average molecular dipole moment decreases,and the amplitude of dominant absorption peak reduces.The combined AA-CGMD method is an effective method for the study of the optical constants of large biomolecular systems.In this paper,the dielectric-function calculation equation at the framework of the Hamiltonian adaptive resolution scheme(H-Ad Res S)is deduced,and the reliability of the above equation is verified using distilled water.In order to eliminate the contingency of the proposed H-Ad Res S dielectric-function calculation scheme,the dielectric functions of ethanol are calculated and compared with all-atom MD simulation results.Comparing all-atom MD and H-Ad Res S calculated dielectric functions and complex refractive indices of water and ethanol,H-Ad Res S results agree well with all-atom MD simulation throughout the study wavebands,demonstrating the feasibility of the H-Ad Res S dielectric-function calculation scheme.