Density Functional Theory Insights Into Surface Wettability Effects On Thermodynamic Properties Of Confined Fluids

Adsorption and transpotation of confined fluids exist in many chemical engineering processes.The geometrical confinement effect may have prominent impact on both physical and chemical proterties of fluids in nanoscale pores.More importantly,the microscopic structures directly influence the macroscopic proterties and determine the process or product efficiency.The thermodyamic properties of confined fluids could be regulated by varying surface wettability of confined spaces.In order to comprehensively understand the intrinsic mechanisms,as well as to provide guidance for process optimization and micro-reactors designing,theoretical investigations of confined fluis are of necessity and urgency.By using Classical Density Fuctional Theory(CDFT),this dissertation investigates the surface wettability effects on thermodynamic properties of confined fluids,including pressure,solubility and solvation free energy,aiming to solve and optimize the existing problems in real systems.This dissertation is composed of following several parts:(1)The set pressure in pressure relief valves(PRVs)varying with the type of sealed medium has been a puzzling problem in the field of chemical machinery and equipment for many years.Here we propose a novel viewpoint to interpret this phenomenon,by which the set pressure difference is ascribed to the additional adsorption pressure of sealed medium adsorbed in the intrinsic nanoscale apertures of PRVs.To demonstrate,two individual types of medium gases(i.e.,saturated steam and air)sealed in different PRVs are investigated,and upon a multiscale model of the apertures in PRVs,the additional adsorption pressures are evaluated by using CDFT.Our calculation shows that the adsorption force of steam is always higher than that of air,resulting in a lower set pressure disregarding the use of different PRVs.The theoretical results are compared with the experimental measurements,displaying qualitatively good agreement,which supports our surmise.Finally,possible solution to reduce the set pressure difference is discussed.This work cast helpful insights for the design and application of PRV.(2)Gas solubility in confined spaces is closely related to heterogeneous catalysis,shale gas extraction and phase separation etc.Even though plenty of experimental and theoretical researches have given pronouncing insights into over-solubility and under-solubility of gas in nanopores,the microscopic mechanism for elaborately regulating gas solubility remains unclear.Herein,we present a theoretical study of the confinement effect in slit-shaped pores on the solubility of Ar in liquid solvents combined with classical density functional theory and machine learning(ML).We demonstrate that for a given confined Ar-solvent system,gas solubility could be changed by varying the pore width or wall potential.ML is adopted to give a criterion to estimate whether a confined Ar-solvent system is enhance-beneficial or reduce-beneficial.Furthermore,the ML results are verified by the CDFT results.Our findings provide theoretical guidance for predicting and regulating gas solubility in nanopores.Besides,the hybrid method in this work could be expanded to other analogous gas-solvent systems for further research.(3)The depletion force exerted on an alkane molecule from surrounding solvent may greatly alter its conformation.Such behavior is closely related to the selective molecular recognition,molecular sensors and self-assembly etc.We develop a multiscale theoretical study on the conformational change of a single alkane molecule confined in water-filled cavitands,in which the quantum and classical density functional theories(CDFT)are combined to determine the grand potential of alkane-water system.Specifically,the intrinsic free energy of the alkane molecule is tackled by quantum DFT,while the solvent effect arising from the solvent density inhomogeneity in confined space is addressed by classical DFT.By varying the alkane chain length,pore size and wettability of inner pore surface,we find that pore confinement and hydrophilic inner surface facilitate the alkane conformational change from extended state to helical state,which becomes more significant as alkane chain length increases.Our findings which are in line with previous experimental observations,provide not only the microscopic mechanism but also theoretical guidance for elaborately manipulating molecular conformation at nanoscale.(4)Atomic Density Functional Theory(ADFT)and Molecular Density Functional Theory(MDFT)are two branches of CDFT.When dealing with complex molecules,MDFT is much more accurate than ADFT.The calculation of direct relation function(DCF)are very complex in the existing MDFT and it slows down the efficiency of numer.By using mean sphere approximation(MSA),we build the relation between intermolecular interaction with molecule distance and molecule orientation,yielding a fast calculation method of DCF for molecular systems.Basing on this method,we develop a novel MDFT for linear molecules,i.e.,nitric oxide(NO).The calculation results of adsorption isothemo of NO by using our MDFT are better than ADFT.This novel MDFT provide theoretical guidance for estabilishing functional framwork for other linear and more complex molecules.