Classical Density Functional Studies On Structural And Thermodynamical Properties Of Fluids In Rough Pores

Phenomena of adsorption,wetting and capillary condensation of fluid near the solid surface are ubiquitous in nature.With the increasing advance of science and technology,such physical phenomena have been widely used in industrial fields such as material self-cleaning,carbon storage,hydrogen storage and super-capacitors.In recent years,structural and thermodynamic properties of confined fluids in nano-pores had become hot topics of the related researchers,and the findings had provided strong support for further design and development of new functional materials.However,most of the existing studies are focused on solid materials with smooth surfaces,but few involves the surface roughness that actual materials may have.To this end,rough slit pores with rectangular bulge are built firstly in this thesis,and the structure and thermodynamic properties of fluid confined in them are systematically investigated.The specific research contents mainly include the following aspects:Firstly,the rough substrate is firstly modeled as a rough surface with rectangular bulges,and the two-dimensional external potential it applies is calculated.In order to facilitate the application of the external potential of the rough substrate in the traditional density functional calculation,an area-weighted free energy average scheme is proposed on the mean field level,which converts the above two-dimensional external potential into one-dimensional effective external potential.Further,this effective potential is applied to the calculation for excess adsorption,which suggests that it can well describe the influence of roughness on adsorption capacity.Secondly,based on the numerical results obtained from density functional calculations,theeffects of roughness on the excess adsorption isotherm,solvation force,surface free energy and thermodynamic response functions are studied.The results show that the height,width,spacing of two neighboring bulges and the width of the rough slit can significantly modulate the above properties of the confined fluid.Specifically,the increase of the height and width of the rectangular bulge can enhance the adsorption capacity of the pores and other thermodynamic properties of the fluid.However,the spacing between two neighboring bulges affects these properties in the opposite way.In addition,it is also shown that the thermodynamic properties of confined fluids usually change in an oscillatory manner with the pore width.Moreover,the period and amplitude of its oscillation are also affected by the geometry of the roughness.Thirdly,employing the two-dimensional external potential obtained above,the longitudinal Henry coefficient and isosteric heat of adsorption of H₂ gas were calculated in the Henry range,in which the gas pressure is sufficiently low.The results show that its longitudinal Henry coefficient is determined by the competition between the strength of adsorption potential and the size of accessible space in the pore,while its isosteric heat of adsorption is affected by the depletion layer around the rectangular bulges.In addition,the adsorption selectivity of CO₂/H₂binary mixture in the rough slit is also studied.The results suggest that the adsorption selectivity are mainly determined by the strength of gas-solid interaction and the compatibility of different gas molecules in their accessible spaces.Fourth,by using the perturbation chain statistical associating fluid theory,the free energy functional of chain fluid is obtained.Based on classical density functional theory,the equilibrium structure and solvation force of the chain fluid confined in rough slit are calculated and studied.The results about confined n-alkane fluids suggest that the geometric morphology of rough substrates can significantly modulate the structure and solvation force of these confined chain fluids.