Density Functional Theory Study On Properties Of Confined Fluid And Material Surface

Compared with bulk fluids, confined fluids have much richerstructures and properties. Meanwhile, many important surface propertiesof the materials that provide confinements can be characterized by theconfined fluids. Therefore, study of confined fluids is of greatsignificance to material science. In this thesis, the structures andproperties of fluids under different geometrical confinement, such as slitpore, cylindrical pore, planar surface, and curved surface, weresystematically investigated using the density functional theoreticalapproach, and the corresponding surface properties of the confinements,for examples, adsorption, pore size distribution, wetting, stability, werealso studied. The main contents are listed as follows:(1) With the accurate direct correlation function derived from theintegral equation theory, an improved density functional theory modelwas proposed, in which the renormalization group transformation wascombined to describe the long-range density fluctuation of confinedfluids. The global phase equilibria of the Lennard-Jones fluids in slit pores were studied. The critical law of the phase diagrams wasdetermined. It is shown that in a small slit pore, the critical exponent β isclose to the two-dimensional Ising exact value1/8. Furthermore, thecritical point shift with the change of the pore size was discussed.(2) The adsorption isotherms of CH4, CF4, SF6in slit carbon pores,and of N2in cylindrical carbon pores, were calculated based on theimproved theoretical model. The results are in good agreement withsimulation data. In particular, the corresponding pore size distributionsare characterized, showing predictions are accurate when compared withgeometrical analysis data and other available data.(3) The three-body intermolecular interactions in real systems wereconsidered to further improve the density functional theory model, andthe prewetting transition of Ar on a planar Li surface was studied. Theprewetting critical exponent β that belongs to two-dimensional prewettingtransition was obtained. The wetting temperature was calculatedsimultaneously by the contact angle method and the chemical potentialmethod.(4) By consideration of the characteristics of high curved surface, anew line tension model was proposed. The line tensions and the contactangles of nanoparticles at equilibrium vapor-liquid interface werecalculated. The effect of lined tension on contact angle was discussed.The contact angle as a function of temperature, nanoparticle diameter, and fluid-nanoparticle interaction strength were systematically studied.The stability of nanoparticle at the vapor-liquid interface was therebycharacterized.(5) Integrated with renormalization group theory and polymerintegral equation theory, the density functional theory model forcolloid-polymer was constructed. The bulk and confined phase diagramsof colloid and ideal polymer mixtures were studied. It is found that thecritical law of the mixtures in a small slit pore conforms totwo-dimensional characteristic. The density distributions ofcolloid-polymer mixtures in the vicinity of a planar wall were studied. Itis found that the colloids tend to aggregate on the surface of the wall.(6) A dynamical density functional theory model for colloid-polymermixtures was constructed. The colloid aggregation in colloid-polymermixtures was studied, showing that the rapid aggregation occurs at thebeginning stage. The sedimentation of the colloid-polymer mixtures wasalso studied. Under appropriate gravitation, a novel "sedimentationreverse" phenomenon will occur, that is, the colloid sedimentates first andthen floats up.