Effects of a tiny sphere inserted in the center of a nano-spherical pore on the interfacial tension and surface adsorption of fluids confined in nanospherical pores

The density functional theory is used to investigate the effects of a tiny sphere (TS) inserted in the center of a nanospherical pore (SP) on the interfacial tension and surface adsorption behavior of confined fluids. Results show that the size and type (hard or attractive) of the inserted TS play important roles in the fluid interfacial tension. The perturbation created in the structure of the confined fluid due to the insertion of a hard TS is not capable of changing the positive sign of the interfacial tension although it does give rise to changes in the interfacial tension and surface adsorption behavior of the pore depending on its size. If, on the other hand, an attractive TS is inserted, the sign may become negative or positive depending on the TS-fluid interactions and TS size. Insertion of a hard TS in the SP, however, causes the interfacial tension to exhibit a positive deviation from its value for the fluid in the SP while this deviation will be negative if an attractive TS is inserted. Investigation of surface adsorption and interfacial tension reveals that, for large SPs, fluid interfacial tension in each thermodynamic state has a linear relationship with adsorption when a TS is inserted. This is while this linear relationship fails for small SPs. Also, it is shown that the locus of deviation from linearity, H, is shifted to higher values of H or Rs as a result of increasing TS radius and bulk density. Finally, it is found that the type (hard or attractive) of the fluid and the inserted TS, however, cannot change the locus of the break point.