Knowledge of interfacial properties is crucial for various industrial applications involving multiphase systems, including those in the petroleum industry and carbon capture and sequestration (CCS) strategies. For instance, these properties influence phase behavior and fluid transport in reservoirs, while understanding CO2-water/brine interfacial properties is essential for securely storing CO2 in underground storage sites.In this thesis, we investigated the interfacial behavior of hydrocarbons, gases, and water/brine mixtures relevant to the petroleum industry and CCS strategies, using the density gradient theory (DGT) and classical density functional theory (DFT). While both methods compute interfacial tension (IFT) based on component density distributions across the interface, they differ in how they define the Helmholtz free energy of the inhomogeneous system. These thermodynamic-based approaches offer detailed interfacial structure information at a significantly lower cost compared to molecular simulations.Our study also considered the effect of impurities, represented by CH4, on the interfacial properties of CO2-fluid mixtures, different oil types, and the presence of salt. Additionally, we compared the interfacial properties of three-phase systems with two-phase systems.The results showed that CO2 had a stronger affinity for water/oil molecules compared to CH4, resulting in an increase in the IFTs of fluid-CO2 mixtures with the addition of CH4. In addition, the shape of the alkane density profile was consistent across all studied alkanes, with the mixture IFTs generally increasing with the n-alkane size. Moreover, cyclic alkanes exhibited relatively higher IFTs compared to linear and branched alkanes in oil-gas mixtures.In the brine mixture study, the results demonstrated that the addition of salt increased the mixture IFT but had no significant effect on the shape of the water, alkane, and gas density profile across the interface.Overall, the DGT method demonstrated good agreement with experimental and MD results for all studied cases. The DFT method, with the selected set of functional energy terms, provided good agreement with experimental and MD results for non-associating mixtures. However, for associating mixtures, the DFT approach showed satisfactory agreement for most cases, except for the water-aromatic mixture, where it significantly overestimated the mixture IFTs.