Interfacial water can be found at the surface of an aerosol, a glass of water, and on the surface of vast oceans. At the molecular level, the structure of water at an interface is typically very different from the bulk structure. In this thesis, we try to reveal the mechanisms behind several physicochemical processes at the interfaces: (i) evaporation of aqueous binary mixtures, (ii) interfacial chemical reactions using the surfactant monolayer-assisted interfacial synthesis (SMAIS) method, and (iii) energy dissipation at the water-graphene interface. To investigate these systems, we employ surface-selective sum-frequency generation (SFG) spectroscopy and charge carrier-sensitive optical-pump terahertz-probe (OPTP) spectroscopy. Applying SFG spectroscopy – a surface-specific vibrational spectroscopy – to investigate the surface of liquid mixtures, we reveal that water’s interfacial alignment becomes randomized upon adding ethanol and methanol to water. In contrast, it becomes ordered upon adding formic acid. This is concluded from an investigation of the H-O-H/H-O-D bending mode of water at the surface. We also provide molecular-level information, such as intermediates and the impact of the surfactant charge on the interfacial chemical reaction of polyaniline and imidine, through in situ SFG measurement. With OPTP spectroscopy, we investigate how the size of the nanographene affects the charge transfer rate between the nanographene and graphene. Such a charge transfer rate is important for photodetector and photocatalysis-related applications. Finally, we track the photoexcited carrier relaxation of graphene in contact with different liquids, clarifying the role of energy and momentum transfer between the graphene electrons and the collective water libration modes.