Gas/liquid and liquid/liquid interfaces, with their unique dynamic molecular behaviours such as high molecular reactivity, specific molecular enrichment, wide range of molecular motions, and distinctive molecular trajectories, have attracted significant research interest. Owing to the inherent fluidity and deformability of liquids, these interfaces exhibit complex dynamic interfacial properties related to wetting, emulsification, phase separation, phase fusion, and environmental versatility, which play pivotal roles in various fields including materials science, chemical engineering, bioengineering, and environmental science. In situ characterization techniques for gas-liquid and liquid-liquid interfaces offer comprehensive insights into the dynamic molecular properties occurring at the heterogeneous interface between the two phases. The gas/liquid and liquid/liquid interface layers have a dynamic region with thicknesses ranging from angstroms to nanometers, separating the homogeneous gas phase from the liquid phase or one liquid phase from another. Their dynamically changing structures are heterogeneous and distinct from the fundamental chemical and physical properties of the two phases. Weak signal intensities often result in low signal-to-noise ratios. The states of the two bulk phases (e.g., impurities and concentration distribution of specific molecules) and the environmental conditions at the interface (e.g., temperature and pressure) can affect the interfacial molecular behaviours, making it challenging to distinguish the location and interface within a very short time. Over the past decade, characterization technologies for gas/liquid and liquid/liquid interfaces have undergone continuous upgrades and iterations. Emerging characterization methods, such as high-resolution imaging, high-speed cameras, interface-functionalized molecular luminescence tracing, imaging with various luminescence light sources, and the construction of multiscale interface models, have been utilized to obtain the interfacial properties of molecular aggregation at the macroscopic level. Spectroscopic characterization strategies, including linear and nonlinear spectroscopy combined with molecular dynamics simulations, were employed to understand the interactions of interface molecules at the molecular level. The integration of multiscale experimental results with theory holds promise for addressing issues such as material transport, phase stability, transformation, and interfacial chemical reactivity at the gas/liquid and liquid/liquid interfaces. In the development of characterization mechanisms, emerging technologies such as field-induced droplet ionization mass spectrometry and liquid gating technology have systematically demonstrated their application advantages in the multiscale characterization of dynamic gas/liquid and liquid/liquid interfaces. The practicality of characterization techniques for dynamic molecular behaviour at gas/liquid or liquid/liquid interfaces is heavily dependent on the advancement of interfacial analysis tools. This article provides an overview of some research achievements in the in situ characterization of the dynamic molecular behaviour at gas/liquid and liquid/liquid interfaces. First, we introduce the properties of the molecular structures and dynamic behaviour patterns. Subsequently, the focus is on progress in macroscopic- and molecular-level characterization techniques as well as innovative methods. Macroscopic-level characterization techniques, including surface/interface mechanical characterization methods and imaging-assisted analysis methods, and molecular-level characterization techniques, including spectroscopic analysis and computational simulation-assisted analysis methods, have enhanced our understanding of the dynamic molecular structures at the surface/interface and behavioural patterns across multiple dimensions. Macroscopic-level characterization techniques are relatively simple; however, they can provide qualitative and quantitative information on the macroscopic aggregation states of interfacial molecules. Molecular-level characterization techniques are more complex but can reveal crucial processes at the interface, deepening our microscopic insight into the interface behaviour. Finally, a comprehensive analysis of the current challenges and future directions in the field is provided, laying the foundation for the future development and application of characterization techniques.
