Insights into the phase behavior at interfaces using vibrational sum frequency generation spectroscopy

Phase separation is ubiquitous at the interface between two distinct phases. Physical transformation during phase separation often plays a crucial role in many important mechanisms, such as lipid phase separation, which is fundamental for transport through biological membranes. Phase separation can be complex, involving changes in the physical state and the reorganization of molecular structures, influencing the behavior and function of materials and biological systems. Surface-sensitive vibrational sum frequency generation (VSFG) spectroscopy provides a powerful tool for investigating these interfacial processes. As a non-linear optical technique, VSFG spectroscopy is sensitive to changes in molecular orientation and interactions at interfaces, making it an ideal method for studying phase separation processes. Here, we review the molecular interaction mechanisms underlying phase separation. We also explore the application of VSFG spectroscopy in studying phase separation processes at different interfaces. In particular, we focus on oil-water interfaces, which are relevant in environmental and industrial contexts; polymer and lipid surfaces, important for materials science and biological membranes; and intrinsically disordered protein systems, which play key roles in cellular function and disease.