Advanced analytical strategies for surfactant characterization at complex biointerfaces: Linking composition, structure, and interfacial function.

Surfactant functionality in complex biointerface-dominated systems is governed by adsorption kinetics, interfacial organization, competitive displacement, phase partitioning, and chemical transformation, rather than by bulk concentration alone. Conventional analytical approaches, which predominantly quantify total surfactant levels or broad classes, therefore fail to capture the fraction that is chemically intact, interfacially available, and functionally active. This review presents an integrated biointerface-centered analytical framework that explicitly links surfactant molecular architecture and interfacial phenomena to analytical strategies appropriate for characterizing adsorption, interfacial organization, chemical transformation, and functional behavior in complex biointerface systems. It integrates distribution-resolved compositional analysis with interface-responsive techniques capable of probing adsorption dynamics, interfacial rheology, membrane perturbation, and real-time biointerfacial interactions. Particular emphasis is placed on resolving compositional heterogeneity, detecting transformation products, and distinguishing active from inactive surfactant populations under non-equilibrium conditions. Emerging multidimensional and AI data-integrated workflows are highlighted for their ability to bridge molecular-level information with interfacial function, including predictive and semi-quantitative approaches that extend analysis beyond the limits of conventional calibration-based methods. The central novelty lies in redefining surfactant characterization as a coupled compositional-functional problem and establishing a decision-oriented strategy that aligns analytical resolution with interfacial complexity and system behavior. This perspective enables a more mechanistic and translationally relevant understanding of surfactants in complex immiscible and biological environments.