Small Angle Scattering Techniques on In Situ Adsorption Studies: A Comprehensive Review.

In situ and operando small-angle X-ray and neutron scattering (SAXS and SANS) have become powerful techniques for studying adsorption processes because they provide real-time information that traditional ex situ methods cannot capture. This review examines the use of these techniques in four major material groups: soft matter and polymers, carbon-based materials, biological systems and ceramics. Unlike conventional before-and-after characterization, in situ methods allow researchers to follow structural changes during adsorption as they happen, revealing adsorption kinetics, intermediate states and pore-filling mechanisms. Operando approaches further connect nanoscale structural evolution with overall adsorption performance, leading to deeper mechanistic understanding. Important developments include monitoring structural rearrangements in polymers during adsorption, studying sodium storage mechanisms in hard carbon anodes using combined small- and wide-angle neutron scattering, observing protein conformational changes at hydrated interfaces, and identifying organic functional groups in mesoporous ceramics in real time. SANS contrast variation, especially through hydrogen/deuterium substitution, allows selective visualization of different components in complex systems, while synchrotron SAXS enables kinetic studies with millisecond time resolution. Despite these advantages, several challenges remain, including model-dependent interpretation of scattering data, difficulties in separating real structural changes from contrast-related artifacts, limitations in accessible timescales and the technical complexity of isotopic labeling. Future research is expected to focus on improved contrast methods, machine learning-assisted data analysis, multimodal approaches combining scattering with spectroscopy or microscopy and the application of these techniques to more complex and disordered adsorbents such as activated carbons, coals and bio-based materials. Overall, this review summarizes current methodologies, compares adsorption-related structural changes across different materials and highlights both current limitations and future opportunities for in situ and operando SAS studies in adsorption research.