Water as an Often-Overlooked Analytical Variable in the Infrared Characterization of Functional Materials. Are We Characterizing the Material or Its Hydration State?

The synthesis and development of new functional materials require reliable characterization of their structure and surface chemistry. Infrared spectroscopy is widely used because it is a rapid, accessible, and sensitive method. Yet experimental simplicity does not ensure interpretive simplicity. Water, ubiquitous in materials, matrices, and laboratory environments, produces intense, state-dependent infrared bands that may be mistaken for intrinsic material features. This affects both the H-O-H bending region at 1600-1660 cm^-1, overlapping with C=O, C=C, C=N, COO^-, and amide I bands, and the broad ν(O-H) envelope used to infer hydroxylation, hydrogen-bond strength, hydrophilicity, defects, and functionalization. In porous, hydrophilic, nanostructured, aqueous-processed, or KBr-dispersed materials, the spectrum therefore describes a material-water-matrix system whose state evolves with preparation and measurement history. This Critical Review examines how sample history, hydration, matrix effects, data processing, and the propagation of assignments through citation affect structural claims, and proposes a hierarchy of discriminating experiments and IR-MIS as a proposed minimum-information reporting framework for auditable IR-based materials characterization.