Colorimetric Revealing of Ethanol–Water Cluster (E-Wc) Transitions in Binary Solution Based on Starch–I2 Crystallization

We have developed a highly sensitive colorimetric probe based on starch-iodine (I₂) crystallization for the precise discrimination of ethanol-water clusters (E-Wc) within binary ethanol-water solutions (E-Ws). This probe enables the identification of specific E-Wc species and their corresponding transition points. Notably, two distinct transition points were identified at ethanol volume fractions of 40-45% and 75-77%. The former corresponds to the structural transition from (H₂O)_m(EtOH) to (H₂O)_m(EtOH)_n, characterized by a significant loss of blue coloration, while the latter signifies the transition from (H₂O)_m(EtOH)_n to (H₂O)(EtOH)_n, as evidenced by alterations in the absorption intensity of the starch-I₂ complex. Mechanistic studies demonstrate that the observed starch-I₂ crystallization is governed by supramolecular E-Wc rather than individual ethanol or water molecules in the binary solution. By leveraging starch-I₂ crystallization as a colorimetric bridge, we establish a direct correlation between E-Wc transitions and the iodine chromogenic effect. This approach enables the visual detection of transitions in colorless supramolecular assemblies, offering new insights into supramolecular science. Furthermore, as a simple, rapid, and visually interpretable detection method, this colorimetric probe holds promising applications in fields such as the food industry and supramolecular science.