Reversible Color Loss in Aqueous Methyl Orange Solutions Induced by Freezing

Aqueous methyl orange (MO) solutions were observed to lose color upon freezing, forming clear ice specimens for some concentrations and pH values. When they were melted back to liquid, the solutions regained their original color intensity. UV–vis spectroscopy was performed on MO samples across a pH range of 1–13 and 10 μM to 1 mM concentration. The loss of visible range absorption was primarily observed from pH 5–13 in solutions with an MO concentration of 10 μM. Density functional theory calculations indicated that the protonation state and stacking arrangement do not account for the loss of visible absorption. Molecular dynamics calculations show that MO interaction energies favor association of the MO molecules with themselves rather than H2O molecules in water or ice. Confocal microscopy and Raman spectroscopy showed that upon freezing, MO concentrated into precipitates that are invisible to the naked eye. The concentration of solutes in the ice microstructure is consistent with the freeze concentration effect; however, we observed precipitates dispersed throughout the entire ice microstructure, both at ice grain boundaries and within the interior of individual ice grains. We report similar observations for methylene blue. In combination, our experimental and computational results indicate that the color loss of aqueous dye solutions upon freezing in ice may be governed by a precipitation mechanism.