A new study published in Spectrochimica Acta Part A by Dominik Heger and colleagues at Masaryk University reveals that phenol's photophysical properties change significantly when frozen, potentially enabling its breakdown by sunlight in icy environments.
In a recent study, researchers at Masaryk University investigated the environmental behavior of phenol, which is a widespread pollutant, when it is embedded in ice. This study, led by Dominik Heger, a researcher at Masaryk University in the Czech Republic, explores how phenol’s photophysical properties change when trapped in frozen environments (1). This study’s findings were published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (1).
Known as carbolic acid, phenol is an aromatic organic compound like alcohols, but with different properties (2). It is considered volatile, owning to its acidic properties (2). Phenol is very corrosive to the skin, eyes, and respiratory tract (2). It is also commonly found in ambient air, water, soil, snow, and even polar ice (1). Its persistence in the environment is largely because of its chemical stability, particularly its low aqueous reactivity and resistance to breaking down under sunlight in typical atmospheric conditions (1).
In this study, Heger and the team investigated the stability of phenol when present in icy settings. Based on what the findings produced, the researchers concluded that phenol may not be as stable in icy environments (1). As part of the experimental procedure, the research team used ultraviolet-visible (UV–VIS) absorption, fluorescence, and Raman spectroscopy to see how phenol behaves when frozen in aqueous solutions or deposited onto ice (1). The findings indicate that freezing phenol significantly alters its photophysical characteristics, particularly through two mechanisms: vitrification (formation of a glass-like substance) and crystallization (1).
When phenol crystallizes in ice, either through freezing concentrated aqueous solutions or via vapor deposition onto the ice surface, a bathochromic shift can be observed in the absorption spectrum above 290 nm (1). This shift moves phenol’s light absorption into a spectral region where natural sunlight in the lower troposphere is more energetic, thereby potentially making the compound susceptible to direct photolysis—the process where molecules are broken apart by light (1).
One of the key findings in the study was where the bathochromic shifts occurred. These shifts occurred in deprotonated forms of phenol (phenolate) and crystalline samples. As a result, the implication is that under specific environmental conditions, particularly in frozen regions, phenol may break down more readily than previously believed (1). As a result, this study provides scientists with a better understanding of how phenol interacts and changes in polar and alpine environments, where snow and ice cover vast areas of the landscape.
Another key finding of the study is the contrast in photophysical behavior between phenol located in freeze-concentrated solutions (FCS) and those on the surface of hexagonal ice crystals. The fluorescence emission spectra revealed that FCS environments have unique solvation properties that can influence chemical reactions, including the acceleration of phenol deprotonation in the excited state (1).
Pure crystalline phenol and phenolate have not been as thoroughly studied. The researchers helped provide valuable reference spectra for future environmental and spectroscopic studies (1). Although the researchers caution that the conditions needed for significant photolysis of phenol in ice, such as sufficient crystallization or deprotonation, may be rare in nature, the study nonetheless underscores the importance of considering phase transitions and microenvironments when modeling the environmental behavior of organic pollutants (1).
By revealing that ice can be more than a passive storage medium for contaminants like phenol, Heger and his team show through spectroscopy how frozen environments influence atmospheric chemistry and long-term pollutant cycling.
