The aqueous solutions of lower alcohols such as methanol, ethanol, 1-propanol and 2-propanol, were studied by fluorescence spectroscopy of pyrene, defining the Py scale for polarity. Sigmoidal curves were used to fit the Py values of aqueous alcohol solutions as a function of the logarithm of water-alcohol mole ratio, i.e., log(WAR). The results from curve fittings were discussed in terms of the structural transitions of aqueous alcohol solutions, as well as the dissociation constants for alcohol- and water-pyrene complexes. The microscopic alcohol and water phases were considered to be saturated with each other, and the structures of dilute aqueous alcohol solutions were found to be more complicated than those of concentrated ones.
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