The non-ideality in binary aqueous systems contributed to the different abilities of solvent entities incorporated in the solvation shell of methylene blue

Abstract

The solvatochromic properties of methylene blue (MB) were investigated in neat water, methanol, ethanol, propanol, dioxane and their corresponding aqueous mixtures. The correlation of the empirical solvent polarity scale (ET) values of MB with solvent composition was analysed using the solvent exchange model of Bosch and Roses to explain the preferential solvation of the probe thiazine dye in the binary mixed solvents. Non-linear solvatochromism of MB was observed in aqueous mixtures of methanol, ethanol, propanol and dioxane. The influence of the composition of the solvating shell in preferential solvation of the solute dye was investigated in terms of both solvent–solvent and solute–solvent interactions, and the local mole fraction of each solvent composition in the cybotactic region of the probe was also calculated. Effective mole fraction variation can provide important physicochemical insights into the microscopic and molecular interactions between MB species and solvent components. The results showed that the MB solvation shell was thoroughly saturated with the solvent complex S12 for dioxane more than ethanol and propanol mixtures, and opposite trends for methanol mixtures, whereas the solvent complex S12 could not incorporate into the MB solvation shell. Data from the binary systems were analysed with KAT parameters using a dual model of basicity and polarity. The results showed that the polarity was better suited for spectral shift in aqueous methanol and ethanol solutions, while the basicity was better for aqueous propanol and dioxane solutions.

Keywords

• thiazine dye
• preferential solvation
• aqueous-solvent aggregates
• kat scale