Near-Infrared Probe for Determination of Solvent Hydrophobicity

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Abstract
The composition of aqueous solvents plays an important role in the spectroscopic characteristics of near-infrared (NIR) dyes. The effects of solvent hydrophobicity on NIR spectral characteristics have been studied with the use of different NIR dyes. An NIR dye (I) that belongs to the diethylthiacarbocyanine family was found to be the most sensitive to changes in the hydrophobicity of the environment. Dye I was used as a model dye to evaluate the analytical utility of this change. In pure water-miscible organic solvents, the absorption maximum for I appears in the region of 807 to 818 nm. If the hydrophobicity of the solvent mixture decreases by an increase in water concentration, an additional, well-separated absorption peak, which can be attributed to the dimer, appears at 698 nm. The relative intensity of these two peaks can be exploited for the determination of the composition of the solvent mixture. The method is evaluated with the use of the polarity parameter (P) for different solvent systems. The described method can be useful for quick spectroscopic determination of P values that are important for several analytical applications, e.g., in spectroscopy or reversed-phase chromatography.