Excess Absorbance as a Novel Approach for Studying the Self-Aggregation of Vital Dyes in Liquid Solution

In the present paper, a simple method for analyzing the self-aggregation of dyes in a solution by a UV-visible absorption measurements is proposed. The concept of excess absorbance is introduced to determine an equation whose coefficients determine the parameters of the aggregation equilibrium. The computational peculiarities of the model are first discussed theoretically and then applied to sodium fluorescein in polar protic and aprotic solvents, as well as in aqueous solutions of methylene blue, which is a cationic dye. Although the experimental responses are very different, the model appears to work equally well in both cases. The model reveals that the trimer is the most likely configuration in both solvents. Furthermore, aggregation is strongly favored for the protic solvent. Interestingly, the model establishes that in aqueous solutions of methylene blue, the tetramer is the predominant form, which has long been assumed and recently demonstrated with sophisticated computational techniques.

Automated summary

A simple method is proposed for analyzing the self-aggregation of dyes in a solution using UV-visible absorption measurements. The concept of excess absorbance is introduced to determine an equation that determines the parameters of the aggregation equilibrium. The model is applied to different solvents and shows that the trimer is the most likely configuration in both cases, while the tetramer is the predominant form in aqueous solutions of methylene blue.