Thionine exhibits a much higher solubility in water than could be expected based on previously published data. Concentrated solutions of thionine up to 0.1 M have been prepared and are red whereas dilute solutions are blue. The absorption spectra of the thionine solutions from 1.0 X 10~5 to 3.5 X 10~2 M have been obtained. The absorption maxima are shifted gradually and continuously to shorter wavelengths as the concentration of thionine is increased. A previously employed monomer-dimer model is not sufficient to explain the experimental results and it is necessary to invoke the formation of higher aggregates (H aggregate) to explain the experimental results. An approach based on the McRae-Kasha exciton model was employed to obtain the sizes of the aggregates in the solutions. Aggregates ranging in size from dimers to 9-mers have been identified.The apparent aggregation number (na) increases with increasing thionine concentration (C, mol/L) above 0.002 M as na = 1.0 + 225C. The stepwise formation constants of the lower aggregates have been calculated as K2 = 4 X 103 M"1 and AT3 =; 4 =; 1.7 X 103 M"1. Increasing temperature causes the deaggregation of the larger aggregates into monomers and smaller aggregates. At a thionine concentration of 0.025 M, the apparent aggregation number in the temperature range 7-40 °C is given by na = 11.6-0.167.
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