Reversal of metachromasy revisited; displacement of Toluidine-blue from alginate by surfactants

The color stability of dyes is crucial in commercial dyeing, whereas their use in scientific research often depends on color changes due to metachromasy. This is the result self-organization of dye molecules into a stacked organization onto a charged template, such as an oppositely charged polyelectrolyte. Literature shows that ‘reversal of metachromasy’, caused by the dye's displacement from its stacked state, can be induced by a.o. surfactants. This is suggested to be a generic effect. We investigated the reversal of metachromasy using the alginate-ortho toluidine blue (TBO) metachromatic complex in a low ionic strength buffered system (phosphate 1 mM, pH = 7). Under these conditions alginate has an ‘apparent’ pK_a of 4.6 and the alginate-TBO complex shows to be 1:1 charge stoichiometric. Displacement by the cationic surfactant TEGO^®trant A100 (1,3-Didecyl-2-methylimidazolium chloride, TEGO) is found to be charge stoichiometric, its pathway depending on initial complex composition. TEGO initially binds to free binding sites on alginate prior to its competition with TBO for which it arranges into similar stacks allowing cooperative binding. Further, the effect of offering TBO alternative binding sites is studied by adding negatively charged surfactants. Binding of TBO to anionic surfactants, was found to depend on the ease of these to form premicellar structures which offer an alternative binding motive for TBO. Premicellar aggregation depends on the surfactant's structure; i.e. size of the ethylene oxide (EO) block, within a homologous series of sodium lauryl ether sulfates (SLES) and from our experiments can be ranked as (EO)₂ > (EO)₄ > (EO)₀ > (EO)₁₂ > (EO)₃₀. Reversal of metachromasy by surfactants, although proceeding via a similar mechanism, is not generic but depends on surfactant structure and its chemical composition.