Synthesis and chemical equilibria of a new 10-methylpyrano-2-styrylbenzopyrylium pigment in aqueous solution and its modulation by different micellar systems

Abstract

The pyranostyrylbenzopyrylium moiety is a new type of benzopyrylium derivative, which has an extra double bond between the benzopyrylium and the phenyl ring B and an additional pyranic ring between C-5 and C-4. In this work, the synthesis and the chemical equilibrium of a new 10-methylpyrano-2-styrylbenzopyrylium in aqueous solutions by UV–Vis spectrophotometry are reported. The introduction of the double bond leads to changes in its chromatic properties, making the pigment exhibit a higher colorant capacity, when comparing with traditional pyranoanthocyanins. There is a stabilization of the color mainly due to the value of the second pKa that increase from 6.5 to 7.3. The modulation of its equilibria network by different micellar systems with different charges (SDS, Triton x-100 and CTAB) was studied. SDS and CTAB had opposite effects on the variation of pKa values, whereas Triton X-100 showed only a slight influence. The stability of each species (AH+, A or A−) in different systems in solution was also studied over 48 h. Overall, the best color stabilization of each species was achieved for the SDS micellar system.