Solubilization of spiropyran in aqueous NaBr solutions of dodecyltrimethylammonium bromide

Abstract

1,3,3-Trimethylindolino-6′-nitrobenzopyrilospiran is solubilized in aqueous NaBr solutions of dodecyltrimethylammonium bromide (DTAB). The dye is soluble in solutions of DTAB having concentrations higher than the critical micelle concentration. The solubilization power of DTAB is 2.08·10⁻³ mol dye per mol DTAB at NaBr concentrations lower than 1.84 M, while it increases linearly with increasing NaBr concentration from 1.84–4.50 M. The constant solubilization power of DTAE is attributable to the spherical micelles, while its higher solubilization power in the presence of NaBr beyond 1.84 M can be ascribed to rod-like micelles. The solubilization capacity of a spherical micelle of DTAB ranges from 0.11–0.18, depending on the NaBr concentration, where the micelle aggregation number varies from 52–86. The rod-like micelle has an increasing solubilization capacity with increasing NaBr concentration, and it can solubilize the dye to such an amount that 136 DTAB molecules along its contour length incorporate one dye molecule. The solubilized dye is partly in the mecrocyanine form, and the solubilization locus has polarity corresponding to a dielectric constant ≈35.