Solvatochromism in aqueous micellar solutions: effects of the molecular structures of solvatochromic probes and cationic surfactants

Solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-p henolate (RB); 1-methyl-8-oxyquinolinium betaine (QB); sodium 1-methyl-8-ox yquinolinium betaine-5-sulfonate (QBS); and 1-methyl-3-oxypyridinium betain e (PB) was studied spectrophotometrically in micellar solutions of the foll owing cationic surfactants: cetyltrimethylammonium chloride, cetyldimethylb enzylammonium chloride, dodecyltrimethylammonium chloride, and dodecyldimet hylbenzylammonium chloride. Microscopic polarity of water at the (average) solubilization site of the solvatochromic probe, E-T in kcal mol(-1), was c alculated from the position of the longest-wavelength absorption band of th e probe. The visible spectrum of PB, the most hydrophilic probe, is not aff ected by surfactants because it is not included in the micellar pseudo phas e. For the other three solvatochromic probes, calculated E-T values depend on the structures of both the probe and the surfactant, namely, its headgro up and long-chain alkyl group. RE, the most hydrophobic probe, samples a mu ch lower microscopic polarity than QB and QBS because it penetrates deeper into the cationic micelle. This conclusion has been confirmed by H-1 NMR. P olarities measured by (zwitterionic) QB and (anionic) QBS differ because th e latter probe exchanges with the surfactant counterion. Calculated E-T val ues refer to micelle-bound probes and are, therefore, different from those reported in the literature, typically determined at [surfactant] less than or equal to 0.05 mol L-1. Effective water concentrations at the solubilizat ion sites of these solvatochromic probes has been calculated by using as re ferences mixtures of water with each the following organic solvents: n-prop anol and dioxane (RB); ethanol, n-propanol, acetonitrile and dioxane (QB an d QBS).