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

Abstract

Solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB); 1-methyl-8-oxyquinolinium betaine (QB); sodium 1-methyl-8-oxyquinolinium betaine-5-sulfonate (QBS); and 1-methyl-3-oxypyridinium betaine (PB) was studied spectrophotometrically in micellar solutions of the following cationic surfactants: cetyltrimethylammonium chloride, cetyldimethylbenzylammonium chloride, dodecyltrimethylammonium chloride, and dodecyldimethylbenzylammonium chloride. Microscopic polarity of water at the (average) solubilization site of the solvatochromic probe, E_T in kcal mol^-1, was calculated from the position of the longest-wavelength absorption band of the probe. The visible spectrum of PB, the most hydrophilic probe, is not affected by surfactants because it is not included in the micellar pseudo phase. For the other three solvatochromic probes, calculated E_T values depend on the structures of both the probe and the surfactant, namely, its headgroup and long-chain alkyl group. RB, the most hydrophobic probe, samples a much lower microscopic polarity than QB and QBS because it penetrates deeper into the cationic micelle. This conclusion has been confirmed by ¹H NMR. Polarities measured by (zwitterionic) QB and (anionic) QBS differ because the latter probe exchanges with the surfactant counterion. Calculated E_T values refer to micelle-bound probes and are, therefore, different from those reported in the literature, typically determined at [surfactant] ⩽0.05 mol L^-1. Effective water concentrations at the solubilization sites of these solvatochromic probes has been calculated by using as references mixtures of water with each the following organic solvents: n-propanol and dioxane (RB); ethanol, n-propanol, acetonitrile and dioxane (QB and QBS).