Microscopic polarities of interfacial regions of aqueous cationic micelles: Effects of structures of the solvatochromic probe and the surfactant

We report on the use of solvatochromic probes to determine the microscopic polarity of water at their (average) solubilization sites in cationic micel les. The following probes were employed: 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-oxypyr idinium betaine (PB). The cationic surfactants studied were cetyltrimethyla mmonium bromide, cetyldimethylbenzylammonium bromide, dodecyltrimethylammon ium bromide, and dodecyldimethylbenzylammonium bromide. The microscopic pol arity, E-T, kcal/mol, depends on structures of both probe and surfactant. P B, the most hydrophilic probe, does not partition into the micellar pseudop hase, whereas RE, the most hydrophobic one, samples a much lower microscopi c polarity than QB and QBS because of its deeper penetration into the catio nic micelle. Polarities measured by (zwitterionic) QB and (anionic) QBS dif fer because the latter probe exchanges for the surfactant counterion. Our E T values refer to micelle-bound probes and are lower than literature values , which have been typically determined at [surfactant] = 0.05 mol L-1. Effe ctive water concentrations at the solubilization sites of these solvatochro mic probes have been calculated by using as references mixtures of water wi th each the following organic solvents: 1-propanol, dioxane (RB); ethanol, 1-propanol, acetonitrile, dioxane (QB and QBS). These water concentrations vary between 22.1 and 47.8 mol L-1, depending on structures of the probe, t he surfactant, and the reference solvent mixture.