Microscopic Polarities of Interfacial Regions of Aqueous Cationic Micelles: Effects of Structures of the Solvatochromic Probe and the Surfactant

Abstract

We report on the use of solvatochromic probes to determine the microscopic polarity of water at their (average) solubilization sites in cationic micelles. 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-oxypyridinium betaine (PB). The cationic surfactants studied were cetyltrimethylammonium bromide, cetyldimethylbenzylammonium bromide, dodecyltrimethylammonium bromide, and dodecyldimethylbenzylammonium bromide. The microscopic polarity, ET, kcal/mol, depends on structures of both probe and surfactant. PB, the most hydrophilic probe, does not partition into the micellar pseudophase, whereas RB, the most hydrophobic one, samples a much lower microscopic polarity than QB and QBS because of its deeper penetration into the cationic micelle. Polarities measured by (zwitterionic) QB and (anionic) QBS differ because the latter probe exchanges for th...