Lp. Novaki et Oa. El Seoud, Microscopic polarities of interfacial regions of aqueous cationic micelles: Effects of structures of the solvatochromic probe and the surfactant, LANGMUIR, 16(1), 2000, pp. 35-41
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.