Luzia P. Novaki

Solvatochromism in pure and binary solvent mixtures: effects of the molecular structure of the zwitterionic probe

The solvatochromic behavior of 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate (WB) was studied by UV–visible spectrophotometry in 32 pure solvents, in binary mixtures of 1-butanol–cyclohexane (BuOH–Cyhx), and of water with methanol, ethanol, 1-propanol, 2-butoxyethanol (2-BE), acetonitrile, 1,4-dioxane and THF. The solvent polarity, ET(33) in kcal mol−1, was calculated from the position of the longest-wavelength intramolecular charge-transfer absorption band of WB and the results were compared with those for 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate [RB, ET(30)] and of 1-methyl-8-oxyquinolinium betaine [QB, ET(QB)]. For pure solvents, ET(33) is a linear function of ET(30), with a slope of practically unity. Steric crowding from the two ortho phenyl rings of RB hinders the formation of H-bonds with solvents, which results in similar susceptibilities of WB and RB to solvent acidity. For binary solvent mixtures, all plots of ET versus the mole fraction of 1-butanol or water are non-linear owing to preferential solvation of the probe by one component of the mixed solvent and, when applicable, to solvent micro-heterogeneity. Preferential solvation due to non-specific and specific probe–solvent interactions was calculated for BuOH–Cyhx and water–acetonitrile. Both solvation mechanisms contribute to the non-ideal behavior in the former binary mixture, whereas probe–solvent specific interactions dominate the solvatochromic behavior in the latter. The composition of the probe solvation shell was calculated. In aqueous alcohols, preferential solvation is by the alcohol. In water–aprotic solvent mixtures, preferential solvation of RB and WB is by the solvent which is present in lower concentration, whereas QB seems to form its own, water-rich solvation shell over a wide range of water concentration. Copyright

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-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, ET 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 ET 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 ET 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).