Regional Issue "Organic Chemistry in Argentina" ARKIVOC 2011 (vii) 369-379
Conclusions
We have determined the binding of o-NA to the micellar interface of n-heptane/AOT/polar solvent reverse micelles using the water, EG, FA, GY, DMA and DMF. The study was performed following the solvatochromic behavior of o-NA in the AOT nonaqueous reverse micelles by using absorption UV-visible spectroscopy. The binding constant can be calculated through the changes in the UV spectra, since the bound molecules have different maxima and are shifted bathochromically from the value in n-heptane. The values of Kb calculated follow the order < water <EG ~ WS = 0 ~ FA < DMF ~ DMA. The results were explained by considering the different parts of the AOT polar headgroup that were involved in the polar solvent-AOT interaction. Moreover it was demonstrated that the o-NA
– AOT hydrogen bond interaction was with the AOT sulfonate moiety. Thus, the more accessibility this group has for hydrogen bonding the larger the o-NA Kb value. Experimental Section
Materials. All chemicals and solvents were obtained from Aldrich or Sintorgan and are 99.9% purity or HPLC grade quality. AOT was dried under reduced pressure, over P2O5 until constant weight. The UV-vis spectra of 1-methyl-8-oxyquinolinium betaine (a solvatochromic probe) in the presence of AOT reverse aggregates showed that the surfactant was free of acidic impurities, which would have greatly reduced the intensity of the solvatochromic B1 band at 502 nm.12,52
Methods. The stock solutions of AOT in the hydrocarbon solvent were prepared by mass and volumetric dilution. To obtain optically clear solutions they were shaken in a sonicating bath and, the polar solvent was added using a calibrated microsyringe. The amount of polar solvent present in the system is expressed as the molar ratio between polar solvent and the AOT (WS = [Polar solvent]/[AOT]) and was kept constant and equal to 2 in every system investigated. To introduce the probe, a 0.01 M solution of o-NA was prepared in methanol (Sintorgan HPLC quality). The appropriate amount of this solution to obtain a given concentration (10-5 M) of the probe in the micellar medium was transferred into a volumetric flask, and the methanol was evaporated by bubbling dry N2; then, the AOT reverse micelles solution was added to the residue to obtain a [AOT] = 0.5 M. The stock solution of AOT 0.5 M and the probe molecules in 10-5 M concentration were agitated in a sonicating bath until the microemulsion was optically clear. To the cell baring 2 mL of o-NA, of the same concentration in n-heptane, was added the appropriate amount of stock solution to obtain a given concentration of AOT in the micellar media. Therefore, the absorption of the molecular probe was not affected by dilution. General. UV/visible spectra were recorded using a spectrophotometer Shimadzu 2401 with a thermostated sample holder. The path length used in absorption experiments was 1 cm.
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