Solvatochromism pyridine-1 -oxide

Johnston et al. l also examined the solvatochromic shift of pyridine N-oxide in an ethane/CjEj (C = 10-13 E = 5) water-in-oil microemuision, also in equilibrium with a lower liquid phase. Contrary to the behavior exhibited by the AOT system, the nonionic microemulsions display a polar environment at low pressures, which becomes progressively less polar as pressure increases. At a pressure of only 50 bar, they reported that the probe s environment resembles that observed in bulk hexane. Added water increases the polarity somewhat, yet a cosurfactant (octanol) is required to produce an environment similar to that in bulk water. The polarity of the ethane/ water/surfactant/cosurfactant system remains essentially constant as pressure increases up to 350 bar. 

To further test pyridine-N-oxide as an indicator, the solvatochromic data are correlated versus micelle radius in Figure 5. The Xmax values for pyridine-N-oxide were determined in solutions of. 025 M AOT in two solvents, n-bctane d n-hexane. For a given Wq the values are very similar in the two solvents. The micelle radii are from the photon correlation experiments of Zulauf and Eicke (, at the same AOT concentration, for a similar solvent, isooctane. It is widely accepted that Wo is a good indicator of the size of reverse micelles, as is evident in the relationship for the two horizontal axes in the figure. There is a relatively linear relationship between micelle size and Xmax for a Wq up to 15. The Xmax this point approaches that of pure water. This is in accord with Eicke and Kvita (28). who indicate that at a Wq of approximately 15, the water pool has the characteristics of free water. These results supply additional evidence of pyridine-N-oxide s hydrophilic nature and utility as an indicator. 

Figure 4. Pyridine-N-oxide as a solvatochromic probe in cyclohexane- effect of Wo = [water]/[AOT]...

Anionic Surfactant AOT has been studied extensively since it forms reverse micelles readily in a variety of organic solvents, even without a co-surfactantf28 >. Figure 6 shows the solvatochromic shifts of 0.0002 M pyridine-N-oxide in solutions of AOT in SCF ethane at 345 bar. No water was added to the system, but it is likely that Wo was 1 given the difficulty of completely dehydrating AOT(22). The pressure was fixed at 345 bar so that all of the AOT solutions would be in the one-phase region(16). Notice how closely the results for ethane match those for... 

The scale was proposed for solute HBD acidity of monomer amphihydrogen-bonding compounds acting as non-self-associated solutes [Taft, Abraham et al., 1985 Kamlet, Doherty et al, 1986a]. In particular, a, values were derived from logfC values for complexation with pyridine N-oxide in cyclohexane this set of values was successively extended through various back-calculations using the solvatochromic equation. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

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