Properties of AOT aqueous and nonaqueous microemulsions sensed by optical molecular probes

Nonaqueous microemulsions of AOT in n-hexane and n-heptane by using six polar solvents as water substitutes such as glycerol (GY), ethylene glycol (EG), propylene glycol (PG), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA) were studied and compared with the corresponding aqueous reverse micelles. The microenvironment generated by these systems was sensed by following the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB). By varying W-s (W-s = [polar solvent]/[AOT]), the existence of an interaction between the hydrogen bond donor solvents EG, PG, and FA and QB was detected by the changes in the absorption spectra. These changes were interpreted as caused by the partition of the probe between the micelle interface and the polar solvent core by hydrogen bond interactions. The hydrogen bond association constants were calculated, being the largest for FA. In the case of GY, as well as for water, QB seems to be anchored at the interface and no partition was detected. For solvents with no hydrogen bond donor ability such as DMF and DMA the polarity sensed by QB increases with W-s being always larger than the polarity of the neat solvent. The value of the critical micellar concentration (cmc) was also investigated by using acridine orange base as absorption and fluorescence probe. The values of the cmc found for these systems are around (7 +/- 2) x 10(-3) M which are higher than those for AOT in comparable aqueous systems.