The aqueous environment in AOT and Triton X-100 (w/o) microemulsions probed by fluorescence

The confined aqueous medium of Triton X-100/cyclohexane-hexanol/water microemulsions was studied and compared with that of AOT/isooctane/water. The microenvironment generated was assessed by following the photophysical behaviour of the cationic dye. acridine orange (AO). This dye presents an acid-base equilibrium in free bulk water (pK(a) approximate to 10.2) which is clearly affected in Triton X-100 microemulsions where the neutral species is stabilised at low omega(0). The addition of water contributes to the appearance of the protonated species. This. however, presents spectral features that show a less polar and/or protic character of the encapsulated water even at high contents. The "anomalous" microviscosity dependence oil the amount of solubilised water (omega(0) = [H2O]/[Surf]) in Triton X-100 microemulsions obtained from steady-state anisotropy data Was used to discuss the existence of bulk free water within these confined media. where at omega(0) = 8, properties seem to change. In AOT, a complex AO-AOT is detected in equilibrium with the protonated AO. at pH = 7. The two-state excited-state formalism was applied to describe the transient data which in the case of AOT introduces a rate constant that accounts for the water quenching (k(q) = 0.51 +/- 0.03 x 10(8) M-1 cm(-1)).