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

Abstract

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 ≈ 10.2) which is clearly affected in Triton X-100 microemulsions where the neutral species is stabilised at low ω₀. 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 on the amount of solubilised water (ω₀ = [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 ω₀ = 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 × 10⁸ M⁻¹ cm⁻¹).