Fluorescence studies of the drug Piroxicam in reverse micelles of AOT and microemulsions of Triton X-100

Abstract

The microenvironment of the polar core of reverse micelles of the anionic surfactant AOT in isooctane and the microemulsions of the non ionic surfactant Triton X-100 in cyclohexane/hexanol were studied by electronic absorption and steady-state fluorescence spectroscopy incorporating an antiinflamatory nonsteroidal drug Piroxicam. Two acid-base equilibria of this probe in water (pK_a = 3.1 and pK_a = 4.8) are strongly and differently affected by the nature of the two interfaces and inner pools of each microheterogeneous system.

In AOT, fluorescence quantum yields vary up to w ₀ = 10 reflecting changes in the microviscosity sensed by the probe and intramicellar pH gradients, reaching a nearly constant value up to w ₀ = 40. In Triton X-100 the interface is polar and protic and interacts with the probe as a Lewis base. Three different spectroscopic species were detected: one attached to the interface at w ₀ = 0, the second one with structured water up to w ₀ = 8 and a third one in free water at large w ₀ values (w ₀ = 16), with increasing microviscosity values.

Fluorescence anisotropy studies with other probes in the same micellar systems are in good agreement with the patterns of microviscosity found in both systems.