Structural changes in W/O Triton X-100/cyclohexane-hexanol/water microemulsions probed by a fluorescent drug Piroxicam

The photophysics of a fluorescent drug Piroxicam was used to probe the intr amicellar region of nonionic microemulsions of Triton X-100/cyclohexane-hex anol/water, using both steady-state and transient (picosecond resolution) t echniques. A protic and polar interface, as inferred from the calculated Ka mlet-Taft parameters (alpha and pi*), interacts with the probe as a Lewis b ase. Three distinct spectroscopic species were detected with pH-dependent c ontributions. Such dependence is noted only above a certain water concentra tion (omega(0) = 8), confirming the existence of water pools thereafter. Tr ansient data lead to similar conclusions. A global analysis of the fluoresc ence decays was made and three different lifetimes were obtained. The short est (tau(1) = 16 ps) and the longest (tau(3) = 2.02 ns) correspond to those already found in free aqueous solution. An intermediate component (tau(2) = 72 ps), whose importance grows with the amount of solubilized water at pH (ext) 4, is thought to represent a less protic and more rigid environment, which is sensed by the probe. An increase in the microviscosity values also detected by fluorescence anisotropy and the lower cr and pi*, calculated 0 .5 and 0.7, respectively, contribute to this statement. Major changes obser ved in the system around omega(0) = 8 are emphasized, taking into account a dynamic light-scattering study, where a dependence of the hydrodynamic rad ius on the observation angle is detected above this water content, (C) 2000 Academic Press.