STRUCTURAL CHANGES IN W/O TRITON X-100/Cyclohexane-Hexanol/Water Microemulsions Probed by a Fluorescent Drug Piroxicam
✍ Scribed by Suzana M. Andrade; Silvia M.B. Costa; Robert Pansu
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 121 KB
- Volume
- 226
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
The photophysics of a fluorescent drug Piroxicam was used to probe the intramicellar region of nonionic microemulsions of Triton X-100/cyclohexane-hexanol/water, using both steady-state and transient (picosecond resolution) techniques. A protic and polar interface, as inferred from the calculated Kamlet-Taft parameters (α and π * ), interacts with the probe as a Lewis base. Three distinct spectroscopic species were detected with pH-dependent contributions. Such dependence is noted only above a certain water concentration (ω 0 = 8), confirming the existence of water pools thereafter. Transient data lead to similar conclusions. A global analysis of the fluorescence decays was made and three different lifetimes were obtained. The shortest (τ 1 = 16 ps) and the longest (τ 3 = 2.02 ns) correspond to those already found in free aqueous solution. An intermediate component (τ 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 α and π * , calculated 0.5 and 0.7, respectively, contribute to this statement. Major changes observed in the system around ω 0 = 8 are emphasized, taking into account a dynamic light-scattering study, where a dependence of the hydrodynamic radius on the observation angle is detected above this water content.