Critical revision of apparent dielectric constants calculations in lipid-water interfaces

In a previous work we used the Stokes shift of two fluorescent probes, dans yldihexadecylamine (DA) and dansylphosphatidylethanolamine (DPE), to study the changes of phospholipid vesicles interfacial polarity on lipid composit ion and temperature (Bernik, D. L., and Negri, R. M., J. Colloid Interface Sci. 203, 97 (1998)), Here, the experimental data are used to critically re visit the procedure commonly followed to obtain the so-called "apparent die lectric constant", epsilon(s)(app), using the Lippert-Mataga equation. We f ound that anomalous values for epsilon(s)(aPP), such as negative or extreme ly large values around 100, can be obtained at the membrane-water interface . The origin of those values is discussed and numerical simulations regardi ng the limitations and assumptions of the procedure are performed. It is sh own that as the Stokes shift procedure renders information on the electric field due to orientational dipoles sensed by the probe, the recovered epsil on(s)(app) values are dependent on the local refractive index estimation. T he results remark on a mathematical basis the cautions that must be taken w hen physical properties are inferred from experimental data in colloidal sy stems. (C) 2000 Academic Press.