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

In a previous work we used the Stokes shift of two fluorescent probes, dansyldihexadecylamine (DA) and dansylphosphatidylethanolamine (DPE), to study the changes of phospholipid vesicles interfacial polarity on lipid composition and temperature (Bernik, D. L., and Negri, R. M., J. Colloid Interface Sci. 203, 97 (1998)), Here, the experimental data are used to critically revisit the procedure commonly followed to obtain the so-called "apparent dielectric constant", epsilon(s)(app), using the Lippert-Mataga equation. We found that anomalous values for epsilon(s)(aPP), such as negative or extremely large values around 100, can be obtained at the membrane-water interface. The origin of those values is discussed and numerical simulations regarding the limitations and assumptions of the procedure are performed. It is shown that as the Stokes shift procedure renders information on the electric field due to orientational dipoles sensed by the probe, the recovered epsilon(s)(app) values are dependent on the local refractive index estimation. The results remark on a mathematical basis the cautions that must be taken when physical properties are inferred from experimental data in colloidal systems. (C) 2000 Academic Press.