Characterization of biosurfactants' micelles formation using fluorescence measurements: sodium taurocholate as case of study.

Abstract

The evaluation of micellization parameters of surfactants that aggregate gradually, such as bile salts, is not trivial. In this work, different probes and data treatment models are tested to set up an analytical method based on fluorescence measurements to determine the critical micelle concentration (CMC) and micellization range ((C) of biosurfactants. Sodium taurocholate (NaTc) is used as example. The fluorescence intensity of five fluorophores has been monitored upon the addition of a concentrated NaTc solution in two different media: water and a biorelevant buffer (maleic buffer pH 6.5, I = 120 mM). Four different data treatment methods have been tested. The micellization process can be evaluated satisfactorily using fluorescent probes such as propranolol and tetracaine, and also monitoring directly the intrinsic fluorescence of NaTc. However, the results obtained with nonpolar probes (pyrene and naphthalene) are more complex to evaluate due to the presence of confluent processes. Although the four models tested for the data treatment are commonly used for this purpose, Carpena's method is the most appropriate as it provides the most accurate CMC and ΔC values. The micellization process is faster in a biorelevant buffer than in water. The study of the micellization of bile salts is not an evident process. After the selection of adequate probes and data treatment methods, the CMC values for NaTC in water and maleic buffer reveal that the biorelevant conditions favour micellization, which in turn may allow faster solubilization of ingested compounds.