MODEL FOR PILE SALT MICELLIZATION AND SOLUBILIZATION FROM STUDIES OF A POLYDISPERSE ARRAY OF FLUORESCENT-PROBES AND MOLECULAR MODELING

The micellar aggregation of the bile salt sodium taurocholate (NaTC) in water, in the concentration range 0-35 mM, was studied by using a ''polydisperse'' array of polycyclic aromatic hydrocarbon probes, ranging from pyrene with four aromatic rings to ovalene with 10 aromatic rings. Measurements of fluorescence spectra, peak intensity, vibronic band intensity ratio, and lifetime were used to follow the solubilization of the probe as a function of NaTC concentration. The results strongly support stepwise aggregation behavior since at least two critical regions of NaTC concentration were reported by each of the probes. For all probes, one of these critical regions occurs at 8-12 mM. The other critical concentration regions, referred to as ''apparent critical micellar concentrations'' (acme), increase with increasing probe size and include 4, 15, and 20 mM. Molecular modeling was used to develop a model for micellar aggregation and probe solubilization for NaTC in water that incorporates experimental results of this and previous studies as well as considerations of molecular dimensions and aggregate conformational energy in the absence of solvent.