The CMC-like behaviour of bile salts as probed by photoexcited Rose Bengal

The onset of the aggregation process in aqueous bile salt (BS) solutions remains a debated question. This study is aimed at investigating this point through the detection of BS monomers and small aggregates using the dye Rose Bengal (RB). Results showed that a few small BS aggregates led to a red shift of the absorption spectrum, while the BS monomer reacted with the photo-induced semi-oxidized dye and decreased its decay rate constant. The dye was dissolved in aqueous solution (10 mmol dm−3 NaOH) of sodium cholate (NaC), deoxycholate (NaDC), chenodoxycholate (NaCDC), dehydrocholate (NaDHC), glycocholate (NaGC) glycodeoxycholate (NaGDC), taurocholate (NaTC), taurodeoxycholate (NaTDC) and the zwitterionic analogue of sodium cholate called CHAPS. BS aggregates were detected at concentrations well below the concentration above which any added bile salt molecules participate in the aggregation process for all trihydroxy bile salts and for CHAPS but not for the dihydroxy bile salts. No aggregation of NaDHC could be observed in the studied concentration range (≤10 mmol dm−3). The presence of 150 and 600 mmol dm−3 sodium ions lowered the concentration below which BS are mainly monomers for dihydroxy but not for trihydroxy bile salts. As already proposed by others, the use of the concept of micellar dissociation concentration (MDC) is suggested here instead of the classical critical micelle concentration (CMC).