A POTENTIOMETRIC TITRATION STUDY ON THE DISSOCIATION OF BILE-ACIDS RELATED TO THE MODE OF INTERACTION BETWEEN DIFFERENT HEAD GROUPS OF NONIONIC SURFACTANTS WITH FREE BILE-SALTS UPON MIXED MICELLE FORMATION IN WATER

By constructing an elaborate set of potentiometric titration together with data analysis system, apparent acid dissociation indices (pK(a)(a pp)) for two bile acids were determined in the mixed surfactant system of bile salts (Sodium Deoxycholate, NaDC, and Sodium Chenodeoxycholat e, NaCDC with nonionic surfactants (Hexaethyleneglycol mono n-dodecyle ther, C(12)E(6), Decanoyl-N-methylglucamide, MEGA-10) in aqueous solut ion at ionic strength 1.5 as a function of mole fraction in the surfac tant mixture. It was found that with increasing the bile salt concentr ation, pK(a)(app) as well as pH showed an abrupt rise at a certain con centration of the bile salt being regardable as a critical micellizati on concentration (CMC) and reached a constant value at the range suffi ciently higher than CMC for each pure bile salt system, meaning that t he dissociation degree of carboxyl group in micelle is smaller than th at in bulk. In the mixed systems of free bile salts with nonionic surf actants, the dissociation state of carboxyl groups in mixed micelles d epends on the species of hydrophilic group of nonionic surfactants as well as on mole fraction in the surfactant mixture.