Surface tension studies of cetyltrimethylammonium bromide-bile salt association

Surface tension measurements have been performed on solutions containing cetyltrimethylammonium bromide and a bile salt, either sodium cholate or desoxycholate, in 50 mM NaCl. For the individual ionic surfactants the interfacial area per surfactant molecule (A(s)) and critical micelle concentration (cmc) were determined. For bile salt solutions a break in surface tension before the cmc is explained as an indication of a change in packing of the anions at the interface from a flat (A(s) > 150 Angstrom(2)) to an upright orientation (A(s) approximate to 41-45 A(2)) and is compared with results from the monolayer studies of Ekwall and Small. For systems containing binary mixtures of the oppositely charged surfactants, the experimentally determined mixed cmc (C*) was used to estimate the mixed micelle composition and the molecular interaction parameter, beta, using the treatment of Rubingh for nonideal mixtures. The mixtures showed significant deviation from ideal mixing, giving an average beta = -4 and -2.7 for the CTAB-NaC and CTAB-NaDOC systems, respectively. For both bile salts, the mixed cmc has a minimum when the bile salt fraction in the mixed micelles is close to 0.3, suggesting that a particularly favorable packing of the micelles is obtained in this composition range. However, using the average value of the interaction parameter in the theory of Rubingh, the cmc values of the systems are predicted within the precision of the experimental findings.