Microstructure and transitions in mixed micelles of cetyltrimethylammonium tosylate and bile salts

Abstract The effect of bile salts viz. sodium cholate (NaC) and sodium deoxycholate (NaDC) on the size and transition of cetyltrimethylammonium tosylate (CTAT) micelles is examined by viscosity, cryogenic transmission electron microscopy (Cryo-TEM), small angle neutron scattering (SANS), nuclear Overhauser enhancement spectroscopy (NOESY) and 1 H NMR. The viscosity of worm-like micelles of CTAT solution strongly decreases as the temperature increases favoring the transformation to spheroidal micelles. At low NaC concentration (5–10 mM), viscosity of 20 mM CTAT solutions decreases, reaching a constant value close to that of water. However, for NaDC, a peak in viscosity is seen followed by a drop around 3 mM concentration. The microstructure of CTAT micelles gets altered by bile salts as revealed by cryo-TEM. As manifested by SANS results, NaC transforms worm-like CTAT micelles into small spherical/ellipsoidal mixed micelles. However, there was no difference in SANS plot of worm-like CTAT micelles in D 2 O and in presence of NaDC; these clearly suggest the presence of worm-like micelles. NaDC behaves differently, showing a maximum in viscosity for 20 mM CTAT at ∼3 mM concentration and phase separation at equimolar ratio. The results are discussed in terms of location of bile salt in mixed micelles determined by NOESY and 1 H NMR experiments.