Investigations on microstructural changes in pH responsive mixed micelles of Triton X-100 and bile salt.

Aqueous solution behaviour of a nonionic surfactant Triton X-100 is investigated in the presence of two bile salts namely sodium deoxycholate (NaDC) and sodium cholate (NaC) at different pH, temperatures and in the presence of sodium chloride and the resultant structural changes to accordingly formed mixed micelles were analyzed by using cloud point (CP), viscosity and scattering techniques. Both the bile salts increased the CP and showed a corresponding decrease in viscosity and apparent hydrodynamic diameter (Dh), which can further be subsided with the progressive addition of sodium chloride and an increase in temperature. Interestingly, in the presence of bile salt below pH ∼5, CP decreased with corresponding increase in viscosity, while a reversed trend was observed above pH∼8. Small angle neutron scattering data reveal that nearly spherical mixed micelles were formed in the presence of bile salt which grow and transform to prolate ellipsoidal ones at pH∼3. These morphological changes are facilitated by the protonation of carboxylic acid group of bile salt and deeper penetration of bile acid molecules into TX-100 micelles at lower pH. Proposed molecular interactions are extremely informative to understand more about these biologically important compounds playing a crucial role in digestion processes.