Structural evolution in catanionic mixtures of cetylpyridinium chloride and sodium deoxycholate

Sodium deoxycholate (NaDC) is a water soluble bile salt commonly used in applications ranging from cell lysis, liposome preparation and isolation of membrane proteins. We present the microstructural evolution in aqueous mixtures of biocompatible cationic surfactant cetylpyridinium chloride (CPC) and bile salt NaDC using dynamic light scattering (DLS), small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS). When the total concentration of the mixture (C-T) is less than 370 mM, associative phase separation occurs, near the equimolar ratio, which vanishes at high concentrations (>370 mM). The associative phase separation observed at low CT has been explained on the basis of competition between electrostatic attraction and entropy of mixing of the components. Pure CPC micelles undergo shape transition from prolate to oblate, as the concentration increases from 50 mM to 400 mM. Small addition of NaDC to 400 mM of CPC leads to marginal size change in the oblate micelles. On the other hand, pure NaDC micelles are prolate ellipsoids for which the micellar size increases by incorporation of CPC. The observed structural transition is explained in terms of the electrostatic binding of bile salts to cationic surfactants and the incorporation of the steroidal skeleton in the bile salt at the micelle core-head group interface. Microstructure evolution in catanionic mixtures comprising biocompatible surfactants offers potential pharmaceutical applications.