Aggregate Structure, Gelling, and Coacervation within the L1 Phase of the Quasi-ternary System Alkyltrimethylammonium Bromide-Sodium Desoxycholate-Water

Abstract

The phase behavior of mixtures of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), with a bile salt, sodium desoxycholate (NaDOC), in water has been studied at 25°C. Mixing of the two surfactants at equimolar composition gave an associative type of phase separation similar to that observed in mixed polyelectrolyte surfactant systems. A partial ternary phase diagram was obtained in the water-rich corner and the limits of the two-phase (2L1) region were determined. The structural properties of the mixed bile salt-CTAB aggregates and their connection with phase separation were investigated by the application of dynamic light scattering (DLS) complemented with cryo-transmission electron microscopy (TEM) studies in the L1 phase surrounding the two-phase island and the gel region. In dilute concentrations both DLS and cryo-TEM techniques provide evidence for the presence of flexible cylindrical micelles at above mole fraction of NaDOC of 0.2. A progressive change in the appearance of the mixed micelles is directly visualized from cryo-TEM studies. At a mole fraction of 0.2 and below, small globular micelles exist; above mole fraction 0.3, flexible cylindrical thread-like micelles are found. A further increase in NaDOC gives structures with decreased extension and increased contortions, in many cases branching with Y-junctions. DLS autocorrelation decay curves have been analyzed using the REPES program which makes a smoothed inverse La-place transformation of the autocorrelation function. The hydrodynamic screening length evaluated using the Stokes-Einstein equation shows in a highly sensitive way the approach to the two-phase region. Critical dynamics behavior, i.e., an approach to a q3 -dependence of the average relaxation rate, is observed in the crossover region close to the coacervation boundary. DLS and rheology measurements were performed at higher concentrations, both in a region with gel-like composition and at several bile salt-CTAB mole fractions. The DLS and rheological properties observed are interpreted using concepts analogous to a solution of entangled polymers. The study was extended to include the influence of varied alkyl chain length and similar trends in aggregation and phase separation were observed when CTAB was replaced by other alkyltrimethylammonium bromides (C18TAB, C14TAB, and C12TAB).