Microstructure evolution in aqueous solutions of cetyl trimethylammonium bromide (CTAB) and phenol derivatives

Surfactant aggregate microstructure changes induced by the addition of a series of phenolic organic dopants (phenol, cresol, 4-ethyl phenol and 4-n- and i-butyl phenol) to a micellar solution of cetyl trimethyl ammonium bromide (CTAB) have been studied using cryogenic transmission electron microscopy (cryo-TEM). As the concentration of each of the dopants is increased, there is a systematic reduction in the curvature of the observed microstructures. For phenol and cresol the microstructure changes from globular micelles to rod-like micelles and then to long worm-like micelles. For 4-ethyl phenol and 4-butyl phenol, the microstructure transforms from globular micelles to worm-like micelles to unilamellar vesicles and then bilamellar vesicles. The concentrations at which these morphological transitions take place reduce as the number of methyl substitutions on the phenolic ring is increased. These microstructure transitions are attributed to changes in the packing parameter of the surfactant complex as dopants penetrate to different degrees in the surfactant palisades layer, resulting from a balance of interaction between the surfactant tails and the dopant aromatic chain and the hydrogen bonding of the hydroxyl group with surrounding water. The alignment of the hydroxyl dipoles reduces the electrostatic field around the CTAB head groups, decreases interlayer repulsive interactions and allows membrane fluctuations to stabilize the bilamellar vesicles. These highly tunable microstructures can be exploited for templated materials synthesis, and have implications for the micellar-enhanced ultrafiltration process. (c) 2006 Elsevier B.V. All rights reserved.