Stomatosomes, blastula vesicles and bilayer disks: Morphological richness of structures formed in dilute aqueous mixtures of a cationic and an anionic surfactant

Cryogenic transmission electron microscopy (cryoTEM) was used to study the structures formed in mixtures of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in dilute aqueous solutions with 0–300 mM NaBr. The DTAB mole fraction, X  , was in the range 0.2–0.4, limited at 25 °C by precipitation of solid DTA–DS at X=0.38X=0.38 without salt to X=0.25X=0.25 at 300 mM NaBr. At a total surfactant concentration of 100 mM the samples separated into two liquid phases (the bottom phase birefringent) within a narrow (±0.01 mole fraction units) composition range. At the mid-point X   varied from 0.32 without salt to 0.22 at 300 mM NaBr. Elemental analysis of C, S, O, and N in the separated phases of a sample with 100 mM NaBr and X=0.26X=0.26 showed the top phase to contain almost only SDS at a low concentration, 14 mM, and the bottom phase 175 mM total surfactant, with X=0.27X=0.27. Elemental analysis on samples without added salt gave erratic results, indicating problems in the physical separation of the phases. The cryoTEM survey of the separated phases revealed similar problems. Without salt both phases showed similar structures, whereas the top phase in the sample with added salt was void of structures larger than small micelles. The cryoTEM survey revealed a variety of structures being simultaneously present in most samples. A general trend with increasing X was an evolution from globular micelles, over disks, bands, branched bands transforming into sparse webs, perforated bilayer structures, and finally smooth bilayers. Increasing salt and total surfactant concentrations resulted in the emergence of structures with smaller mean curvature at lower X. Perforated bilayers were found in samples with 100 mM or less of added salt, and usually persisted to DTAB contents where precipitates appeared. The porous bilayers seemed to derive from sparse webs of band-like structures, and the hole size decreased with increasing X   and salt concentration. Two types of recurrent structures were noticed: blastula aggregates, seemingly an intermediate structure transforming crumpled bilayers into vesicles of similar size (diameter 400–500 Å), observed over a broad range of conditions, and at 100 mM total surfactant concentration and 50 mM added salt or more a type of regular disks with a diameter of 180±30 Å180±30 Å.

0.24 ⩽ [DTAB]/([SDS] + [DTAB]) ⩽ 0.34: From disks to perforated vesicles.Figure optionsDownload as PowerPoint slide