Spontaneous Transformations between Surfactant Bilayers of Different Topologies Observed in Mixtures of Sodium Octyl Sulfate and Hexadecyltrimethylammonium Bromide

Abstract

The influence of adding salt on the self-assembly in sodium octyl sulfate (SOS)-rich mixtures of the anionic surfactant SOS and the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) have been investigated with the two complementary techniques, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy. We are able to conclude that addition of a substantial amount of inert salt, NaBr, mainly has three effects on the structural behaviors: (i) the micelles become much larger at the transition from micelles to bilayers, (ii) the fraction of bilayer disks increases at the expense of vesicles, and (iii) bilayer aggregates perforated with holes are formed in the most diluted samples. A novel form factor valid for perforated bilayer vesicles and disks is introduced for the first time and, as a result, we are able to directly observe the presence of perforated bilayers by means of fitting SANS data with an appropriate model. Moreover, we are able to conclude that the morphology of bilayer aggregates changes according to the following sequence of different bilayer topologies, vesicles → disks → perforated bilayers, as the electrolyte concentration is increased and surfactant mole fraction in the bilayer aggregates approaches equimolarity. We are able to rationalize this sequence of transitions as a result of a monotonous increase of the bilayer saddle-splay constant (k(c)(bi)) with decreasing influence from electrostatics, in agreement with theoretical predictions as deduced from the Poisson-Boltzmann theory.

Explaining the growth behavior of surfactant micelles

“…The growth behavior hNi / / 1=2 mic , derived by using mole fraction rather than volume fraction in the expression for the entropy of mixing, implies a maximum polydispersity equal to r N /hNi = 1/ ffiffiffi 2 p % 0.71. The latter result is inconsistent with experimental observations that r N /hNi is frequently found to be close to or larger than unity for long rodlike or wormlike micelles [10,47,48,65,66]. A relative standard deviation exceeding unity implies an extent of growth stronger than predicted by the spherocylindrical micelle model [cf.…”

“…This lack of agreement between the spherocylindrical micelle model and experimental results has also been found when geometrical shapes of micelles have been experimentally determined with small-angle neutron scattering (SANS) [10,[43][44][45][46][47][48]. In accordance, in contrast to a simple one-dimensional growth behavior, several common surfactants have been found to self-assemble to form triaxial tablet-shaped micelles, with a distinct thickness, width and length that may grow (or shrink) with respect to both width and length.…”

Mixing Block Copolymers with Phospholipids at the Nanoscale: From Hybrid Polymer/Lipid Wormlike Micelles to Vesicles Presenting Lipid Nanodomains

“…Instead, we have developed a more simple approach to try to describe the scattering of hybrid vesicles of polymers with lipid domains, using only classical expressions of scattering amplitudes and form factors that can be easily processed with proven fitting methods. We derived a model recently proposed by Bergström et al 32 to describe mixtures of surfactants bilayers with salt creating holes inside the bilayers. In this simplified model of perforated vesicles, several assumptions were made: i) the vesicles were considered as infinitely thin circular shells with radius R, of form factor: Eq A 1 the holes being considered as symmetrical disks with radius and amplitude:…”

“…So, we considered a model of hybrid vesicles composed of a bilayer polymer vesicle with bilayer 'disks' of lipids as shown on Figure 10: a vesicle with inner radius and thickness , containing disks of us and membrane thickness . In order to account for the respective disk/vesicle thicknesses, we introduced the scattering amplitude of a bilayer cross-section 32 given by:…”

“…A4. Moreover, the scattering amplitudes of the different contributions were weighted by the corresponding volumes, instead of the areas in the work of Bergström et al 32 As a result, the 'hybrid vesicle' model has several parameters:…”