Small-Angle Neutron Scattering (SANS) Study of Vesicles and Lamellar Sheets Formed from Mixtures of an Anionic and a Cationic Surfactant

Abstract

The various bilayer structures formed from aqueous mixtures of an anionic (SDS) and a cationic surfactant (DTAB) with identical hydrocarbon C 12 chains at 40 ˚C have been investigated using small-angle neutron scattering (SANS) as well as static light scattering (SLS). The SANS data were analyzed using a paracrystal lamellar model with respect to the layer distance distribution and the number of layers in a single cluster. Unilamellar or oligolamellar vesicles form in the most diluted samples in the absence of added salt where the number of layers in a single cluster is found to be 1-3. Beyond the compositions where micelles form (30:70 < [SDS]:[DTAB] < 70:30), we observe a transition from vesicles to stacks of lamellar sheets upon increasing the overall surfactant concentration, indicated by an abrupt increase of the number of layers in a single cluster from 1-3 to infinity. Combined SANS and SLS data for samples containing vesicles ([SDS] + [DTAB] ) 0.25 wt % in the absence of added salt) could be fitted with a model for unilamellar vesicles using a structure factor for sticky hard spheres, indicating that the vesicles attract each other and form clusters. However, at [SDS] + [DTAB] ) 0.125 wt %, the vesicles appeared to be too large for the size distribution to be determined from our SANS and SLS data. In 0.1 M NaBr, the vesicles were clearly destabilized and either micelles or lamellar sheets form at most compositions and concentrations where vesicles predominate in the absence of added salt.

Vesicles of a New Salt‐Free Cat‐anionic Fluoro/Hydrocarbon Surfactant System

“…In the correct ratios, vesicles may be established spontaneously and are thermodynamically stable species. [19][20][21] has also been demonstrated by 19 F NMR spectroscopy ( Figure 9). For the L 1 -phase sample with C 14 DMAO (100 mmol L…”

Thermodynamics of Unilamellar Vesicles: Influence of Mixing on the Curvature Free Energy of a Vesicle Bilayer

“…It may be noted that, according to Eq. [13], a remains dependent on the monolayer curvature 1/r also for the special case where both ξ and v are independent of r .…”

“…By employing the various curvature-dependent expressions [6], [12]- [13], [16]- [18] in order to calculate γ (r ) ≡ ε(r )/a(r ) from Eq. [14], and then collecting terms proportional to 1/r and 1/r 2 , respectively, expressions for k 1 and k 2 in terms of the various free energy contributions are obtained from which…”

“…As a matter of fact, the geometrical packing constraints as manifest in Eq. [13], together with an unfavorable hydrocarbon/water interfacial tension, give rise to the main contribution to the bilayer bending constant. The resulting contribution equals k geom bi ≡ N ε /4π , where ε is the curvature-independent part of the free energy per monomer of expelling N surfactant monomers as a planar bilayer is bent to form a geometrically closed vesicle at a fixed hydrocarbon/water interfacial area.…”

Lipid based drug delivery systems: Kinetics by SANS

“…Due to rapid aggregation, the dispersions are formed by oligolamellar up to multilamellar vesicles. The SANS data were analyzed using a paracrystal lamellar model with respect to the layer distance distribution and the number of layers in a single cluster [28]. In this model, scattering intensity is given by…”