Self-Assembly in Mixtures of an Anionic and a Cationic Surfactant: A Comparison between Small-Angle Neutron Scattering and Cryo-Transmission Electron Microscopy

The self-assembly in SOS-rich mixtures of the anionic surfactant sodium octyl sulfate (SOS) and the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) has been investigated with the complementary techniques small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). Both techniques confirm the simultaneous presence of open and closed bilayer structures in highly diluted samples as well as the existence of small globular and large elongated micelles at higher concentrations. However, the two techniques sometimes differ with respect to which type of aggregates is present in a particular sample. In particular, globular or wormlike micelles are sometimes observed with cryo-TEM in the vicinity of the micelle-to-bilayer transition, although only bilayers are present according to SANS and the samples appear bluish to the eye. A similar discrepancy has previously been reported but could not be satisfactorily rationalized. On the basis of our comparison between in situ (SANS) and ex situ (cryo-TEM) experimental techniques, we suggest that this discrepancy appears mainly as a result of the non-negligible amount of surfactant adsorbed at interfaces of the thin sample film created during the cryo-TEM specimen preparation. Moreover, from our detailed SANS data analysis, we are able to observe the unusually high amount of free surfactant monomers present in SOS-rich mixtures of SOS and CTAB, and the experimental results give excellent agreement with model calculations based on the Poisson–Boltzmann mean field theory. Our careful comparison between model calculations and experiments has enabled us to rationalize the dramatic microstructural transformations frequently observed upon simply diluting mixtures of an anionic and a cationic surfactant

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

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 (<i>k̅</i>_<i>c</i>^bi) with decreasing influence from electrostatics, in agreement with theoretical predictions as deduced from the Poisson–Boltzmann theory

Remarkable Viscoelasticity in Mixtures of Cyclodextrins and Nonionic Surfactants

We report the effect of native cyclodextrins (α, β, and γ) and selected derivatives in modulating the self-assembly of the nonionic surfactant polyoxyethylene cholesteryl ether (ChEO₁₀) and its mixtures with triethylene glycol monododecyl ether (C₁₂EO₃), which form wormlike micelles. Cyclodextrins (CDs) generally induce micellar breakup through a host–guest interaction with surfactants; instead, we show that a constructive effect, leading to gel formation, is obtained with specific CDs and that the widely invoked host–guest interaction may not be the only key to the association. When added to wormlike micelles of ChEO₁₀ and C₁₂EO₃, native β-CD, 2-hydroxyethyl-β-CD (HEBCD), and a sulfated sodium salt of β-CD (SULFBCD) induce a substantial increase of the viscoelasticity, while methylated CDs rupture the micelles, leading to a loss of the viscosity, and the other CDs studied (native α- and γ- and hydroxypropylated CDs) show a weak interaction. Most remarkably, the addition of HEBCD or SULFBCD to pure ChEO₁₀ solutions (which are low-viscosity, Newtonian fluids of small, ellipsoidal micelles) induces the formation of transparent gels. The combination of small-angle neutron scattering, dynamic light scattering, and cryo-TEM reveals that both CDs drive the elongation of ChEO₁₀ aggregates into an entangled network of wormlike micelles. ¹H NMR and fluorescence spectroscopy demonstrate the formation of inclusion complexes between ChEO₁₀ and methylated CDs, consistent with the demicellization observed. Instead, HEBCD forms a weak complex with ChEO₁₀, while no complex is detected with SULFBCD. This shows that inclusion complex formation is not the determinant event leading to micellar growth. HEBCD:ChEO₁₀ complex, which coexists with the aggregated surfactant, could act as a cosurfactant with a different headgroup area. For SULFBCD, intermolecular interactions via the external surface of the CD may be more relevant

Characterization of Oil-Free and Oil-Loaded Liquid-Crystalline Particles Stabilized by Negatively Charged Stabilizer Citrem

The present study was designed to evaluate the effect of the negatively charged food-grade emulsifier citrem on the internal nanostructures of oil-free and oil-loaded aqueous dispersions of phytantriol (PHYT) and glyceryl monooleate (GMO). To our knowledge, this is the first report in the literature on the utilization of this charged stabilizing agent in the formation of aqueous dispersions consisting of well-ordered interiors (either inverted-type hexagonal (H₂) phases or inverted-type microemulsion systems). Synchrotron small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) were used to characterize the dispersed and the corresponding nondispersed phases of inverted-type nonlamellar liquid-crystalline phases and microemulsions. The results suggest a transition between different internal nanostructures of the aqueous dispersions after the addition of the stabilizer. In addition to the main function of citrem as a stabilizer that adheres to the surface of the dispersed particles, it has a significant impact on the internal nanostructures, which is governed by the following factors: (1) its penetration between the hydrophobic tails of the lipid molecules and (2) its degree of incorporation into the lipid–water interfacial area. In the presence of citrem, the formation of aqueous dispersions with functionalized hydrophilic domains by the enlargement of the hydrophilic nanochannels of the internal H₂ phase in hexosomes and the hydrophilic core of the L₂ phase in emulsified microemulsions (EMEs) could be particularly attractive for solubilizing and controlling the release of positively charged drugs