Search

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt:  a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.

To understand the strong viscoelastic response showed by aqueous solutions of erucylbis(hydroxyethyl)methylammonium chloride (EHAC) in the presence of potassium chloride (KCl), steady-state rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) experiments were performed. This cationic surfactant has the ability to self-assemble into giant wormlike micelles. The effect of surfactant concentration, added salt, and temperature were investigated. The surfactant solutions have a gellike behavior at room temperature and become Maxwellian as the temperature is increased. It was found that the low-shear viscosity has a strong dependence on salt concentration and temperature. Small-angle scattering indicated the formation of wormlike micelles. The high-Q range was fitted using the Kratky−Porod wormlike chain model, and a cross-sectional radius of gyration (Rg,xs) of 21 Å was obtained. Additionally, cryo-TEM images revealed changes in the structure of the entangled network with the addition of salt.

The ability of a number of amphiphilic di- and triblock copolymers bearing short blocks of repeating lipid-mimetic units to sterically stabilize egg-phosphatidylcholine (EPC) liposomes has been studied by means of light scattering, cryogenic transmission electron microscopy (cryo-TEM), and fluorescence spectroscopy. The hydrophobic blocks contain 1−4 lipid-mimetic units, whereas the hydrophilic blocks consist of poly(ethylene glycol) (PEG) of average molecular weights close to 5000 Da. The effect of the chain architecture was investigated as well. The structural changes and membrane permeability were studied as a function of the amount of the incorporated copolymer. The structural investigations, performed by means of cryo-TEM, revealed that the maximum amount of the copolymer that can be incorporated in the EPC membrane without affecting the liposome integrity increases with increasing number of the lipid-mimetic anchors per single copolymer chain. This helps the formation of a dense and thick PEG protective layer which, in turn, is expected to enhance the liposome longevity. At the same time, the membrane permeability, investigated using a hydrophilic dye as a probe, was only slightly affected by the incorporation of the copolymers. The structural changes and membrane permeability are discussed in light of the shape of the macromolecules and the phase propensity of the steric stabilizers.

The lactosyl carborane [1,2-dicarba-closo-dodecarboran(12)-1-ylmethyl](β-d-galactopyranosyl)-(1→4)-β-gluco-pyranoside (LCOB) is an amphipilic boron-containing compound intended for boron neutron capture therapy. In this study cryogenic transmission electron microscopy was used to investigate the effects of LCOB on the structure and properties of extruded egg phosphatidylcholine liposomes. The results showed that LCOB concentrations up to at least xLCOB = 0.25 could be included in the preparations without any noticeable effects on the liposomal size or structure. Inclusion of LCOB concentrations corresponding to xLCOB = 0.44 or more gave, however, rise to significant changes in the liposome size distribution and the overall sample structure. In fresh samples no signs of phospholipid solubilization were detected until the molar fraction of LCOB reached 0.82. At this concentration the boronated compound induced, however, formation of open liposomes and threadlike micelles and, furthermore, with time the sample displayed a macroscopic phase separation. More details on the molecular interaction between LCOB and the liposome membranes were obtained by electron spin resonance spectroscopy. Two different n-doxyl stearic acid spin probes were used to measure the dynamics and the degree of order in the mixed systems. The results indicated that LCOB insertion into the phospholipid bilayer increased the packing order in the vicinity of the water/hydrocarbon interface whereas the inner hydrophobic region of the bilayer was unaffected.

The structural features of vesicles, micelles, and other aggregates spontaneously formed from a fluorinated surfactant (the ammonium salt of perfluoropolyether, PFPE) and a hydrogenated surfactant (n-dodecylbetaine) in dilute water solution were characterized by means of cryo-transmission electron microscopy (cryo-TEM) at different betaine mole fractions. The size distribution of the aggregates was found to depend critically on surfactant composition. In a narrow range of betaine molar fractions (xbet = 0.76−0.79), two different populations of unilamellar and spheroidal vesicles with mean radii of 70−120 and 20−30 nm, respectively, coexisted with globular and, in some cases, threadlike micelles. In the same region, vesicles with openings in the bilayer and disk-shape fragments began to appear. Further increase in betaine molar fraction resulted in an increase of the number of globular micelles and discoid aggregates and, finally, to the complete PFPE solubilization into mixed micelles. The large vesicles disappeared at xbet = 0.81, whereas the small vesicles (mostly with open bilayers), occurred up to xbet = 0.86. At xbet < 0.75 no micelles were detected, and the sample consisted of uni- and multilamellar vesicles with high polydispersity. Increasing the total surfactant concentration gave rise to a significant increase of the vesicle size without modification of the size distribution of the aggregates themselves.

The interactions of five poly(oxyethylene)−poly(oxypropylene)−poly(oxyethylene) (PEO−PPO−PEO), Pluronic, copolymers and phosphatidylcholine liposomes of varying composition have been studied. Structural studies were performed by means of cryo-transmission electron microscopy (c-TEM) and reveal that inclusion of low amounts (~2 mol %) of Pluronics gives rise to significant morphological changes of the liposome preparations. Pluronics with large poly(oxyethylene) (PEO) blocks, such as F127, F108, and F87, induce the formation of bilayer disks, whereas those with comparably short PEO blocks, P105 and P85, tend to to promote a reduction in the liposome size. Inclusion of cholesterol in the liposomal preparations reduces the incorporation of copolymers in the lipid bilayer and thus reduces, and in some cases even abolishes, the morphological changes observed in the absence of cholesterol. The effect of the copolymers on liposome permeability was also investigated. All investigated copolymers were found to increase the leakage of carboxyfluorescein from preformed liposomes. This was true also in the case of cholesterol-containing liposomes despite the fact that no change in the liposome structure could be observed by means of c-TEM. The magnitude of the induced leakage was found to correlate well with the hydrophobicity, as measured by the cmc, of the respective Pluronic. By raising the temperature or decreasing the solvency of the copolymer in the solution, the effect of the copolymer on liposome leakage was found to increase significantly.