Model Catanionic Vesicles from Biomimetic Serine-Based Surfactants: Effect of the Combination of Chain Lengths on Vesicle Properties and Vesicle-to-Micelle Transition (original) (raw)
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The Journal of …, 2006
The aqueous self-assembly of a novel lysine-derived surfactant with a gemini-like architecture, designated here as 12-Lys-12, has been experimentally investigated for the amphiphile alone in water and in a mixture with dodecyltrimethylammonium bromide (DTAB). The neat surfactant forms interesting micrometer-sized rigid tubules in the dilute region, resulting in very viscous solutions. For the catanionic mixture with DTAB, various single and multiphase regions were identified (up to a total surfactant concentration of 1.5 wt %) by means of combined polarizing light microscopy, cryo-TEM, and NMR. In the DTAB-rich side, for a mixing molar ratio in the range 2 < DTAB/12-Lys-12 < 4, a region of stable, unilamellar vesicles can be found. Furthermore, it was found that upon addition of 12-Lys-12 to pure DTAB solutions, the mixed micelles grow and beyond a given mixing ratio, vesicles assemble and coexist with small micelles. The transition is not continuous, since there is a narrow mixing range where phase separation occurs. Self-diffusion measurements and cryo-TEM imaging show that the average vesicle radius is on the order of 30-40 nm.
Spontaneous Vesicle Formation of Single Chain and Double Chain Cationic Surfactant Mixtures
The Journal of Physical Chemistry B, 2007
The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; X2 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;or=1, where X2 is the mole fraction of DDAB in the DTAB-DDAB mixture, while vesicle particles were formed directly from monomer solution at 0 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; X2 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.1. Furthermore, the transition from vesicle to micelle was found at 0 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; X2 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.4 at higher concentrations. An addition of DTAB to DDAB solution lowered considerably the DDAB concentration of the vesicle formation, which is attributable to asymmetric distribution of DTAB molecules between inner and outer monolayers of the vesicle bilayer. The shape and size of aggregates were obtained from surface tension, cryo-TEM, and light scattering data.
Lamellar Gels and Spontaneous Vesicles in Catanionic Surfactant Mixtures
Langmuir, 2006
Caillé analysis of the small-angle X-ray line shape of the lamellar phase of 7:3 wt/wt cetyltrimethylammonium tosylate (CTAT)/sodium dodecylbenzene sulfonate (SDBS) bilayers shows that the bending elastic constant is κ) (0.62 (0.09)k B T. From this and previous results, the Gaussian curvature constant is κ j) (-0.9 (0.2)k B T. For 13:7 wt/wt CTAT/SDBS bilayers, the measured bending elasticity decreases with increasing water dilution, in good agreement with predictions based on renormalization theory, giving κ o) 0.28k B T. These results show that surfactant mixing is sufficient to make κ ≈ k B T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles to be equilibrium structures. The measurements of the bending elasticity are confirmed by the transition of the lamellar phase of CTAT/SDBS from a turbid, viscoelastic gel to a translucent fluid as the water fraction is decreased below 40 wt %. Freeze-fracture electron microscopy shows that the gel is characterized by spherulite defects made possible by spontaneous bilayer curvature and low bending elasticity. This lamellar gel phase is common to a number of catanionic surfactant mixtures, suggesting that low bending elasticity and spontaneous curvature are typical of these mixtures that form spontaneous vesicles.
Journal of colloid and interface science, 2007
The vesicle-micelle transition in aqueous mixtures of dioctadecyldimethylammonium and octadecyltrimethylammonium bromide (DODAB and C 18 TAB) cationic surfactants, having respectively double and single chain, was investigated by differential scanning calorimetry DSC), steady state fluorescence, dynamic light scattering (DLS) and surface tension. The experiments performed at constant up to 1.0 mM total surfactant concentration reveal that these homologue surfactants mix together to form either mixed vesicles and/or micelles, or both of these structures in equilibrium, depending on the relative amount of the surfactants. The main transition melting temperature T m of the mixed DODAB-C 18 TAB vesicles is larger than that for the neat DODAB in water owing to the incorporation of C 18 TAB in the vesicle bilayer, however, little amount of C 18 TAB having a minor effect on the T m of DODAB. The surface tension decreases sigmoidally with C 18 TAB concentration and the inflection point lies around x DODAB ≈ 0.4, indicating the onset of micelle formation owing to saturation of DODAB vesicles by C 18 TAB molecules and formation of vesicle structures. At low C 18 TAB concentrations When x DODAB > 0.5 C 18 TAB molecules are mainly solubilized by the vesicles bilayers, while at high C 18 TAB concentrations but when x DODAB < 0.25 micelles are dominant. Fluorescence data of the Nile Red probe incorporated in the system at different surfactant molar fractions indicate the formation of micelle and vesicle structures. These structures have apparent hydrodynamic radius R H of about 180 and 500-800 nm, respectively, as obtained by DLS measurements.
Journal of Colloid and Interface Science, 2008
A transition from micelles to vesicles is reported when salts are added to a catanionic micellar solution composed of sodium dodecylcarboxylate (SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. The counterion binding and increase in aggregate size was monitored by mass spectrometry, rheology and dynamic light scattering measurements, whereas the vesicles were characterized by freeze-fracture and cryo-transmission microscopy experiments. The effect of counterions on the formation of vesicles was studied and compared to a previously studied catanionic system with a sulfate head group, SDS/DTAB. As in the latter case, no anion specificity was found, while large differences in the hydrodynamic radii of the formed objects were observed, when the cation of the added salt was varied. A classification of the cations could be made according to their ability to increase the measured hydrodynamic radii. It is observed that, if the sulfate headgroup of the anionic surfactant is replaced by a carboxylic group, the order of the ions is reversed, i.e. it follows the reversed Hofmeister series. Different morphologies are observed as the ionic strength of the system is increased. The aggregates are analogous to those found in the SDS/DTAB system.
Responsive Vesicles from Dynamic Covalent Surfactants
Angewandte Chemie International Edition, 2011
Herein we describe the use of dynamic covalent surfactants to create dynamic vesicles that are highly responsive to changes in their environment. Synthetic vesicles have drawn much attention because of their close similarity to biological cells, of which the membranes are generally composed of doubletailed phospholipid surfactants. In living systems, bilayer membranes are highly dynamic, amongst others as a result of embedded proteins and carotenoids, which is essential for endo-and exocytosis, cell signaling, and cell division. In contrast, synthetic vesicles tend to be very static, especially when composed of double-chain surfactants. A few specific exceptions exist, including dynamic systems comprised singletailed surfactants, thus forming vesicles with critical aggregation concentrations (CAC) several orders of magnitude higher than those of their double-chain surfactant analogues. The building blocks for these systems are limited to surfactants with relatively short tails, and vesicles are only formed within narrow pH windows. Morphological transitions have been effected using surfactants that are capable of covalent structural modification in an aggregate environment. However, these transitions tend to be unidirectional, and do not allow formation from and reversal to isotropic solution. [7] The low solubility of double-chain surfactants limits spontaneous vesicle formation in water, making sonication, film hydration, and solvent injection techniques necessary to induce vesicle formation. Double-chain surfactant systems that are capable of switching reversibly between a nonaggregated state and an aggregated, vesicular state remain largely unexplored.
Langmuir, 2006
Mixed catanionic surfactant systems based on amino acids were investigated with respect to the formation of liquid crystal dispersions and the stability of the dispersions. The surfactants used were arginine-N-lauroyl amide dihydrochloride (ALA) and N R -lauroyl-arginine-methyl ester hydrochloride (LAM), which are arginine-based cationic surfactants; sodium hydrogenated tallow glutamate (HS), a glutamic-based anionic surfactant; and the anionic surfactants sodium octyl sulfate (SOS) and sodium cetyl sulfate (SCS). It is demonstrated that in certain ranges of composition there is a spontaneous formation of vesicular, cubic, and hexagonal structures. The solutions were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and turbidity measurements. Vesicles formed spontaneously and were found for all systems studied; their size distribution is presented for the systems ALA/SCS/W and ALA/SOS/W; they are all markedly polydisperse. The aging process for the system ALA/SOS/W was monitored both by turbidity and by cryo-TEM imaging; the size distribution profile for the system becomes narrower and the number average radius decreases with time. The presence of dispersed particles with internal cubic structure (cubosomes) and internal hexagonal structure (hexosomes) was documented for the systems containing ALA and HS. The particles formed spontaneously and remained stably dispersed in solution; no stabilizer was required. (Cubosome and hexosome are USPTO registered trademarks of Camurus AB, Sweden.) The spontaneous formation of particles and their stability, together with favorable biological responses, suggests a number of applications.