Vesicle?surfactant interactions: effects of added surfactants on the gel to liquid-crystal transition for two vesicular systems (original) (raw)
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A calorimetric study of the gel-to-liquid crystal transition in catanionic surfactant vesicles
Thermochimica Acta, 2002
Dilute solutions of catanionic vesicles formed by the mixed single-chained (sodium dodecylsulphate, SDS) and doublechained (didodecyldimethylammonium bromide, DDAB) surfactants have been investigated by differential scanning calorimetry. It is for the first time reported a gel-to-liquid crystal phase transition temperature, T m , in this type of mixed vesicles. The SDS-rich vesicles (at X SDS = 0.71) show a concentration-dependent T m in the range 9-16 • C. Addition of salt is seen to have an effect on T m similar to that observed with increasing surfactant concentration, both inducing a decrease in T m. These results differ from those obtained for neat DDAB vesicles. The observed effects in the two types of vesicles are rationalised in terms of headgroup electrostatic interactions which may have influence on the chain packing and phase transition temperature.
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.
Some concepts on design of surfactant gels and vesicles
Bulletin of Materials Science, 1994
It is conjectured that anionic-cationic surfactant combination can be regarded as equivalent to a double chain surfactant and using molecular packing considerations it is shown that vesicles, viscoelastic solutions and liquid crystals can be designed by the proper choice of chain lengths of the pair. Using these concepts new systems are designed, from mixtures of cetyltrimethyl ammonium bromide and sodium alkyl sulfonates, to produce both viscoelastic gels and vesicles.
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 Surfactants and Detergents, 2019
One of the most challenging characteristics not fully understood yet of the cationic surfactant salt dioctadecyldimethylammonium (DODA +) halide is concerned with the effects of the counterion (usually Br − and Cl −) on the surfactant assembly into vesicle structures in aqueous solution. These counterions play a key role in the self-organization of DODA + into bilayer structures. Differential scanning calorimetry (DSC) and dynamic light scattering (DLS) techniques were used here to investigate systematically the effects of the single salts NaCl and NaBr, respectively, on the thermal behavior and structural organization of the cationic dioctadecyldimethylammonium bromide and chloride (DODAB and DODAC) in water. The results undoubtedly indicate that the added Br − or Cl − , respectively, into DODAC and DODAB aqueous dispersions, replaces partially the bound counterions (Cl − and Br −) from the vesicles, yielding formation of DODA-B/DODAC mixed vesicles. As a conclusion, single salts may be used to tune the thermal and structural characteristics of cationic vesicles.
Journal of Colloid and Interface Science, 1996
vided into three different stages. At low concentration the The solubilization of small unilamellar lecithin vesicles by some surfactant merely distributes between the lipid and water different alkyl sulfate surfactants (C 10 SO 0 4 , C 12 SO 0 4 , and phases and partitions into the vesicle membrane without C 14 SO 0 4 ) was investigated by means of light scattering and cryocausing any major alterations in the bilayer architecture. transmission electron microscopy. All surfactants were found to Then, at a critical, and for each system specific, lipid/surfacinduce vesicle growth at subsolubilizing concentrations and a tant molar ratio the bilayer becomes saturated with surfactant transformation into small globular lipid/surfactant mixed micelles and a breakdown of the vesicular structure is initiated. The at high surfactant concentrations. The surfactant chain length, third and final stage is characterized by the complete solubilihowever, was found to have a profound influence both on the zation of the lipid component into lipid/surfactant mixed amount of surfactant needed for solubilization of the lipid bilayer and on the type of structures formed during the vesicle to micelle micelles. transition. For C 10 SO 0 4 a coexistence between vesicles composed
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.
Size and Charge Modulation of Surfactant-Based Vesicles
The Journal of Physical Chemistry B, 2011
Nonstoichimetric mixtures of two oppositely charged surfactants, such as sodium dodecylsulfate and hexadecyltrimethylammonium bromide or tetradecyltrimethylammonium bromide and tetraethylammonium perfluorooctanesulfonate, a fluorinated species, form vesicles in dilute concentration regimes of the corresponding phase diagrams. Vesicles size and charge density are tuned by changing the mole ratio between oppositely charged species, at fixed overall surfactant content. They are also modulated by adding neutral electrolytes, or raising T. In the investigated regions, mixtures made of sodium dodecylsulfate/hexadecyltrimethylammonium bromide show ideality of mixing, the other non ideality and phase separation. The formation of unilamellar vesicles occurs in the sodium dodecylsulfate/hexadecyltrimethylammonium bromide mixture, but not in the other. DLS, viscosity, and electrophoretic mobility quantified the above effects. Surface charge density, surface tension, elasticity, and osmotic pressure concur to the stability of unilamellar vesicles and a balance between the above contributions is demonstrated. The results are relevant for practical applications of vesicles as carriers in biomedicine.