Micelle–Vesicle Transition of Fatty Acid Based Ion-Pair Surfactants: Interfacial Evidence and Influence of the Ammonium Counterion Structure (original) (raw)
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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.
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.
Thermoreversible Vesicle-to-Micelle Transitions in Surfactant-Salt Mixtures
2006
Mixtures of the cationic surfactant, CTAB and the organic compound, 5-methyl salicylic acid (5mS) spontaneously self-assemble into unilamellar vesicles at room temperature. Upon heating, these vesicles undergo a thermoreversible transition to wormlike micelles. This phase transition results in a 1000-fold increase in the solution viscosity with increasing temperature. Small-angle neutron scattering (SANS) measurements show that the phase transition from vesicles
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.
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
The Journal of Physical Chemistry B, 2007
The self-organization of a single-tailed amino acid based chiral surfactant sodium N-(4-n-octyloxybenzoyl)-L-valinate (SOBV) has been studied in water. A number of techniques like surface tension, fluorescence probe, dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) have been utilized for characterization of the self-assemblies. The amphiphile forms large spherical vesicles of 400-600 nm diameters in dilute aqueous solution. However, the vesicles get transformed into spherical micelles with increase of surfactant concentration or upon addition of relatively low amount (20 mM) of NaCl or KCl. This is the first example of salt-induced vesicle to micelle transition (VMT) in a single surfactant system. The vesicles are stable in the temperature range of 30-70°C. Cleavage of intermolecular hydrogen bonds among the amide groups in the presence of salt appears to be the plausible cause for the VMT.
The curious world of hydroxide surfactants. Spontaneous vesicles and anomalous micelles
The Journal of Physical Chemistry
Double-chained cationic surfactants typified by dodecyldimethylammonium bromide are insoluble in water, forming lamellar liquid crystal phases. They form vesicles only on prolonged sonication. If the halide ion is replaced by a hydroxide, the resulting surfactants are highly soluble and form spontaneously a clear solution which appears to comprise a mixture of small micelles and fairly monodisperse vesicles. The distribution of particle size changes with added base or with partial titration with acid (HBr, HC1, HF) which can sometimes yield vesicles with an initially unsymmetric distribution of anions. Evidence for these structures from quasi-elastic light scattering (QELS) and viscosity measurements and an account of their extraordinary properties are presented.
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.
2000
The microstructures in mixtures of two dimeric (gemini) surfactants, the dimethylene-1,2-and eicosamethylene-1,20-bis(dimethyldodecylammonium bromide), referred to as 12-2-12 and 12-20-12, have been investigated at 25°C by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmission electron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 forms vesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentration then rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlike micelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % and progressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity data clearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was strongly adsorbed by the vesicles. Spectrophotometry and DLM showed that, under the conditions used, the vesicles were nearly eliminated at above 0.7 wt % 12-2-12. The cryo-TEM observations were performed on samples vitrified two months after preparing the mixtures. The progressive increase of the 12-2-12 content in the mixture resulted first in vesicle growth (0.1 wt %), followed by vesicle breakage into smaller vesicles (0.26 wt %), the formation of disklike micelles (0.4-0.75 wt %), then of ring-like micelles and short elongated threadlike micelles (1 wt %), the growth of those threads (1.5 wt %), and finally the formation of a network (2 wt %), where threads and rings were interconnected. The network contained also a few isolated rings. The same structures were observed when the mixtures for cryo-TEM observation were vitrified 5-7 days or two months after mixture preparation, for the mixtures containing 0.5 wt % or less, and 1 wt % or more 12-2-12. Aging of the systems was observed for mixtures containing 0.65-0.75 wt % 12-2-12. When examined 5-7 days after preparation the mixtures showed long rigid rodlike micelles and irregular ribbons. Those structures disappeared when the mixtures were allowed to equilibrate for two months. The results are discussed and a model is presented to explain the observed behavior.