An Onion Phase in Salt‐Free Zero‐Charged Catanionic Surfactant Solutions

Abstract

Mixtures of cationic and anionic (catanionic) single-chain surfactants can readily form bilayers in aqueous solutions, [1] in which uni-and multilamellar onion phases (the so-called vesicle phase) are often observed to be in equilibrium. [2] Since vesicles represent simple model systems for biological membranes and have practical applications (for example, for controlled drug or DNA release), [3] investigations of vesicle phases are of considerable interest in different areas, including surfactants, materials, and life sciences. Recently, two new self-assembled structures of controlled size (nanodisks and regular hollow icosahedra) were observed in dilute catanionic surfactants with H + and OH À counterions by Zemb and coworkers. [4][5][6] Such so-called "true" catanionic systems, with a nonswelling but finite uptake of water, and with a spacing of the same order as described in the current study were studied and documented by Jokela et al. [7] It was also later established by Rand, Parsegian, and Leiken [8] that the lamellar phase at maximum swelling of salt-free catanionic systems with a zero osmotic pressure, that is, the repulsive hydration interaction is compensated by van der Waals force at that point. The molar ratio r of the anionic to cationic components controls the structural surface charge and, hence, controls the long-range repulsive interaction independently of the weight volume fraction (f), which in turn controls the average colloidcolloid distance. The salt-free catanionic systems can be represented in a ternary phase diagram whose two independent variables are f and r.[6]Herein we report, for the first time to our knowledge, the discovery of a "true", salt-free concentrated catanionic uniand multilamellar onion phase that differs from the catanionic surfactant systems with excess salt that are formed by the combination of the counterions, as evident from our freezefracture transmission electron microscopy (FF-TEM) observations and small-angle X-ray scattering (SAXS) measurements. This molecular catanionic couple comprises the longest hydrocarbon chains described to date, so it was essential to determine if the carbon chains were in a frozen (gel) or liquid state. The size of the unilamellar vesicles ranges from about 20 to 700 nm and that of the large onions are several micrometers. The interlamellar spacing between the bilayers of onions is about 35 nm, thus suggesting rather compact packing of the bilayers. The high osmotic pressure sustains the highly stable colloidal suspension of the catanionic onion phase. The observations of the onion phase may prove valuable and stimulating to fellow specialists, not least as "true" catanionic surfactant systems do not seem to be exhaustively investigated yet.The "true" salt-free catanionic vesicle phase was obtained by mixing aqueous solutions of trimethyltetradecylammonium hydroxide (TTAOH) and oleic acid (OA; see Figure 3). The stock solution of TTAOH (pH 12-13) was prepared from the commercial bromide form (TTABr) by anion exchange with a strong b...

Sponge Phase Producing Porous CeO2 for Catalytic Oxidation of CO

“…In previous studies, all samples were found to exhibit shear-thinning behavior over the studied shear-rate range, which was typical for L a -phase solutions. [27,32,33] When c DEHPA was within the range 300-320 mmol L À1 , we observed partial shear-thickening behavior within the shear-rate range of 50-100 s À1 , for which the planar lamellar structure was confirmed by the above FF-TEM images. It was previously reported that vesicles could be prepared from the classical planar lamellar phase through shearing, with an improvement of viscosity.…”

“…Freeze‐fracture (FF) TEM is a powerful tool that can provide direct information on self‐assembled surfactant structures in their hydrated state 27. The microstructure transition in the lamellar phase at c L ‐Arg =300 mmol L −1 with different c DEHPA values is shown in Figure 5.…”

“…The rheological behavior of the two samples was usually found in gels and solutions of densely packed vesicles, as has been reported for many systems. [27,40] With continuously increasing c DEHPA , G' and G'' decreased gradually to less than 10 Pa and showed a weak frequency dependence. When c DEHPA increased to 320 mmol L À1 , G' was lower than G'' at low frequency and higher than G'' at high frequency, with planar-lamellar phase behavior.…”

“…Freeze-fracture (FF) TEM is a powerful tool that can provide direct information on self-assembled surfactant structures in their hydrated state. [27] The microstructure transition in the lamellar phase at c l-Arg = 300 mmol L À1 with different c DEHPA values is shown in Figure 5. When c DEHPA was 180 mmol L À1 , polydisperse vesicles with diameters ranging from 100 to 500 nm were found (Figure 5 a).…”

One-step procedure for the preparation of functional polysaccharide/fatty acid multilayered coatings

“…It is worth conducting further studies, with the aim of exploring the potential of these responsive, compartmentalized membranes as functional coatings for food preservation, hierarchical nanoreactors, and transport membranes and to extend the deposition process to different substrates. The method presented in this work is not restricted to polymer/surfactant mixtures, and we expect that similar multilayered coatings can be prepared applying an analogous rapid, single-step procedure to a large variety of spontaneously assembled, hybrid, layered systems in bulk solution, such as: catanionic 58,59 , surfactant/polyoxometalates 60 , fullerene/polyoxometalates 61 , and DNA/lipid multilayer 62,63 vesicles.…”