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...

Vesicles of a New Salt‐Free Cat‐anionic Fluoro/Hydrocarbon Surfactant System

“…In particular, self-assembled structures, such as disk-like micelles and regular hollow icosahedral aggregates, in true salt-free aqueous cat-anionic surfactant solutions have been observed by freeze-fracture transmission electron microscopy (FF-TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). [6][7][8] Focusing on vesicle formation, we have developed three ways to prepare a charged vesicle phase that is not shielded (salt-free cat-anionic surfactant systems): 1) By adding small amounts of ionic surfactants to the L 3 phase of nonionic surfactants and cosurfactants [9] (it is theoretically argued that the vesicle phase is formed in these systems by the influence of charge density on the Gaussian bending constant [10] ); 2) by mixing a cationic surfactant with OH À as the counterion and an anionic one with H + as the counterion, but with one of the two components in small excess; [6][7][8]11] and 3) by metal-ligand coordination, that is, by mixing an anionic surfactant with M 2 + (M 2 + = Zn 2 + , Ca 2 + , Ba 2 + , etc.) as the counterion and coordinated surfactants to form a charged vesicle phase without any cosurfactant and with the charges not shielded because the solutions are salt-free.…”

Salt-free vesicle-phases and their template effect

“…In this case, the zwitterionic surfactant containing N→O group and the strong acidic surfactant containing -SO 3 H group, in which the two groups combine to form catanionic surfactant mixtures [18] . Recently, a uni-and multilamellar high viscoelastic onion-phase was obstained in TTAOH/Oleic acid system in which interlamellar spacing between the bilayers of onions is about 35 nm (Figure 1), suggesting rather compact packing of the bilayers [19] .…”

“…As a result, the vesicles obtained by this method often have low yield values and precipitates are usually produced without vesicle formed when the two surfactants mixed at equal mole. On the contrary, there are no precipitates forming in salt-free surfactant systems and some novel aggregates can be obtained in the absence of the excess salts and the shield induced by the salts [12][13][14][15][16][17][18][19] . Hence, our group also performed some studies on salt-free systems, and got some interesting results.…”

Metal-ligand coordinated Ca(DS)2/C14DMAO/H2O system: Phase behavior and rheological property

“…However, precipitates usually form near equimolar mixed ratios of cationic and anionic surfactants because of the screening of salts on the charged aggregates. In recent years, salt-free surfactant systems have come to the front because electrostatic interactions between the aggregates are not screened and very rich phase behaviors can be observed [4][5][6][7][8][9][10][11][12][13][14][15][16]. To obtain salt-free systems, two routes are often employed.…”

“…To obtain salt-free systems, two routes are often employed. One is to mix alkyltrimethylammonium hydroxide (C n TAOH) or alkyldimethylamine oxide (C n DMAO) with acids having long hydrophobic tails [4][5][6][7][8][9][10][11][12]. The other is to construct metalligand coordinated systems by mixing multivalent metal surfactants with C n DMAO [13][14][15][16].…”