Onion phase

El-Rassy, H., Belamie, E., Livage, J. and Coradin, T. (2005) Onion phases as biomimetic confined media for silica nanoparticle growth. Langmuir, 21, 8584-8587. 

Linden, E. van der, Hogervorst, W.T., and Lekkerkerker, H.N.W. (1996). Relation between the size of lamellar droplets in onion phases and their effective surfaee tension. Langmuir, 12,3127-3130. 

The nomenclature used to describe vesicular structures is sometimes variable, but the following definitions are used here. The term vesicle is restricted to the unilamellar vesicle (ULV) architecture depicted in Fig. la. Liposomes are vesicles formed from biologic surfactants such as lecithin and cholesterol, and were first studied by Bangham et al. over 35 years ago [2]. MultilameUar vesicles (MLVs or onion phases) consist of multiple surfactant bilayers forming concentric shells around an aqueous core (Fig. lb). MLVs are generally much larger than ULVs, and can be as large as several microns. 

Fig. 3 I rceze-fraclLire TEM of the onion phase in the system tetradecyllrimethylaminoniurn bromide-tetradocyldiinethyl-araincoxidc-hexanol, showing largo mullilamellar vesicles (bar=l rm). Kcproduced Ifom Ref, fl8] with permission cf Aeadcmic Press.

LMV) is encountered in the so-called onion phase, where each LMV includes tens to himdreds of concentric vesicles and where the LMVs arc in contact, as seen in Fig. The onion phase is used in slow-release for-... 

Despite most of the myelin studies having focused on growth and late timescale behavior the mechanism for their formation is still unknown. Penetration scans using onion phase (lamellar phase is presheared into multilamellar vesicles or onions) have shown that myelin formation can be suppressed (48), This implies that there formation is sensitive to bilayer organization in the lamellar phase. Dissolution of these onion phases also have interesting and exotic behavior (48, 49),... 

It is easy to see that the size of the onions decreases dramatically if the innermost shell is not filled, which amounts to dropping the first term in the series (12). To generalize the theory, one has to takep to be variable and express the total bending energy in terms of the radii of the outermost and innermost onion skins. We refrain from this exercise in view of many other uncertainties. For instance, the nonspherical shapes of the outermost skins, because of their additional bending energies, should cause the phase transition from the onion phase to the planar multilayer system to be discontinuous. 

Figure 1.9 Freeze-fracture electron micrograph showing the onion phase of vesicles present in the tetradecyldimethylamineox-ide/hexanol/water system (bar = 1 pm). Reproduced from reference 58 with permission of Academic Press/Elsevier.

The final section of the volume contains three complementary review articles on carbon nanoparticles. The first by Y. Saito reviews the state of knowledge about carbon cages encapsulating metal and carbide phases. The structure of onion-like graphite particles, the spherical analog of the cylindrical carbon nanotubes, is reviewed by D. Ugarte, the dominant researcher in this area. The volume concludes with a review of metal-coated fullerenes by T. P. Martin and co-workers, who pioneered studies on this topic. 

The diuretic and cholagog effect of Ononis arvensis motivated the development of a new two-dimensional paper chromatographic and TLC method for the measurement of onion in its roots and aerial parts. Samples were dried, ground and extracted with methanol (70 per cent, w/v) for 2 h. The supernatant was diluted and used for TLC separation on a cellulose stationary phase. The first eluent was 3 per cent formic acid, and the second was n-butanol-acetic acid-water (4 1 5, v/v) for both TLC and paper chromatography. Spots were scraped off, extracted with methanol and the absorption was measued at 260 nm. It was found that the average ononin content in roots and aerial parts was 0.153 0.0278 (per cent) and 0.498 0.045 (per cent), respectively. Because of the simplicity, the method was... 

Fig. 2.55. Gradient reversed-phase HPLC analysis of flavonoids in white onions (a) and celery (b). ODS column of 150 X 3.9mm i.d particle size 5pm. Mobile phase 20min gradient of 15-35 per cent acetonitrile in water adjusted to pH 2.5 with TFA. Fowrate lml/min. Upper and lower traces represent samples before and after hydrolysis, respectively. Detection wavelength 365 nm. IS = internal standard Qc = quercetin Ap = apigenin Lt = luteolin. Reprinted with permission from A. Crozier et al. [159],...

Solid-phase borate complexation coupled with RP-HPLC has been employed for the measurement of polyhydroxyflavones in human blood plasma, vegetables and redwine. The chemical structures of polyhydroxyflavones included in the investigation are shown in Fig. 2.87. Vegetables were homogenized, centrifuged and the supernatant was applied for analysis. Human plasma was heparinized before analysis. The outer skins of onion were... 

The separation and identification of two novel anthocyanins from red onion, Allium cepa, have also been reported. Scales of 2.37 kg red onion were cut and extracted twice with methanol containing 0.5 per cent TFA. The extract was filtered, extracted with ethyl acetate and the aqueous phase was further purified in an ion-exchange and an SEC column. SEC separation was performed in a Sephadex LH-20 column (100 X 5 cm i.d.) applying methanol-water-TFA (39.6 60 0.4, v/v) as the mobile phase at a flow rate of 2.5 ml/min. 

Liposomes can be created by shaking or sonicating phospholipids in water. Low shear rates create multilamellar liposomes, which have many layers like an onion. Continued high-shear sonication tends to form smaller unilamellar liposomes. In this technique, the liposome contents are the same as the contents of the aqueous phase. Sonication is generally considered a gross method of preparation, and newer methods such as extrusion are employed to produce materials for human use. 

Nested fullerene-type structures with other inorganic layered materials have been investigated recently. Tenne et al (1992) have shown that MoSj is formed in a nested (onion-like) structure when MoOj is reduced by HjS in the gas phase at 1070-1220K (Feldman et al, 1995). Similar results have been obtained with WSj. The idea seems to be more general, being applicable to other inorganic layered structures, as... 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

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