Vesicles onion phase

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),... 

Fascinating kinetics is observed when the systems are driven out of equilibrium. Typically, this is accomplished by mechanical treatment, by a sudden change of composition, or by a temperature jump. The most intensively studied mechanically induced transition is the formation of multilamellar vesicles ( onions ), which occur on shearing of a lamellar phase [32-36]. A sudden change of composition may lead to the formation of myelins [36-39]. Mechanical pinch-... 

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.

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... 

Lamellar phases are known to exist in at least two configurations. In addition to a planar or continuous lamellar phase, where the bilayers are ordered in sheets [113], lamellar phases also exist with the bilayers ordered in closed concentric shells [114-116]. The structural units in the latter phase are often referred to as onions , multilamellar vesicles (Fig. 

In dispersions stabilized on the basis of phospholipids, an additional fraction of very unstable particles was detected in the cryo-preparation. These particles possibly have a spherical shape as particles with an onion-like structure were detected in the freeze-fracture preparations. Due to the layered structure of the smectic phase, a cylindrical particle shape should be energetically more favorable than a spherical one. This may also be the reason for the higher sensitivity of spherical particles towards the electron beam. In formulations with polymeric stabilizers (e.g. poloxamer, polyvinyl alcohol and Tween 80), such highly unstable particles have not been detected yet. Polymer-stabilized smectic nanoparticles possess a more round, paving-stone-like particle shape. In dependence on the stabilizer system, additional colloidal structures (e.g. vesicles and micelles) formed by the excess of emulsifiers could be detected as well (Fig. 10.11). 

The next molecular organisation is more complex. This involves many layers of alternating molecules, intercalated with layers of water—i.e. lamellar phase. It is possible for rafts of these layers to float around in water, but for most real products, they present themselves as fuUy enclosed multi-layered onion-like droplets. The number of layers can nm into hundreds, and these so-called lamellar drops (or vesicles) can be up to several microns in diameter. 

Subsequently, Hoffmann, Meyer and coworkers identified spontaneous vesicles in a ternary system containing dodecyldimethylaminoxide as surfactant and hexanol as cosurfactant [21]. Increasing the cosurfactant concentration at fixed surfactant content, they found a continuous range of lamellar phases. Small unilamellar vesicles were followed by onions, a mixture of onions and extended bilayers, and finally extended bilayers only. The phases... 

This ascertains that the combustion of the surfactant mainly occurs at 285°C. Below this temperature, the silver nanopaiticles are not exposed to the reactive gas phase because the surfactant still covers them. Actually, Ag Ws proved to be active only from 350°C [4]. Importantly, a fraction of the surfactant is only removed at higher temperature, since a peak is also observed around 415°C. So further cleaning of the surface at higher temperature could likely lead to higher accessibility of the NPs and higher activity. Conversely, it was noticed that the activity of the NPs at 350°C decreased after that the catalyst was brought to 400°C [4]. Sintering is a plausible explanation to this observatiom The control on the size and the shape of the particles -that is one of the main interest of the synthesis via onion vesicles [5] - would then be lost. 

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