Vesicle DODAB

PS (anionic) or 4 nmol of SA, BisHOP, DOTMA, DOTAP, DODAB, or DC-CHOL (cationic) are also added (molar ratio 4 2 1). For cationic man-nosylated liposomes, 4 pmol of DOGP-4 a Man was included at the molar ratio of 4 2 1 1. Greater amounts of charged lipids can be added depending on the amount of vesicle surface charge required. 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

Single-compartment DHP and DODAB (5) vesicles Size-quantized CdSe particles generated in situ in vesicles... 

Figure 7. An idealized model for CdS sensitized photo reduction of water by PhSH in aqueous DODAC or DODAB vesicles. The exact position of the colloid represented here as generated on the outside surface of the vesicles, is based on fluorescence quenching experiments performed in anionic DHP vesicles, assuming similar inters actions of the CdS particles with both types of vesicles.

Due to the relatively high viscosity of surfactant vesicle and microemulsion systems (refer to data on DODAB and CTAB/50J BuOH in Table X), their use in HPLC will be limited since lower flow rates would be required which would lengthen the required time for a separation. Additionally, most surfactant vesicular (112) as well as some micellar solutions are optically opaque which limits the wavelength range available for spectroscopic detection unless a postcolumn dilution step is employed (219). 

The problem with using surfactant-modified stationary phases in LC is that the surfactant will usually slowly elute (bleed) from the support thus resulting in different retention behavior of solutes with time. This is why most applications are in the area of GC or GLC. An exciting recent advance has been reported by Okahata, et al (181). Namely, a procedure has been developed for immobilizing a stable surfactant vesicle bilayer as the stationary phase in GC. A bilayer polyion complex composed of DODAB vesicles and sodium poly(styrene sulfonate) was deposited on Uniport HP and its properties as a GC stationary phase evaluated. Unlike previous lipid bilayers which exhibited poor physical stability, the DODAB polyion phase was stable. Additionally, the temperature-retention behavior of test solutes exhibited a phase transition inflection point. The work demonstrates that immobilized surfactant vesicle bilayer stationary phases can be employed in GC separations (181). Further work in this direction will likely lead to many such unique gas chromatographic supports and novel separations. 

Figure 4.2 a) The formation of a didodecyldimethylammonium bromide (DODAB) vesicle by sonication of planar bilayers (BLM). 

Similar results were obtained when the zinc porphyrin was bound to the outer vesicle surface and a quinone bolaamphiphile was integrated within a DHP or DODAB vesicle membrane. The quenching constant of the porphy-... 

Finally it should not be omitted to mention the fact that the rigid sphere fullerene, Ceo, can also be dissolved in vesicle membranes . When dissolved in hexane, chloroform or 1,2-dichloroethane, a narrow, concentration-independent absorption band at 334 nm (e 52000) was produced. In vesicles (lecithin, DODAB, DHP), the fullerene adsorption becomes concentration dependent whereby band-broadening, bathochromic shifts (343-360 nm) and loss of extinction (e 10000 4000) were observed in more concentrated solutions. 50 clearly aggregates within the vesicle membranes, a step not observed in micellar solutions. 

The most important transformation of vesicles, however, is their spontaneous growth where forming tubes. Sometimes large DODAB vesicles with a queue (Figure 4.35) can be observed under the light microscope. A membrane which is curved in one dimension is directly connected here to a membrane curved in two dimensions. Thereafter, observations show that the vesicle shrinks and the queue growsthus constituting the most simple and... 

Incorporation of monomers with similar characteristics to the hydrophobic tails of the surfactants involved (typically alkane chains of DODAB and DMPC) tends to suppress phase separation somewhat, and results in either multi-polymer bead aggregates (e.g., necklaces) or parachutes containing an elliptical rather than a spherical latex bead. Copolymerization of butyl methacrylate with ethylene glycol dimethacrylate in DODAB vesicles resulted in polymer necklaces where the polymer beads appear randomly dispersed in the vesicle bilayer [15] in contrast to the polymer shells observed by Hotz and Meier [10] for the same reaction in DODAC vesicles. Similarly, polymerization of octadecylacry-late, another straight-chain monomer, in DODAB vesicles produced parachutes with extremely elHpsoidal polymer beads in contrast to the rather spherical beads observed commonly for the polymerization of aromatic monomers such as styrene in DODAB [12]. Presumably these differences are caused by an increased compatibility between the surfactant bilayer and the monomer chosen. 

Polymerization of divinyl benzene in DODAB vesicles also resulted in phase separation where broccoH parachutes were observed [15]. The latex beads in these cases appear as clusters of fuzzy latex particles. Phase separation appears to be sHghtly suppressed by the cross-linking, but the incompatibiUty of the aromatic monomer with the surfactant bilayer apparently dominates the final morphology as phase separation is obvious. Interestingly, when the alkyl chain surfactant is replaced with an aromatic polymerizable one, phase separation is completely suppressed [16]. With the A1 and A2 surfactants illustrated in Fig. 4, German et al. were able to successfully copolymerize styrene and divinyl benzene. The authors concluded that using a polymerizable amphiphile is a prereq-... 

The BLM vesicles are usually prepared from amphiphiles with two long alkyl chains. This leads to 32 or so CH goups per molecule, which is enough to produce the desired insolubility. Typical examples are natural phospholipids (see Table 2.2.3) and dimethyldioctadecylammonium bromide (DODAB). The inner... 

Figure 17. Fluorescence intensity ratio IJI (Jo = intensity in the absence of DODAB) of anthracene and pyrene (both at 0.02 mM) as a function of the molar fraction of DODAB in aqueous mixtures of DODAC and DODAB vesicles. Surfactant concentration 9.6mM.

FIG. 5 Idealized models for CdS-sensitized photoreduction of water in aqueous vesicle suspensions. The vesicles are from (a) DHP (b) DODAC or DODAB (c) polymerized n-Ci5H3iC02(CH2)2N+(CH3)[CH2QH4CH = CH2]Cr (d) DODAC and thiol-functionalized surfactant. (From Refs. 4 and 11-13.)... 

Akinetic study was also performed in a variety of vesicular solutions (DDAB, DODAB, DODAC [NaOH] = 2.25mM, 25 °C). Interestingly, the vesicles possess stronger catalytic reaction environments than the micelles. The rate-determining proton transfer from carbon to the hydroxide ion was accelerated up to 850 fold in di- -dodecyldimethylam-monium bromide (DDAB) vesicles. This is evidence that the reaction sides are less aqueous than those in micelles, as anticipated. Application of the pseudophase model afforded the bimolecular rate constants in the vesicles (kves). For the different vesicles, ves is significantly higher (ca. 12 times for DODAB) than the second-order rate constant in water. This shows that the catalysis is due to both a medium effect and a concentration effect. It was assumed that there was a fast equilibrium for substrate binding to the inner and outer leaflets of the bilayer, in accord with the fact that no two-phase kinetics were found. 

Vesicle catalysis followed the order DDAB > DODAC > DODAB, with values of about 850, 550, and 160, respectively. This looked an unexpected result because the rate constants did not respond in the usual way to an increasing chain length of the tails. But it was recognized that the high catalytic efficiency of DDAB with the shortest alkyl chains was the result of vesicles being in the liquid-crystalline state at the reaction... 

Fig. 12 AFM contact mode image showing a typical assembly of DODAB bilayer patches (a). The patches were self-assembled from a vesicle suspension and imaged in pure water. To the right, a series of approaching force curves are shown that were taken along a line across one of the patches in 5 mM LiN03(b). The different electrostatic interaction - repulsive on the mica substrate at the beginning and end of the line and attractive on the DODAB patch - is visible. (The figure is reproduced with kind permission from M. Radmacher [204].)...

Figure 3.6. Cryo-lransmission electron micrograph of a vesicle population obtained after extrusion at 60°C of 10 mM DODAB the scale bar corresponds to 100 nm (from Martin Jung, Ph.D. Thesis, Technische Universiteit Eindhoven, The Netherlands, 2000)...

Fig. 5 Cryo-TEM micrographs of crosslinked nanocapsules. Left. Nanocapsules obtained by encapsulating DODAB vesicles (obtained by extrusion through 100 nm pore size filters) using RAFT copolymer PlBAg-co-AAg) and MMA t-BA EGDMA monomer at molar feed ratios of (a) 2 1 0.14, (b) 1 2 0.1, (c) 1 2 0.16, and (d) 1 2 0.33. Right. Crosslinked P(MMA-co-AA) nanocapsules obtained after acid hydrolysis of the same four crosslinked P(MMA-co-t-BA) nanocapsules shown on the left. Sample vitrification was performed at pH 7.2 in liquid ethane. Reproduced from [52] with permission from the Royal Society of Chemistry...

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