CTAB bilayer

The resulting GNRs are coated with a CTAB bilayer that prevents aggregation and facilitates the formation of relatively stable nanorod suspensions in aqueous solution. Smith et al. recently showed that very small amounts of iodide present as an impurity in CTAB samples purchased from different chemical suppliers have a dramatic influence on nanorod growth. In some cases, nanorods did not... 

Positively charged CTAB bilayer stabilizes the nanorods... 

The kinetic behavior of these systems is consistent with the supposition that substrate and/or catalyst molecules are freely moving around among the micelles and the bilayer vesicles much faster than the rate of reaction. However, Kunitake and Sakamoto (1978) showed that the rate of the intravesicle reaction was much faster than that of the intervesicle reaction, when p-nitrophenyl palmitate was used as substrate. Table 6 compares the rates of the intra- and intervesicle reactions in 2C12N+2C, bilayer and in CTAB micelles. A large rate difference (> 200-fold) was found in the bilayer system for the combination of cholest-Im and p-nitrophenyl palmitate. Slow transfer among vesicles of tightly bound p-nitrophenyl palmitate causes the rate difference. 

Figure 11.1 Molecular structures and schematic illustrations of micelle and bilayer membranes (a) micelle, (b) bilayer membrane and vesicle, (c) CTAB, and (d) 2CnN. ...

The basic mode of mesophase formation is as described above for CTAB. However, as one might expect, things are not quite so straightforward and there are various types of mesophase which can be formed from various types of surfactant. The surfactant structure can be varied so that fluorocarbon chains can be employed in place of the hydrocarbon variety, while anionic (e. g. -SOa") and the neutral (e.g. -(0CH2CH2) -0H) polar headgroups are often used. These surfactants can then form a variety of different mesophases as a function of (mainly) concentration in the solvent of choice (normally water). These phases are the lamellar (L ) phase, a simple bilayer phase, and variations on the cubic (li, I2, Vi, V2) and hexagonal (H, H2) phases. For these last phases, the subscript 1 implied a normal phase as found in a water-rich system, while the subscript 2 implied a reversed phase as found in an oil-rich system. For the cubic phases, the letter F implied a micellar phase (e. g. 

DSC studies have also been undertaken to investigate the effect of iontophoretic transport of the P-blocker, propranolol hydrochloride on the lipid bilayers of the stratum comeum, pre-treated by two penetration enhancers, sodium lauryl sulfate (NaLS) and hexadecyl trimethylammonium bromide (CTAB). DSC results indicated that NaLS was included into the intercellular lipids during iontophoresis and also showed a slight decrease in the two higher transitions in the SC. Lowering of these lipid endotherms is an indication that the bilayer has increased in fluidity. In the case of CTAB, DSC revealed an increase in the initial skin transition, attributed to the insertion of CTAB into the lipids responsible for this transition. However, pre-treatment of the SC with NaLS proved to be successful in enhancing the flux of propranolol hydrochloride across the SC, while this was not the case for CTAB [17]. 

The rod- or wire-like morphology is common in some other systems, though the reasons of their formation may not be the same as in the case of copper described above. Synthesis of specifically nanorods of CdS and CdSe has been reported by Chen et al. [251] from a CTAB / cyclohexane / water system. The amount of cyclohexane seemed to control the morphology. Another possible factor was the degree of sonication that might have led to the formation of bilayer vesicles. 

A surfactant packing parameter between 3 and 1 is likely to lead to bilayer structures [22,23], These tend to be either lamellar bilayers or vesicles, which are closed spheroidal packages of surfactant [5], A n/uolc between 3 and 1 is found in double-chain surfactants such as dialkyl-dimethylammonium salts or phosphatidylcholines of chain length 12-20 (Fig. 2). Such surfactants are insoluble in water and form bilayers when cast as films. Common single-chain surfactants such as sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) have n/ao/c < i, are soluble in water, and form micelles. 

At this point it is useful to discuss the general aggregative property of dialkylammonium bilayers in comparison with other ammonium aggregates. As shown in Table II, single-chain ammonium amphi-philes such as CTAB(conventional surfactant) form globular, fluid micelles. The double-chain ammonium amphiphiles can produce bilayers... 

More recent studies of amine adsorption mechanisms at surfaces have been carried out using atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and SFVS techniques (Bakshi et al. 2004 Castro et al. 1986 C hemyshova et al. 2000 Fuerstenau and Renhe 2004 Paruchuri et al. 2004 Schrodle and Richmond 2008 Subramanian and Ducker 2000 Velegol et al. 2000). AFM studies show that spherical micelles of cetyltrimethylammonium bromide/cetyltrimethylammonium chloride (CTAB/CTAC) surfactants form at silica surfaces near the critical micelle concentration (CMC) (Bakshi et al. 2004). Compared with tertiary and quaternary amines, primary amines, such as DDA, would have a different molecular structure at the silica surface. For example, AFM studies suggest that primary amine forms a featureless bilayer at mica surfaces. 

The effect of added alcohols upon the microviscosity of micelles was studied by Turro et al. [76]. The low-molecular alcohols reduce the microviscosity of CTAB micelles. The high-molecular alcohols do not influence the microviscosity. Schinitzky et al. observed a phase transition of a bilayer membrane by the fluorescence depolarization technique [75]. 

Riemersma [192] suggests that phosphate groups belonging to phosphoino-sitides, phosphatidic acid and other anionic lipids were involved in the ionic interaction of the surfactant head groups while the alkyl chain actually penetrated the membrane bilayer. At a certain concentration the membrane would form mixed micelles with the surfactant cations leading to higher permeability and cytolysis. However, both anionic and cationic surfactants induce lysis and their mode of action cannot be identical. Bradford et al. [194] examining the solubilization of microsomal constituents observed that both CTAB and... 

CTAB forms a bilayer structure around silver nanorods (formed in the previous step) during the growth of nanorods [302], El-Sayed and coworkers provided two reasonable explanations for the formation of faceted and rod-shaped particles first, the growth rates vary at different planes of the particles and second, particle growth completes with the capping action of stabilizers [303]. 

CTAB micelles + SOS monomers Mixed micelles (in less than 1 ms) Mixed micelles Floppy bilayer aggregates (in s to minutes)... 

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