Micelles hemimicelles

To conclude, we should also point out that condensed phases containing the organic compound of interest can be formed at the particle surfaces. Such phenomena occur in cases involving ionic organic compounds that form micelles/hemimicelles... 

Key words Insoluble surfactants -adsorption at electrodes - Lang-muir-Blodgett films - micelles -hemimicelles... 

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

The adsorption isotherm obtained for dodecylsulfonate (CDS) on alumina is given in Figure 2. This isotherm is similar to that obtained in the past for sulfonate/alumina systems (4). This isotherm behaves in an s-shaped manner (6) revealing its four characteristic regions of adsorption 1) Electrostatic interaction 2) Lateral association (hemimicellization), 3) Electrostatic hindrance and 4) micellization. 

Keywords Fluorescence probing Hemimicelle Micellar fluidity Micelle Organized assemblies in solution and interfaces Polarity parameter Pyrene... 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

These fluorescent probes have been successful in reporting the structural parameters of surfactant assemblies such as micelles [103], reverse micelles [104], ternary systems [105], swollen micelles [106], microemulsion [107], vesicles [108], liposomes [109], hemimicelles [110], monolayers [111] and bilayers [111]. 

Fig. 36 Schematic diagram of surfactant-adsorption on silica surface as monomer, hemimicelle, and at surfactant concentration above CMC where pyrene is bound to micelle...

Surface aggregates formed by ionic surfactant adsorption on oppositely charge surfaces have been shown to be bi layered structures (1.) and are called admicelles<2) in this paper, though they are sometimes referred to as hemimicelles. The concentration at which admicelles first form on the most energetic surface patch is called the Critical Admicellar Concentration (CAC) in analogy to the Critical Micelle Concentration (CMC), where micelles are first formed. Again, in much of the literature, the CAC is referred to as the Hemimicellar Concentration (HMC). 

Region 2 is characterized by a marked change in the slope of the adsorption isotherms. This results from the onset of association of the hydrocarbon chains of the surfactant ions adsorbed in the Stem plane. The mean separation distance of adsorbed ions under these conditions is about 70 A., which approximates the mean separation distance in bulk at the c.m.c. In such adsorption phenomena, there is a relationship between this asociation and the formation of micelles in bulk solution. For example, electrokinetic studies (1) on quartz at neutral pH showed that alkylammonium ions associate in the Stem plane when their bulk concentration is approximately one hundredth of the c.m.c. This association which has been called hemimicelle formation (3), gives rise to a specific adsorption potential which causes the adsorption to increase markedly and brings about a reversal in the sign of the potential at the Stem plane. The hemimicelle concentration, that is the bulk concentration necessary... 

Although the partitioning of HOCs to surfactant micelles has been well studied, HOC partitioning to sorbed surfactants (e.g., hemimicelles, admicelles) has received much less attention. Holsen et al. (1991) examined the sorption of several HOCs on ferrihydrite coated with sodium dodecyl sulfate (SDS) and found that the HOCs with the lowest water solubility... 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

Surface micelles of fluorinated amphiphiles have potential as templates for the elaboration of metallic nanodot arrays. Hexagonal arrays of gold and silver nanodots, a few nM in size, have been observed by AFM to form when a thin layer of metal was deposited through vacuum evaporation on a silicon wafer covered with hemimicelles of the F8H16 diblock (E. Charrault, P. Muller, M. Maaloum, M.P. Krafft, C. Petit, P. Petit, unpublished). 

LB monolayers and Y-type bilayers lead us to bilayers, hemimicelles, and micelles (Fig. 4.12). A bilayer of phospholipid amphiphiles forms the cell wall, which surrounds each living cell (prokaryotic and eukaryotic) the ionic outer layers contact the bulk solution (blood, serum, etc.), while the... 

Several papers reported on the spatial determination of photoactive molecule in organized assemblies (76-79), and Pallavicini and coworkers reviewed on the use of luminescence as probe in self-assembly of multicomponent fluorescent sensors (80). Also, luminescence quenching studies on [Rufbpyls] in sodiiun dodecyl sulfate (SDS) micelles and hemimicelles by using a variety of quenchers were reported by Tiuro and coworkers (81) and then reviewed by De Schryver and coworkers (82). 

The first pathway is the formation of mixed micelles or hemimicelles, composed of polymer-bound hydrophobes comicellized with surfactant molecules. Intermolecular physical cross-links often enhance the viscosity of the micellar solutions. The second pathway is intramolecular comicellization so that the hydrodynamic size of the associates contracts. 

A more plausible explanation for anionic surfactant adsorption on silica gel is found in the presence of about 0.2 wt.% alumina in the silica gel. The alumina pz-c is about pH=8.0, so it would be positively charged at the pH of the experiments. If the alumina is uniformly distributed through the silica, all of the adsorption could be accounted for provided a close-packed mono-layer of surfactant is formed on the alumina. This circumstance would also be consistent with the shape of the toe of the isotherm. Gaudin and Fuerstenau (25) advanced the idea of hemimicelle formation (two-dimensional micelles on a surface) to account for similar observations in flotation processes. 

Fig. 4.24. ESR spectra of 16-doxyl stearic acid in solloids (hemimicelles), micelles and ethanol-glycerol mixtures and comparison of corresponding rotational correlation times and viscosities.

Hemimicelle An aggregate of adsorbed surfactant molecules that may form beyond monolayer coverage, the enhanced adsorption being due to hydrophobic interactions between surfactant tails. Hemimicelles (halfmicelles) have been considered to have the form of surface aggregates or of a second adsorption layer with reversed orientation, somewhat like a bimolecular film. In bilayer surfactant adsorption, the term ad-micelles has also been used (20). 

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