Hemi-micelles

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

These effects suggest the role of hemi-micellization as well as the enhanced solvent power of the interfacial region for the longer chains ... 

The surfactants associate at the oxide surface to form hemi-micelles with their hydrophobic groups exposed to the aqueous phase at low concentrations, but at higher concentrations, with the hydrophilic groups turned outwards. Hematite coated with various proteins (ovalbumin, y-globulin, lysozyme) adopted either the iep of the proteins or a value between that of the oxide and the protein and displayed modified coagulation behaviour (Johnson and Matijevic, 1992). 

The surfactant sodium dodecyl sulphonate (Ci2H2sS03Na) and its sulphate adsorb electrostatically on hematite at low solute concentrations (Han et al., 1973). Hydro-phobic effects operate at high concentrations due to the incompatibility of the hydrocarbon part of the molecule with water. This involves condensation of the alkyl chains at the surface (hemi-micelle interactions), which lowers the free energy of the system and reverses the surface charge. 

At the concentration (CHm) when hemi-micelles start to form it is possible to use the Stern-Graham equation for calculation of the energy

group from the water phase to the hemi-micelle102) ... 

Hemi-micelles are extended half-cylindrical objects similar to micelles, except that the amphiphiles are aggregated only in a semicircle, instead of a full circle, and the hemi-micelles must rest on a solid support. 

At higher ionic surfactant concentrations. Van der Waals interaction between hydrocarbon chains and hydrophobic bonding results in aggregation, forming clusters called hemi-micelles (12.17-21). The aggregation numbers of hemi-micelles are not well established estimates range from 2 to -250 (15.22.23). Surface charge is reduced more rapidly than at lower solution concentrations and is ultimately reversed as solution concentration and adsorption increase so the adsorption bond includes a chemical or specific contribution. 

The composition of the hemi-micelle in Reaction 3 was determined by optimizing the fit to the adsorption density at high concentration and high pH, recognizing an experimentally observed reversal in zeta potential (lj0, and the high-density packing of molecules in hemi-micelles observed by Waterman et al. (34). 

The relationship between adsorption and interfacial properties such as contact angle, zeta-potential and flotation recovery is illustrated in Figure 39.2 for cationic surfactant dodecylammonium acetate/quartz system (5). The increase in adsorption due to association of surfactants adsorbed at the solid-liquid interface into two dimensional aggregates called solloids (surface colloids) or hemi-micelles occurs at about 10 M DA A. This marked increase in adsorption density is accompanied by concomitant sharp changes in contact angle, zeta-potential and flotation recovery. Thus these interfacial phenomena depend primarily on the adsorption of the surfactant at the solid-liquid interface. The surface phenomena that reflect the conditions at the solid-liquid interface (adsorption density and zeta-potential) can in many cases be correlated directly with the phenomena that reflect the... 

Fuerstenau, D.W. Streaming potential studies on Quartz in solutions of aluminum acetates in relation to the formation of Hemi-Micelles at the Quartz-solution interface. J. Phys. Chem. 1956,60(7), 981-985. 

The following mechanism was put forward [31] to explain this autocatalysis (1) permeation by cosurfactant (amide) of the water-AOT-toluene interfacial regions as a result of partitioning equilibria with concomitant increase in polarity and dielectric constant in these regions (2) diffusion of swollen micelle to proximity of electrode surface (3) collision of swollen micelle with the electrode surface (de facto hemimicelle formation) or with a hemi-micelle on the electrode surface and diffusion of amide through the AOT interfacial region within the electron transfer distance of the electrode (4) irreversible oxidation of amide. 

The applications of IL-based surfactants have not been exclusively associated to their utilization as extraction solvents in extraction and/or preconcentration schemes. In fact, another interesting approach consists of utilizing IL-based surfactants as the modifiers of extraction of sorbents when linked to the solid supports forming hemi-micelles or admicelles. The modified sorbents perform better in the solid-phase extraction (SPE) approaches than the conventional sorbents. Gangula et al. termed this methodology as admicellar or hemimicellar SPE in a review published in 2010, covering mainly applications with conventional surfactants [58]. 

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