Hydrophobicity micelles

Surfactants have been investigated extensively in CE for the separation of both charged and neutral molecules using a technique based on the partitioning of the analyte molecules between the hydrophobic micelles formed by the surfactant and the electrolyte solution, which is termed micellar electrokinetic capil-... 

The application of MEKC for chiral separation is primarily used when the enantiomers of interest are neutral. In conventional CE without micelles, neutral enantiomers will be swept along with the electro-osmotic flow (EOF) because they carry no ionic charge. If a neutral CD is present and forms a complex with these CDs, they will still move with the EOF. Thus, it is necessary for neutral enantiomers to create the potential for differential migration so that the overall complexes and free enantiomers will not just be swept along with the EOF. For this reason, the use of MEKC which utilizes ionic micelles for differential migration (through hydrophobic interaction) modified with CDs for enantioselectivity was applied [9]. The mechanism is as outhned for MEKC however, because there are hydrophobic micelles inherently present in the electrolyte, additional interactions between the enantiomers and the micelle over those with just a CD may, of course, occur which will normally influence any observed separation. 

Molecules which are weakly soluble in water, can be solvated in the hydrophobic micelle nucleus. This is an important technical process for instance for detergents. As levelling agent in textile chemistry micelles act as buffers for dyestuffs, the dyes are solubilized in the micelle nucleus and this equilibrium holds the "free dye concentration in the water phase more or less constant during the dyeing process (1 9,50). 

Separation in Micellar Electrokinetic Chromatography (MEKC) is based on partitioning of the analyte molecules between the aqueous run buffer and the core of micelles, which are contained in the run buffer. The technique is essentially a hybrid between CE and liquid chromatography (LC). The run buffer and micelles are moved through the capillary by an applied electric field. The analytes are dragged with the bulk solution. Similar to LC, the analytes partition between two phases, in this case two mobile phases, the hydrophilic run buffer and the hydrophobic micelles. Unlike other electrophoresis modes, MEKC can distinguish between different neutral compounds according to their hydrophobicity. 

It is possible to measure to by adding a hydrophilic compound such as methanol to the buffer. Methanol does not partition into the hydrophobic micelles core. It is eluted at to- Totally hydrophobic compounds can be used to measure tmc- For example, the dye Sudan III is completely solubilised by the micelles and passes the detector at tmc-... 

The addition of dyes in the initial reaction mixture affords dye-doped silica cores. According to their solubility, in fact, they partitimi between water and hydrophobic micelles, the latter fraction remaining physically entrapped in the silica network. Derivatizing the dye with a trialkoxysilane group leads to its co-condensati(Mi with TEOS, resulting in robust luminescent systems. Thus, this method allows the physical or covalent entrapment of dozens of molecules to a small silica core, providing very bright nanosystems. 

The first molecular interaction model was developed by Nagarajan [3, 4, 5]. For the description of the microstructure of the complex the necklace model was adopted. The free-energy expression developed earlier for micelles was modified in order to incorporate the interaction with the polymer. This interaction was described by two parameters. One of them was the micelle-core area shielded by the polymer and the other one was an interaction parameter due to the hydrophobic contribution of the polymer segments interacting with the core. The shielding of the micelle core has two opposite effects on the micelle formation. On the one hand, it reduces the contact area between the hydrophobic micelle core and water on the other hand, it increases the polar head group interactions. Since the shielding parameter is some kind of mean value, at present no a priori method for the estimation of this area is available ... 

The location of TMB within the micelles was studied by Kevan et al. [132] with the spin-echo technique. In the deuterated SDS solution in the spin-echo spectrum of TMB" one can observe both the proton modulation from the hydrogen atoms of SDS and the deuterium atoms of water. The conclusion was that TMB" is located at the micelle-water interface. As discussed above, parent TMB is located in hydrophobic micelle nuclei. So the location of TMB changes within its lifetime and it transfers from the hydrophobic region of the micelle to the interface. The evolution of TMB" in CTAB and SDS micelles was studied by Beck and Brus with pulse Raman spectroscopy [132]. They found that TMB is formed in CTAB micelles with a delay and yields TMB and TMB in the millisecond range. TMB formed in SDS micelles is stable for several days [134]. The anionic micelles seem to stabilize TMB" and the cationic ones to destabilize it. 

Oxidation of the precursor anion in these models can be catalysed by cobalt (and other transition metals) [25]. The enzyme can also be modelled to some extent by surfactant solutions [26]. Quaternary ammonium salts (e. g. cetyl trimethylammonium bromide) are better than neutral detergents. Anionic surfactants are ineffective. The reason for the activity is two-fold. The rate of oxidation is increased, and the fluorescence of the product, which is very weak in pure water, is considerably enhanced in the hydrophobic micelles. 

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