Micelles palisade layer

Inside the column, solutes are affected by the presence of micelles in the mobile phase and by the nature of the alkyl-bonded stationary phase, which is coated with monomers of surfactant (Fig. 1). As a consequence, at least two partition equilibria can affect the retention behavior. In the mobile phase, solutes can remain in the bulk water, be associated to the free surfactant monomers or micelle surface, be inserted into the micelle palisade layer, or penetrate into the micelle core. The surface of the surfactant-modified stationary phase is micelle-like and can give rise to similar interactions with the solutes, which are mainly hydrophobic in nature. With ionic surfactants, the charged heads of the surfactant in micelles and monomers adsorbed on the stationary phase are in contact with the polar solution, producing additional electrostatic interactions with charged solutes. Finally, the association of solutes with the nonmodified bonded stationary phase and free silanol groups still exists. 

The larger effect of PVP relatively to POE suggests a better penetration of PVP than POE in the SDS micelle palisade layer. 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

FIG. 6 Representation of spherical water-containing reversed micelles solubilizing a polar molecule (p) in the micellar core (A) or an amphiphilic molecule (a) in the palisade layer (B). 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

Exploiting the properties of aqueous surfactant solutions in which the surfactants aggregate to form micelles consisting of apolar cores comprised of the hydrophobic tail groups stabilized by coronae formed by the hydrophilic surfactant heads (Fendler and Fendler, 1975 Bunton, 1991). The apolar core plays the role of the organic solvent, whereas the palisade layer can provide a medium of intermediate polarity. 

The rough water-hydrocarbon surface of the core introduced in Figure 8.3c suggests that the core of the micelle should really be considered as two distinct regions an inner core that is essentially water-free and a hydrated shell between the inner core and the polar heads. This partly aqueous shell is sometimes called the palisade layer. The extent to which the hydrocarbon chains protrude into the water is problematic, but we can get an idea of the volume of the palisade layer as follows. 

In another related study (using hexadecyltrimethyl ammonium bromide micelles), isopropyl benzene was solubilized, and the chemical shifts of aromatic and alkyl protons were observed. The results suggest that the isopropyl benzene molecules are oriented such that the isopropyl groups are buried more deeply in the core of the micelle, while the benzene ring is in the more hydrated palisade layer. This plus the conclusion of Item 3 is consistent with the description presented in Section 8.3, which located the benzene in a relatively polar portion of the micelle. 

What is a palisade layer How does one determine if there is water penetration in a micelle and, if there is any, where the water molecules are ... 

The outer-core region of the micelle, commonly referred to as the palisade layer, may provide a medium of intermediate polarity that affects the energetics of transition state formation. The primary influence of micelles is to concentrate... 

An increase in the phenanthrene partition coefficient for SDS micelles is observed with increasing ionic strength at a fixed pH of 6 (Table 2). A conceptual model has been proposed to describe the effects of electrolyte addition on the partitioning of nonpolar compounds such as phenanthrene into the core (or deep region within the palisade layer) of ionic surfactant... 

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

In the palisades layer, i.e., between the hydrophilic groups and the first few carbon atoms of thehydrophobic groups that comprise the outer regions of the micelle core. 

More deeply in the palisades layer, and In the micelle inner core. 

In aqueous systems, non-polar additives such as hydrocarbons tend to be intimately associated with the hydrocarbon core of the micelle. Polar and semi-polar materials, such as fatty acids and alcohols are usually located in the palisades layer, the depth of penetration depending on the ratio of polar to non-polar structures in the solubilizate molecule. 

Nonionic micelles have a hydrophobic core surrounded by a shell of oxyethylene chains which is often termed the palisade layer (Fig. 6.28). This layer is capable of mechanically entrapping a considerable number of water molecules, as well as those that are hydrogen-bonded to the oxyethylene chains. Micelles of nonionic surfactants tend, as a consequence, to be highly hydrated. The outer surface of the palisade layer forms the shear surface that is, the hydrating molecules form part of the kinetic micelle. 

Solubilisates that are located within the micellar core increase the size of the micelles in two ways. Micelles become larger not only because their core is enlarged by the solubilisate but also because the number of surfactant molecules per micelle (the aggregation number) increases in an attempt to cover the swollen core. Solubilisation within the palisade layer, on the other hand, tends not to alter the aggregation number, the increase in micellar size resulting solely from the incorporation of solubilisate molecules. 

To date, there are many synthesis recipes for MCM-48 available. The basic concept for these syntheses is to control the effective surfactant-packing parameter g between the limits 1/2 and 2/3 in other words, to increase the palisade-layer volume of micelles. The following are some successful synthesis strategies. 

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