Polar microdomain

This transition from reverse micelles to a tridimensional H-bond network has a direct consequence on third-phase formation. Moreover, the structure of the solution does not depend on the nitric acid concentration. Third-phase formation is thus prevented. Significant variations in extraction properties can be expected concurrently with this micelle-to-cosolvent microstructural transition. Without octanol, polar microdomains are clearly separated from the apolar solvent by an interface, whereas in the second system, the transition between polar and apolar areas is spatially more extended and probably creates an open structure as in a network. Nevertheless, a systematic study with structural determination in relation with the extraction ability is not yet available in the literature. Regarding the efficiency of the extractant solution containing modifiers, the key issue is also the competition for complexation between the complexing agent and the cosurfactant head-group. 

The spectroscopic probe pyridine-N-oxide was used to characterize polar microdomains in reverse micelles in supercritical ethane from 50 to 300 bar. For both anionic and nonionic surfactants, the polarities of these microdomains were adjusted continuously over a wide range using modest pressure changes. The solubilization of water in the micelles increases significantly with the addition of the cosolvent octane or the co-surfactant octanol. Quantitative solubilities are reported for the first time for hydrophiles in reverse micelles in supercritical fluids. The amino acid tryptophan has been solubilized in ethane at the 0.1 wt.% level with the use of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT). The existence of polar microdomains in aggregates in supercritical fluids at relatively low pressures, along with the adjustability of these domains with pressure, presents new possibilities for separation and reaction processes involving hydrophilic substances. 

For systems with added water (Figure 7, Wq = 12.5 and 20), an extremely polar microdomain was observed with a polarity close to that of bulk water at only 60 bar. The existence of such a microdomain is novel and interesting however, the... 

The same result is observed for SCF ethane in the one-phase region from 58 to 285 bar. This indicates that water must be present in conjunction with surfactant to give highly polar microdomains seen in Figure 4. 

Highly polar microdomains exist in reverse micelles of AOT and nonionic polyethylene oxide surfactants in ethane, even below 100 bar, both with and without cosolvents. Without cosolvents these domains are likely very small since values of Wo are small. The addition of the cosolvent octane provides a means to take up large amounts of water over a wide pressure range. The polarities in the interior of the micelles approach that of bulk water. The existence of polar microdomains in supercritical fluids at relatively low pressures presents an opportunity for new separation and reaction processes involving hydrophilic substances. 

Neurons constitute the most striking example of membrane polarization. A single neuron typically maintains thousands of discrete, functional microdomains, each with a distinctive protein complement, location and lifetime. Synaptic terminals are highly specialized for the vesicle cycling that underlies neurotransmitter release and neurotrophin uptake. The intracellular trafficking of a specialized type of transport vesicles in the presynaptic terminal, known as synaptic vesicles, underlies the ability of neurons to receive, process and transmit information. The axonal plasma membrane is specialized for transmission of the action potential, whereas the plasma... 

Schematic representation of an asymmetric cell division. This figure shows a hematopoietic stem cell under the influence of cell components within its niche. Between the stromal endosteal and subendosteal cell layers, the hematopoietic stem cell is stimulated into distinct polarities that split cytoplasmic components into microdomains. The cell division that is able to promote a different distribution of these intracellular elements, generates distinct offspring. The asymmetric division explains the phenomena of simultaneous self-renewal and differentiation.

As with most discoveries, these ideas then passed through a lull before takeoff. Then the Chicago-New York axis of Yang and Turro in the USA investigated the first example (6) in the host-guest mould [10]. Their focus was on the demonstration of exciplex emission of (6) in aqueous solution due to protection by an encapsulating /J-cyclodextrin. Ever since Weller s experiments, exciplex emissions had been observed to fall off rapidly in intensity as solvent polarity was increased [4], A formally related example due to Tazuke from 1982 must, however, be pointed out, where a hydrophobic polymer microdomain in mixed and neat aqueous solution allows exciplex emission from pyrene and dimethyl-aniline pendants [11], The importance of (6) in the present context stems from the correlation of pH-dependent emissions from the naphthalene moiety and from the exciplex. Co-occurrence of externally switchable photophysics and supramolecular phenomena would later become common in the research literature [12]. 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

Figure 7. Variable polarities of microdomains in reverse micelles of anionic surfactant (0.01 M AOl) in ethane (T = 35 C)...

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