Asymmetric wetting

Fig. 6 Illustration of surface energy effects on the self-assembly of thin films of volume symmetric diblock copolymer (a). Sections b and c show surface-parallel block domains orientation that occur when one block preferentially wets the substrate. Symmetric wetting (b) occurs when the substrate and free surface favor interactions with one block B, which is more hydrophobic. Asymmetric wetting (c) occurs when blocks A and B are favored by the substrate and free surface, respectively. For some systems, a neutral substrate surface energy, which favors neither block, results in a self-assembled domains oriented perpendicular to the film plane (d). Lo is the equilibrium length-scale of pattern formation in the diblock system...

Instead of observing the change of the morphology as a function of the film thickness, surface boundaries could also be used to control the wetting layer morphology at interfaces, the surface topographies, and the microdomain period [148]. In the case of symmetric or asymmetric wetting of the block copolymer at... 

A series of studies of microscopic thin hquid films, both symmetric (foam and 0/Wemulsion films) and asymmetric (wetting films), from aqueous solutions of... 

In contrast, the wetting films are relatively thicker and their thickness depends on the concentration of the AB polymeric surfactant This behavior is due to the different adsorption and orientation of the polymeric surfactant molecules at the solid-liquid and liquid-air interface of the asymmetric wetting film the results suggest the formation of adsorption bUayers at the solid interface, and the steric repulsion of the loops and tails of the polymeric surfactant determined the film thickness. 

Priest C, Albrecht TWJ, Sedev R, Ralston J (2009) Asymmetric wetting hysteresis on hydrophobic microstnictured surfaces. Langmuir 25 5655 5660... 

One report (13) describes the procedure for spinning dry asymmetric ceUulose acetate fiber with a bore skin. Such fibers are spun in a modified dry-spinning process in which a volatile Uquid (methyl formate) is used as the ceUulose acetate solvent. The bore coagulating Uquid is isopropyl alcohol, which is subsequentiy removed. The advantages of these dry fibers over most ceUulose acetate membranes are that they can be stored dry, they are wet-dry reversible, they can be sterilized and packed dry, and they are ready for use without removal of preservatives. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

When a droplet is deformed asymmetrically, the ratchet motions of the droplet can be induced as demonstrated on the vibrated gradient surface and on a saw-shaped electrode on which the wetting was changed by electrowetting [48]. 

Figure 16.7 Schematic drawing of the asymmetric electrode pattern. The gold electrode was covered with a Fc-alkanethiol monolayer. The wetting of the gold electrode was switched from wetting to repulsive and vice versa by changing the electrochemical potential of the electrode.

The wet cell pellet (7-8 g) obtained was stored at 20 °C until it was used for asymmetric reduction. 

Rh2(S-TBSP)4 8 and Rh2(S-DOSP)4 9 (Tab. 14.3) [40, 45]. A very unusual feature of the prolinate-catalyzed cyclopropanations is that the reactions proceed with much higher asymmetric induction when hydrocarbon solvents are used instead of dichloromethane [40, 45]. Room-temperature cyclopropanations of styrene with Rh2(S-TBSP) or Rli2(S-D0SP)4 typically occur with 90-92% enantioselectivity, while the Rh2(S-DOSP)4-cata-lyzed reaction at -78°C occurs in 98% enantiomeric excess (Tab. 14.3) [40]. The rhodium prolinate catalysts are very easy to handle, being stable to air, heat, and moisture although it has been reported that the enantioselectivity can decrease if the cyclopropanation is conducted in wet solvents [46]. 

The main task in technical application of asymmetric catalysis is to maximize catalytic efficiency, which can be expressed as the ttn (total turnover number, moles of product produced per moles of catalyst consumed) or biocatalyst consumption (grams of product per gram biocatalyst consumed, referring either to wet cell weight (wcw) or alternatively to cell dry weight (cdw)) [2]. One method of reducing the amount of catalyst consumed is to decouple the residence times of reactants and catalysts by means of retention or recycling of the precious catalyst. This leads to an increased exploitation of the catalyst in the synthesis reaction. 

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