Carriers monodisperse

Other studies have shown how monodispersed silica particles can be used as carrier material for pigments and dyes (68-73). Various reactive dyes, containing a sulfonic acid group, can be attached to the surface or incorporated into the growing silica particles (71). The resulting pigment particles have better color properties due to the con-... 

However, there are a number of difficulties associated with the synthesis of colloidal semiconductor particles. The preparation of stable, monodispersed, well-characterized populations of nanosized, colloidal semiconductor particles is experimentally demanding and intellectually challenging. Small and uniform particles are needed to diminish non-productive electron-hole recombinations the mean distance by which the charge carriers need to diffuse to reach the particle surface from which they are released is necessarily reduced in small particles. Monodispersity is a requirement for the observation of many of the spectroscopic and electro-optical manifestations of size quantization in semiconductor particles. Small semiconductor particles are difficult to maintain in solution in the absence of stabilizers flocculations and Ostwald ripening... 

We have shown that fractions collected from broad particle distributions can be reinjected into the sedimentation FFF device for a second run (18). The emerging peaks are relatively narrow, reflecting their small particle size range. Quite obviously, a reinjected fraction run in a carrier of a different density will emerge at a different volume because of the effect of Ap on retention parameter X (see Equation 3). The shift in retention volume (see next section) can be used to calculate the density of the particulate material exactly as outlined for monodisperse populations. 

One of the most attractive features of colloidal semiconductor systems is the ability to control the mean particle size and size distribution by judicious choice of experimental conditions (such as reactant concentration, mixing regimen, reaction temperature, type of stabilizer, solvent composition, pH) during particle synthesis. Over the last decade and a half, innovative chemical [69], colloid chemical [69-72] and electrochemical [73-75] methods have been developed for the preparation of relatively monodispersed ultrasmall semiconductor particles. Such particles (typically <10 nm across [50, 59, 60]) are found to exhibit quantum effects when the particle radius becomes smaller than the Bohr radius of the first exciton state. Under this condition, the wave functions associated with photogenerated charge carriers within the particle (vide infra) are subject to extreme... 

Figure 4.7 shows TOF as a function of the particle size of a monodisperse supported platinum catalyst Pt/Al203 in the reaction of deep methane oxida tion. One can see that equation (4.91) is sufficient to describe the experimental data when the Pt particles are more than 2 nm in size. When they are smaller, TOF deviates from the monotonous dependence (4.91). The reason may be a considerable change in the chemical composition of the anchored active com ponent due to the strong interaction of this component with the carrier surface. 

As model samples for the verification of the conventional SdFFF as a concentration methodology monodisperse polystyrene latex beads (Dow Chemical Co.) with nominal diameters of 0.357 fum (PSl) and 0.481 /Ltm (PS2) were used. They were either used as dispersions containing 10% solids or diluted with the carrier solution (triple-distilled water -r 0.1% (v/v) detergent FL-70 from Fisher Scientific Co. -l- 0.02% (w/w) NaNj) to study sample dilution effects. Diluted samples in which the amount of the polystyrene was held constant (1 /u-L of the 10% solids) while the volume in which it was contained was varied over a 50,000-fold range (from 1 to 50 mL of carrier solution) were introduced into the SdFFF column. During the feeding step, the flow rate was 5.8 mL/h for the PSl polystyrene, and 7.6 mL/h for the polystyrene PS2, and the channel was rotated at 1800 rpm for the PSl sample and at 1400 rpm for the PS2 sample. In the sep-I aration (elution) step, the experimental conditions for the two samples were as follows ... 

Our coverage had to be more limited and has been restricted to those papers in which curves of the types described above were presented. Unfortunately, the catalytic results are not always fully comparable. In fact, most works do not distinguish initial rate results (on clean surfaces) from steady-state rates (on semipoisoned surfaces), even though most refer to low conversions. Further, if a true particle size effect is to be evidenced, the metal particles must correspond as much as possible to the following criteria they must be (1) monodisperse, (2) fully reduced, (3) unpoisoned (by liquid solvents or adventitious species), (4) unperturbed (by the carrier), and (5) unpromoted (by Ca, K, etc.). 

Sivakumar S, Bansal V, Cortez C, et al. (2009) Degradable, surfactant-free, monodisperse polymer-encapsulated emulsions as anticancer dmg carriers. Adv Mater 21 1820-1824... 

Following the early work by Thorsen et al., focused on the formation of monodisperse aqueous droplets in an organic carrier fluid performed on a microfluidic chip, and then followed by others works, the breakup mechanism responsible of droplet formation was later analyzed by Garstecki et al. ° showing that when is order of 1 the dominant contribution to the dynamics of breakup at low capillary numbers is not dominated by shear stresses, but it is driven by the pressure drop across the emerging droplet. 

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