Precipitation and redispersion

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

Finally, the precipitation and redispersion of the silver nanocrystals was found to be nearly reversible. After precipitating the largest nanocrystals of a polydisperse dispersion by lowering the system pressure from 414 bar to 276 bar, and then repressurizing to 414 bar, 90% of the silver nanocrystals redispersed. Reversible nanocrystal flocculation has potential value in fine-tuning size-dependent separations with minor variations in pressure. Reversible solvation conditions are difficult to achieve using a conventional anti-solvent approach. 

The redispersion of the stabilized ITO nanoparticles leads to a decrease in stability of the nanoparticles, measured as decreased fraction of stable particles after the hrst redispersion cycle. This is due to the weak interactions between the stabilizer and the particle surface, as shown above by the ITC measurements. By precipitation and redispersion of the particles, some stabilizer molecules were detached from the surface because of the addition of the organic solvent. The detachment still small enough to achieve redispersion of the nanoparticles during the redispersion steps was verified by TGA and is plotted in Fig. 9 (right). In case of the first re-agglomeration step, the amount of bound stabilizer on the surface is still sufficient to achieve full redispersion of the nanoparticles. However, after three precipitation-redispersion cycles, the amount of coupled stabilizer molecules is so low that only 35 % of the particles are stable [20]. 

Polymerization in dilute solution also allows a very straightforward isolation and purification of the microgels. After polymerization, the resulting microgels can be conveniently precipitated from the reaction solution by using suitable nonsolvents for the microgel molecules. The resulting powders can be filtered off, dried, and redispersed in suitable solvents when needed. 

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

XANES to ensure the quality of the synthates. Three batches of ferrihydrite were synthesized and precipitates were washed 5-6 times to ensure a chloride-free synthate. Ferrihydrite precipitates were redispersed in 200 mL of double deionized (DDI) water at (1) room temperature (25°C), as well as preheated in water baths to temperatures of (2) 50°C and (3) 75°C. For all of these slurries, pH was kept constant at 10 using 1M KOH. 40 mL samples were pipetted from each reaction vessel after 0, 1,2, 3, and 7 days. Slurries were centrifuged, washed three times with DDI water and air dried for analyses (BET, XRD, and XANES). BET analyses were used to evaluate the decrease in surface areas with increasing crystallinity, and XRD and XANES were used to detail the structural and speciation changes in iron. 

In another preparation [43], ammonia gas was passed through a solution of SnCU, the precipitate rinsed well (to remove Cl, which caused the final films to be porous—a useful observation since porous films are sometimes preferred over compact ones), and redispersed in concentrated HNO3 to give a semitransparent sol at a pH of 5-7. Films were deposited from this solution onto Si (100) by heating at 60-100°C for 4 hr, followed by 100-200°C for 6-12 hr. The initial lower-temperature step was necessary to obtain nucleation if omitted, no film was deposited. Films ca. 200 mn thick were obtained with a crystal size of 3.5 mn. 

The estimation of casein in milk by refractometric techniques appears to hold some promise. The casein may be precipitated, washed, and redispersed to yield a solution suitable for refractometry (Brereton and Sharp 1942 Schober etal. 1954). Another method involves computation from the difference between the refractive indices of two samples, one made alkaline to dissolve the casein and the other treated with copper sulfate to precipitate it (Hansson 1957). Heating the milk... 

The dialyzed protein from the outer level was filtered and acidified with 1 N HCl. Maximum precipitation occurred at about pH 5.5, leaving a clear supernatant fluid. The precipitate was washed with water and redispersed in 6 M urea. The clear supernatant liquid at pH 5.5 was filtered and further acidified. A cloudy precipitate forms at about pH 5, the precipitation attaining a maximum near pH 4.5. This settles out on standing. Thus the primary solution of the epidermal proteins in 6 M urea is divided into two main fractions—a coarse, flocculent precipitate at pH 5.5 and a fine, granular precipitate at pH 4.5. The first precipitate can be dissolved in urea, dialyzed and reprecipitated at pH 5.5 many times. For the present purposes the processes were repeated three times. The precipitate at pH 4.5 can be redissolved by adjusting the pH to 7 with NaOH these processes were also repeated three times. 

The materials from the middle and inner levels which had clotted in the sac were shaken and the clear fluid poured away from the retracted clots. This fluid gave no precipitate at pH 5.5, but cloudy precipitates at about pH 4.5. These were dissolved and reprecipitated three times as above. The main clot-like precipitates were redispersed in 6 M urea, filtered and dialyzed these processes were repeated three times. In this way two main fractions were prepared for the middle and inner levels as was done in the case of the outer levels. With each successive reprecipitation of the main precipitate less and less of the fraction precipitating at pH 4.5 is obtained. 

Both sc-ethane and SC-CO2 provide density tunable dispersibility for nanocrystals. Partially fluorinated ligands enabled the first example of a sterically stabilized nanocrystal dispersion in pure CO2. The nanocrystals show LCST phase behavior with increased dispersibility at higher solvent densities. Additionally, arrested precipitation to synthesize nanocrystals in SC-CO2 has been developed. The technique yields chemically robust nanocrystals that are fully passivated with fluorinated ligands allowing for collection and redispersion of the particles without any change in size or polydispersity. The nanocrystal size produced depends on both the solvent density and length of the ligand, with smaller less polydisperse particles formed at conditions of adequate steric stabilization. 

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