Solvent precipitation

Further reduction in the volume of solvent required can be achieved by heat treatment of the exopolysaccharide at 100-130°C for 1-15 minutes before solvent precipitation. 

List the following stages involved in recovery of exopolysaccharides by solvent precipitation and subsequent packaging in an appropriate order. 

Dissolution/reprecipitation processes were evaluated for the recycling of poly-epsilon-caprolactam (PA6) and polyhexamethyleneadipamide (PA66). The process involved solution of the polyamide in an appropriate solvent, precipitation by the addition of a non-solvent, and recovery of the polymer by washing and drying. Dimethylsulphoxide was used as the solvent for PA6, and formic acid for PA66, and methylethylketone was used as the non-solvent for both polymers. The recycled polymers were evaluated by determination of molecular weight, crystallinity and grain size. Excellent recoveries were achieved, with no deterioration in the polymer properties. 33 refs. 

Species with low solubility in water may exist in solid form (e.g., Ag2S, BaS()4) or liquid form (e.g., chlorinated solvents). Precipitation reactions and immiscible-phase separation are important processes affecting this type of speciation. 

Silylation reactions with silyltriflates progress rapidly at 0 - 20°C, side reactions are therefore suppressed and yields are generally high. Triethylamine seems to be the best auxiliary base, because triethylammoniumtriflate, which is insoluble in nonpolar solvents, precipitates in liquid form. Progress and end of the reaction may therefore be observed by increase of the second (salt) phase. 

The study concluded that Once wash steps are optimized, samples prepared by solid phase extraction are cleaner than those prepared by protein precipitation. Samples prepared by extraction with a Multi-SPE plate resulted in lower LOQs than samples prepared by solvent precipitation. Drug recoveries were acceptable (>80%) for both the SPE and the solvent precipitation methods. Well-to-well reproducibility of samples was slightly better with extraction with a Multi-SPE plate. Evaporation and reconstitution, while more time-consuming, yield better chromatographic performance, allow analysis of lower concentration samples, and require optimization for good analyte recovery. 

Fig. 3.3 Fractionation protocol for GSE based on successive solvent precipitations (adapted with...

The lithiation shown in Eq. (10) takes places at 20°C in ra-pentane. Under these conditions, the reactivity of the lithimn alkyl is reduced to such an extent that only the most reactive bond in 53 (and in 59, respectively) is acted upon and therefore the selective exchange of the phosphorus hydrogen with lithium takes place. The phosphides formed can easily be separated and purified because they dissolve with difficulty in nonpolar solvents. Precipitation of these phosphides is, however, incomplete and can fail to occur if the reaction solutions contain significant concentrations of partially alkylated silylphosphanes such as (Me3C)P(SiMej)2 and MeP(SiMe3)2. 

In this paper, a description will be given of the previous work carried out to find ways to reduce metallic trace elements in the filtered extract to low levels. This will be compared with new work where solvent precipitation was carried out on filtered extract solution as an alternative method of producing low levels of trace elements. 

Liquefaction to produce filtered extract solution was carried out in a 2-1 autoclave under the conditions described previously and elsewhere (9). Solvent precipitation experiments using the filtered extract solution were carried out in a 0.2-1 autoclave. The extract and solvent were mixed together cold, in the autoclave, and heated to the extraction temperature in a fluidised sand bath. The mixture was agitated at the required temperature for 20 minutes and then filtered through a pressure filter containing a nomex cloth, at the extraction temperature, using the pressure in the autoclave or an iq lied nitrogen pressure. In some eiq)eriments, the solvent was injected, under pressure, into extract solution already at the required temperature. 

An alternative to adjusting process conditions is to have a secondary solvent precipitation stage. Toluene was found to be the best solvent tried and, again, low levels of trace elements were obtained. Using this method, however, there is a loss of product, but this is mainly high molecular weight species, which are difficult to hydrocrack and are probably responsible for deposition of carbonaceous material on the catalyst. 

The structure of MAO is poorly defined and varies with preparation conditions, but it performs well in activating the metallocene initiator. The use of MAO is complicated by its lack of long-term storage stability. It is usually supplied by manufacturers as a cloudy solution of MAO in toluene MAO has very low solubility in aliphatic solvents. Precipitation is often observed on long standing, especially if the container is frequently opened and exposed to moisture and oxygen. This precipitation, if not too extensive, may not affect the utility of the MAO as a coinitiator. A modified MAO, known as MMAO, offers some improvement in storage stability and improved solubility in aliphatics. MMAO is prepared by controlled hydrolysis of a mixture of trimethylaluminum and triisobutylaluminum. 

There is no general rule for the prevention or avoidance of occlusion. In some cases, a change of solvent/precipitant system may help to achieve this goal. Raising the drying temperature is also beneficial. 

In some cases the molecular-weight distribution can be determined by turbi-dimetric titration, a technique which is based on the fractional precipitation. A precipitant is added to a very dilute solution of the polymer, and the resulting turbidity is measured as a function of the amount of added precipitant the preparative separation of the fractions is thereby avoided. If the polymer is chemically homogeneous, the mass distribution function can then be calculated. Tur-bidimetric titration is also suitable as a means for establishing the best fractionation conditions (e.g., choice of solvent/precipitant combination, size of fractions, etc.), before carrying out a full-scale fractionation by precipitation. 

Fractional extraction is free from the disadvantages encountered in fractional precipitation. Here, the technique consists in extracting the polymer with a series of solvent/precipitant mixtures, the proportion of solvent being increased stepwise. Since one begins with the poorest solvent mixture - in contrast to fractional precipitation - the first fraction contains the low-molecular-weight com-... 

Cross-Fractionation. Complex polymers contain more than one broad property distribution, if molecular weight and composition are the only two property distributions present then an example of cross-fractionation would be the separation of the polymer first according to molecular weight and the separation of each single molecular weight fraction obtained according to composition. This cross-fractionation provides a two-dimensional answer to a two-dimensional distribution problem. It has typically been accomplished for polymers using solvent/non-solvent precipitation. 

Proton donor Proton acceptor Solvent Precipitation Ref. 

The solvent precipitation technique is alto used to encapsulate material, ... 

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