Precipitation-solvent evaporation method

For this, the PLGA nanospheres were either prepared in the presence of PEG-PLA copolymers by the precipitation-solvent evaporation method or were prepared without any stabilizing agent and subsequently coated by incubation in aqueous solutions of PEG-PLA. 

PLGA nanospheres were prepared by the precipitation-solvent evaporation method in the presence of PEG PLA polymers (Stolnik et al, 1995). The resulting PEG-coated particles had an average particle size between 120 and 140nm and a polydispersity index between 0.08 and 0.13, indicative of a relatively narrow size distribution. 

In solvent evaporation method, the lipid is dissolved in a volatile organic solvent (e.g., cyclohexane or chloroform), which is dispersed in an aqueous phase to form an emulsion (Figure 58.5c). The lipid precipitates with the evaporation of the solvent and forms nanoparticle dispersion. The solid lipid nanoparticles are harvested after complete evaporation of the solvent. ... 

Alternatively, coacervation or solvent extraction is often used to produce microspheres (Fig. 3). The protein and polymer emulsion is stirred with a nonsolvent for the polymer such as silicone oil, resulting in the formation of embryonic microspheres (for a review, see Lewis, 1990). The nonsolvent extracts the methylene chloride or ethyl acetate firom the polymer phase, causing precipitation of the polymer and entrapment of the protein in the polymer matrix. To remove the nonsolvent, a volatile second nonsolvent (e.g., heptane) is added, and the microspheres are allowed to harden in the nonsolvent. After repeated extraction with the volatile nonsolvent, the final microspheres are then dried. While this method offers the advantage of avoiding contact between the protein phase and an aqueous phase as in the solvent evaporation method, the additional solvents utilized in this process are often difBcult to completely remove and are a safety and toxicity concern. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Purify the derivatized dendrimer using gel filtration (size exclusion chromatography) on a desalting column or through use of ultrafiltration spin-tubes (for G-4 and above). For smaller dendrimers, the derivatives may be purified by repeated precipitation from a meth-anolic solution by addition of ethyl acetate, dioxane, or benzene. The SPDP-dendrimer may be dried by lyophilization (if in water or buffer) or by solvent evaporation in vacuo (if the precipitation method was used). 

Because the instability of the N-oxide metabolite, which was subjected to decomposition during sample preparation (solvent evaporation during offline SPE), online SPE LC/MS became the method of choice for the application. Hsieh et al. (2004) built a system with two TFC cartridges and one analytical column, and another system with two TFC cartridges and two analytical columns for GLP quantitative bioanalysis of drug candidates. A Turbo C18 (50 x 1.0 mm, 5 /.mi, Cohesive Technologies), an Xterra MS C18 (30 x 2.0 mm, 2.5 /mi), and a guard column were used. Protein precipitation preceded injection. The cycle times for the two systems were 0.8 and 0.4 min. 

One of the first methods for making capsules involved polymer coacervation. In this method, macromolecules are dissolved in either the dispersed or continuous phase of an emulsion and are induced to precipitate as a shell around the dispersed phase. Coacervation can be brought about in several ways, such as changes in temperature or pH, addition of salts or a second macromolecular substance, or solvent evaporation (Bungenberg de Jong 1949). 

The separation of the stereoisomers was done by fractional crystallization modifying the method described in ref. (14). After filtering the catalyst and evaporating the majority of water ethanol was added and pure cis isomer was precipitated. Then the precipitate was filtered and washed with the ethanol. After cooling the mother liquor the cis isomer precipitate was removed again, and the solvent evaporation, precipitation, filtration and washing cycle was repeated twice. 

Two complementary experiments show that the orientation and hiding of one or the other face of the steroid ring of cholate can occur when mixtures of lecithin and bile salt are considered. One of these experiments was performed by Etienne (4), who observed the following facts incidentally while extracting lipids from the serum lipoproteins by Delsal s method. This method utilizes a mixture of methanol and methylal (1 to 4) in the cold. The proteins are precipitated, while the lipids are dissolved in the methanol-methylal solvent mixture. If this solution of the lipids is evaporated, the residue is soluble in nonpolar solvents, such as chloroform. However, if sodium cholate is added to the lipoproteins before their extraction, the residue obtained after the methylal-methanol solvent evaporates is insoluble in chloroform. More precisely, while cholesterol and the triglycerides of the lipidic residue are extracted by chloroform, all of the lecithin remains insoluble, associated to the bile salt. The explanation is probably as follows. During evaporation, methylal with its low boiling point (44°C.), evaporates first, and the solvent becomes more and more concentrated with methanol and the residual water from the lipoprotein aqueous solution. Therefore, in the lecithin plus... 

Precipitation of the cast liquid polymer solution to form the anisotropic membrane can be achieved in several ways, as summarized in Table 3.1. Precipitation by immersion in a bath of water was the technique discovered by Loeb and Souri-rajan, but precipitation can also be caused by absorption of water from a humid atmosphere. A third method is to cast the film as a hot solution. As the cast film cools, a point is reached at which precipitation occurs to form a microporous structure this method is called thermal gelation. Finally, evaporation of one of the solvents in the casting solution can be used to cause precipitation. In this technique the casting solution consists of a polymer dissolved in a mixture of a volatile good solvent and a less volatile nonsolvent (typically water or alcohol). When a film of the solution is cast and allowed to evaporate, the volatile good solvent evaporates first, the film then becomes enriched in the nonvolatile nonsolvent, and finally precipitates. Many combinations of these processes have also been developed. For example, a cast film placed in a humid atmosphere can precipitate partly because of water vapor absorption but also because of evaporation of one of the more volatile components. 

In synthetic laboratories, the most common method to obtain crystals is crystallization from solution. In a way this is also the most difficult process to control. The solvent evaporation rate can be affected by several factors such as ambient humidity, temperature and ventilation, thermal excursion during the 24 h in the case of open air room temperature crystallizations, and vibrations of the building (footsteps, traffic, lifts, etc.) and of the refrigerators. There is no unique and transferable recipe for the crystallization of a substance Philip Ball concluded his Nature Editorial in 1996 by saying, the precipitation of good single crystals remains a black magic [68]. 

The solvent emulsification/evaporation method involves lipid precipitation in O/W emulsions. Solid lipids are dissolved in a water-immiscible organic solvent (e.g., cyclohexane) followed by emulsification in an aqueous medium. Upon evaporation of the solvent, the nanoparticle dispersion is formed due to lipid precipitation. Residue of organic solvents is the major problem of this method [94], However, the microemulsion and solvent emulsification/evaporation methods can be performed conveniently in the laboratory without specific apparatuses. 

In the solvent method the separation of the solubilised or dispersed material from the solvent phase can be explained by precipitation or phase change induced by solvent evaporation, addition of electrolyte, pH modification or heat treatment (Krochta and McHugh 1997). Such treatments can be adjusted to enhance film formation or specific properties. For composite emulsion-based films or coatings a lipid material and most likely a surfactant, is added to the solution, which is then heated above the lipid melting point and homogenised. The prepared solution is then applied on an appropriate support and the solvent evaporates. 

Three main ways exist for making homogeneous blends of polymers mechanical mixing, mixing in a common solvent, and in situ polymerisation. Mixing in a common solvent has been the commonest method in academic studies though this method, including the recovery of the blend by solvent evaporation or precipitation of the polymers in a non-solvent, would have very limited industrial application. 

It is important to note that unlike in precipitation method, both polar solvent, such as acetone, and nonpolar solvents, such as diethyl ether and hexane, produced needlelike crystals of ibuprofen when saturated solutions were cooled to 5°C over 120 min. Dichloromethane produced cubic, whereas acetonitrile produced spherical agglomerated crystals by this method. Solvent evaporation of ethanol gave platy... 

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