Lyophilic substances

A large amount of water is added to the dehydrated material in order to cause it to swell the swollen structure is preserved when the material is frozen and subsequently dried in vacuo (in the frozen state) to a low moisture content. Some leaching occurs during the treatment with water and this, undoubtedly, further contributes to the increase in the porosity of the solid. Drying of the lyophilized substance can.be completed in a relatively short time in a vacuum oven at an elevated temperature, or at room temperature in the presence of an efficient water adsorbent. 

The alkaline product from the wood ash was a crude solution of sodium and potassium carbonates called "lye". On boiling the vegetable oil with the lye, the soap (sodium and potassium salts of long chained fatty acids) separated from the lye due to the dispersive interactions between the of the fatty acid alkane chains and were thus, called "lyophobic". It follows that "lyophobic", from a physical chemical point of view, would be the same as "hydrophobic", and interactions between hydrophobic and lyophobic materials are dominantly dispersive. The other product of the soap making industry was glycerol which remained in the lye and was consequently, termed "lyophilic". Thus, glycerol mixes with water because of its many hydroxyl groups and is very polar and hence a "hydrophilic" or "lyophilic" substance. 

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

The term steric stabilization has been used by colloid scientists to describe how a lyophilic substance, located on the surface of a lyo-phobic colloid, can prevent aggregation of the dispersion. The phenomenology of steric stabilization has been recognized and put to use over many millenia one notable example is the use, by the ancient Egyptians, of casein as a steric stabilizer of carbon (lamp black) in the production of inks for writing on papyrus. Only in the last 50 years or so has a scientific understanding of steric stabilization mechanisms emerged. 

The lyophilization procedure, as described, is suitable for materials such as vegetables, that do not usually contain much nonaqueous volatile matter. For other materials, rich in these volatiles, a modification of the procedure would be required. The volatiles could in some cases be extracted with a solvent and dried separately (28), or the substances evolved in the drying could be collected and analyzed for the amount of nonaqueous material. 

Lyophilization is a similar technique and is, in fact, evaporation at reduced temperature under vacuum. In some cases, an aqueous sample can be frozen and the vapor pressure of the ice is sufficient to produce a relatively rapid rate of evaporation. It can also be used effectively where the substances of interest have vapor pressures that are sufficiently high at room temperature, to cause substance loss under normal evaporation procedures. As the vapor pressure of a substance is exponentially related to the temperature, a relatively small reduction in temperature can reduce the vapor pressure of the sample components sufficiently to render any loss during evaporation relatively insignificant. This technique is gentler than evaporation and,... 

Currently available CRMs for Cr(lll)/(VI) species There exist a lyophilized water certified for Cr(III)/Cr(VI) and a binder-free glass fiber filter loaded with welding dust certified for Cr(VI) and total Cr (Vercoutere et al. 1998 Christensen et al. 1999) issued by the BCR. They consist of a set of specimens for single use. There is a need for more CRMs, such as a Cr(VI) in industrial effluents and in river water containing, e.g. humic substances. 

Nemati, F., Cave, G. N, Couvreur, P. Lyophilization of substances with low water permeability by a modification of crystallized structures during freezing. Assc. Pharm. Galenique Ind., Chatenay Malabry, 3, p. 487-493, 1992... 

Beck and Brink [28] have described a sensitive method for the routine assay of cobalamins in activated sewage sludge. The method involves extraction with benzyl alcohol, removal of interfering substances using a combination of gel filtration and chromatography on alumina, concentration of the extract by lyophilization, and direct determination of total cobalamin by high-speed liquid chromatography, in comparison with cobalamin standards. 

Proteins must be stable during processing, storage, and reconstitution (if lyo-philized). Although liquid formulation is preferred for protein biopharmaceuticals, it may not always be the most stable presentation. The biopharmaceutical protein may need to be lyophilized to maintain stability. Lyophilization involves the removal of water from a frozen substance by sublimation and water vaporization under vacuum.17 But in some cases, this process may itself cause protein instability. 

Aprotinin is a polypeptide consisting of a chain of 58 amino acid residues, which inhibits stoichiometrically the activity of several proteolytic enzymes such as chymotrypsin, kallikrein, plasmin, and trypsin. Aprotinin is obtained from bovine tissues and purified by a suitable process. It is stored as a bulk solution or lyophilized powder. The amount of two related substances des-Ala-des-Gly-aprotinin and des-Ala-aprotinin is determined by CZE with a 100% analysis. The relative migration times are 0.98 for des-Ala-des-Gly-aprotinin and 0.99 for des-Ala-aprotinin, and the specified limits are 8.0 and 7.5%, respectively. 

Concentration of a sample in a SpeedVac, a centrifuge with open rotor running in vacuum, is not lyophilization because the sample is not frozen therefore, this method does not fit for macromolecules with defined structures. The reconstitution (dissolving) of protein samples concentrated in a SpeedVac is often more difficult and comparable to drying with air. But it is the method of choice for concentration of solutions of low-molar mass substances, e.g., peptides, since the sample is collected at the bottom of the tube by means of centrifugal force. 

Concentration. This group consists of those processes in which water is removed and the dissolved substances are left behind. Examples are freeze concentration, lyophilization (freeze-drying), vacuum distillation, and membrane processes such as reverse osmosis and ultrafiltration. A common disadvantage of these methods is that inorganic species are concentrated along with the organic constituents. 

Two general classes of methods can be functionally defined for preparing concentrates of organic substances. Concentration methods involve the removal of water (e.g., lyophilization, freeze concentration, vacuum distillation, reverse osmosis [RO], and ultrafiltration) and result in a more highly concentrated aqueous solution of organic contaminants. Isolation methods are those methods in which the organic substances are physically removed from the aqueous solution, for example, adsorption onto a solid substrate followed by desorption (I). 

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