Solvents multicomponent systems

Phase-Inversion Process. Most tortuous-pore membranes are made by a casting process known as "phase inversion." Figure 2.2 is a simplified schematic of a casting machine which makes cellulose ester membranes. Typically, a casting solution made up of the polymer and a multicomponent solvent system is metered onto a stainless steel belt or web. The belt passes through a series of environmental chambers usually containing water vapor at elevated temperatures. The more volatile solvents evaporate and the water vapor precipitates the polymer around the less volatile solvent which becomes the "pore-former." Subsequently, (not shown in Figure 2.2), after the membrane is formed, the residual solvents are washed out of the pores, surfactants are added, and the membrane is dried. 

In principle the adsorption equilibria for a multicomponent solvent system plus sample can be enormously complex, and no practical understanding of such systems can be hoped for without certain simplifying assumptions. Throughout the present treatment we will assume that no solvent demixing occurs as a result of selective adsorption of one or more solvent components. This is a reasonable assumption for most chromatographic systems, and separations in which solvent demixing is likely to occur can be anticipated, as discussed below. Further approximations will prove necessary as we proceed. 

Approximate Rf Values for Chloral Hydrate in Multicomponent Solvent Systems... 

The multicomponent solvent system consisted of benzene-chloroform-acetone-methanol (20 92.5 15 7.5) developed over a distance of 15 cm (2x in some cases, a third development was necessary), which enabled very good separation of coextracted compounds with both 10-DAB III... 

The general principle of temperature-dependent multicomponent solvent systems (thermomorphic solvent system (TMS)) is to conduct the hydroformylation... 

The dyes are spotted 2 cm above the edge of the plate in 2-3 pi portions of their solutions. The chromatograms are developed until the solvent front has migrated 10-12 cm. Multicomponent solvent systems should be thoroughly shaken in a separatory funnel before use. 

This served to further fractionate the azo pigments of the B type as obtained from step II. Reverse-phase partition chromatography columns were prepared from silicone-treated celite and from a multicomponent solvent system, pH 3.4. The latter contained n-octanol, diisopropyl ether, ethyl acetate, methanol, and aqueous 0.2 M acetic acid in proportions of 1 2 2 3 4 by volume. 

The complex distribution system that results from the frontal analysis of a multicomponent solvent mixture on a thin layer plate makes the theoretical treatment of the TLC process exceedingly difficult. Although specific expressions for the important parameters can be obtained for a simple, particular, application, a general set of expressions that can help with all types of TLC analyses has not yet been developed. One advantage of the frontal analysis of the solvent, however, is to produce a concentration effect that improves the overall sensitivity of the technique. 

Very little work has been done in this area. Even electrolyte transport has not been well characterized for multicomponent electrolyte systems. Multicomponent electrochemical transport theory [36] has not been applied to transport in lithium-ion electrolytes, even though these electrolytes consist of a blend of solvents. It is easy to imagine that ions are preferentially solvated and ion transport causes changes in solvent composition near the electrodes. Still, even the most sophisticated mathematical models [37] model transport as a binary salt. 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

The mobile phase plays an important role in the separation of components. Often multicomponent mixtures are required to achieve the desired separations. Much work has been done on the TLC separation of, for example, amino acids. Numerous solvent systems have been developed for such purposes, and more than one solvent system is usually necessary before separation of all of the components is achieved. Two-dimensional chromatography is often required such a separation is shown in Fig.2.2. 

There are several common forms of solid epoxy adhesives. These include film, tape, powder, and preformed shapes. Certain formulations are better suited for specific forms. For example, casting of tape or film adhesive from solvent solutions lends itself to working with multicomponent hybrid systems, where each resin can be solubilized and blended together in a universal solvent. B-staged systems are generally more brittle and better suited for powders or preformed adhesives. 

The hydrophilic delivery system described in this review can be extended to drugs with a low water-solubility (e.g., doxorubicin). Such compounds may be incorporated in CT/TPP nanoparticles by means of dextran sulfate complex prior to entrapment [54] or by dissolving them in a polar solvent (acetone, ethanol or acetonitrile) as demonstrated for the relatively hydrophobic peptide cyclosporin A [26,81]. It is quite possible that this approach would work in a multicomponent polymer system as well. 

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological properties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. 

The aim of the present paper is (a) to derive relations for the activity coefficients in multicomponent mixtures in terms of the KB integrals, (b) to obtain on their basis an expression for the solubility of a solute in an ideal multicomponent solvent, (c) to use the obtained equations to predict the solubilities in real systems, and (d) to compare the predicted solubilities with the experimental ones. 

The fundamental principle of separation for modem DuCCC is identical to classic countercurrent distribution. It is based on the differential partitions of a multicomponent mixture between two countercrossing and immiscible solvents. The separation of a particular component within a complex mixture is based on the selection of a two-phase solvent system, which provides an optimized partition coefficient difference between the desired component and the impurities. In other words, DuCCC and HSCCC cannot be expected to resolve all the components with one particular two-phase solvent system. Nevertheless, it is always possible to select a two-phase solvent system, which will separate the desired component. In general, the crude sample is applied to the middle of the coiled column through the sample inlet, and the extreme polar and nonpolar components are readily eluted by two immiscible solvents to opposite outlets of the column. 

Rf values can be disturbed by side effects or demixing of the multicomponent solvent used. In order to obtain reproducible Rf values, much attention must be placed on the reproducibility of the system. 

Systematic studies on the selection of the best mobile phases to assure the best micropreparative separation of analyzed taxoids, especially of 10-DAB III, as well as its less polar derivatives baccatin III, paclitaxel, and cephalomannine obtained from the extracts of fresh and dried needles and stems of Taxus baccata L. by Glowniak and Mroczek have been undertaken [2]. The TLC investigation on silica gel included solvent systems with one and two polar modifiers, multicomponent mobile phases, as well as some multiple development techniques and gradient elution. As polar modifiers, methanol, acetone, dioxane, ethyl acetate and ethylmethylketone, as well as their mixtures, have been reinvestigated, but dichloromethane, chloroform, benzene, toluene, heptane, and their mixtures were used as solvents. 

Isolation Process in the Cross-Over Region. Chimowitz and Pennisi (13) developed a process for the separation and isolation of components from mixtures by operating in the multicomponent temperature-solubility cross-over region. The cross-over point of a pure component (dy/dT)p = 0 represents the pressure at which the dependence of solubility on temperature reverses itself. At lower pressures, the solubility is principally dependent on solvent density - raising temperature decreases density and thus solubility decreases. At higher pressures, solubility is principally dependent on solute sublimation pressure raising the temperature increases sublimation pressure and thus solubility increases. The cross-over point is therefore unique for each solute-solvent system. When there are two solutes, cross-over points occur at different pressures. At an intermediate pressure, the temperature can therefore be manipulated to deposit either component. 

The results of the crystallization trials are summarized in Table 11-3. The acetone/water binary phase diagram was used to approximate equilibrium conditions for the solvent systems at the freeze crystallization conditions in the absence of a multicomponent phase diagram for the acetone/water/imipe-nem/NaHCOp system. 

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