Solvent critical compositions

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

As Eq. (3) sh vs, the critical composition (ticn can be controlled by the asymmetry of chain lengths. Particularly interesting is the limit Na = N, Nb = I (which physically is realized by polymer solutions, B representing a solvent of variable quality). Checking the deviations from the mean field predictions, Eq. (3), further contributes to the understanding of the statistical mechanics of mixtures. 

The composition of the nonsolvent-solvent mixture representing the critical composition of the ternary system with polymer of infinite molecular weight (see Fig. 123,a) possesses unique significance. Scott and Tompa have shown that the composition at the critical point in the limit of infinite molecular weight is specified by... 

In Fig. 17, the twisting power is plotted against the solvent composition in the dioxane-EDC mixed solvent system at various temperatures. It is clear that a linear additive law for the twisting power is realised at each temperature. The critical composition of the solvent at which compensation occurs shifts to higher dioxane content as the temperature is increased. 

Figures 7 and 8 show that k is not sensitive to pressure as the pressure is much higher than the phase separation points of the mixed solvents. The figures also demonstrated that as the composition of the mixed solvents is far from the critical composition, the effect of pressure on the h is very limited in entire pressure range. It can be concluded that, to control k effectively by pressure, both pressure and composition should be close to the critical point of the mixed solvents at this temperature. The main reason is that the effect of pressure on the density of the mixed solvents is not considerable outside the critical region of the solvents.

At the critical point the mole fraction of CO2 Xi is 0.888 (Figure 9). In Figure 9 the part of the curve with Xi < 0.888 is the bubble point curve, and a homogenous mixture above the bubble point can be regarded as a subcritical fluid. The part of the curve with X] > 0.888 is the dew point curve, and a homogeneous mixture above the dew point is a vapor or a supercritical mixture. The mixed solvent near critical region at fixed temperature is defined as the solvent of which the composition and pressure are close to the critical composition and critical pressure ofthe mixture. 

If several plates have to be developed with the same solvent system in one day, the total amount required can be prepared in one batch provided that the solvent compositions are uncritical. This is recommended even for quantitative work in order to maintain reproducible hRf values. Critical solvent systems are discussed in Section 4.1.3 Problematical Solvent System Compositions . 

Fig. 3. The phase separation process in a nematic solvent. System composition liquid crystal, 98%wt silicone oil (Aldrich), 2%wt. a Some droplets form after the quench (picture 20 s after the quench), b The droplets diffuse randomly and coalesce in the initial stages of the phase separation (picture 42 s after the quench), c Coalescence stops once a critical size is reached. The droplets begin to form small linear aggregates oriented along the alignment direction of the liquid crystal (picture 55 s after the quench), d The chains grow as time evolves (picture 120 s after the quench). Scale bar 60 im...

The critical solvent composition (esc) for PDMS standards was determined using a silica column (pore size 100 A, average particle size 5 pm, 200 x 4 mm ID) and toluene/Ao-octane mixtures. When plotting the retention factor k versus solvent/nonsolvent composition, the esc is indicated by the intersection point demonstrated in Figure 31.4a. Under these conditions, PDMS samples of different molecular weights elute at the same retention time. Any separation of an unknown sample into different peaks can be ascribed to a heterogeneity other than molecular weight distribution. 

The sense of cholesteric twist, in the liquid crystal of PBLG in dioxane, is opposed to that in CH Cl, as pointed out by Robinson. We measured the cholesteric pitch of PBLG in various mixed solvent systems, and estimated the sense of cholesteric twist in the individual solvent. If two solvents which make the sense of cholesteric twist of PBLG opposite to each other, are mixed, the cholesteric pitch of PBLG in mixed solvent will diverge at the critical composition. It is found that the sense of cholesteric twist of PBLG in dioxane and chloroform is opposite to that in dichloromethane, dichloroethane and benzene. 

This shows at once that if e>0 (const. < 1), the ratio x/x will be very small if n is large. One of the phases will then always be very poor in pol5nner substance. In particular, when we consider the equilibrium between the pure polymer (x = 1) and the solution, we will find that x is extremely small the polymer is practically insoluble. This explains why it was believed that macromolecules could be dissolved only when heat was evolved on mixing (compare p. 61). As soon as e is negative (const. > 1), the polymer shows complete miscibility. Intermediate cases of limited solubility do not exist unless e happens to be very small (critical temperature or critical composition of the solvent). With compact molecules, e.g., globular proteins, E will be proportional to the surface of the particle rather than to its mass, but qualitatively the result will be exactly the same. 

As a rule, in a mixed mobile phase a solvent peak appears near the void volume of the column. The appearance of the solvent peak may due to one of several effects, the first of which is the preferential solvation of polymers [172]. After dissolution in a mixed solvent, the polymer binds into its solvation shell one part of mixture to a larger extent. After the separation of the solvated polymer from rest of injected solvent, the solvent peak appears on chromatogram as was demonstrated by SEC [172] and under suitable condition [173] its area, or height, may be correlated with coefficient of preferential solvation [ 172]. An evaporation of one component from the sample bottle or displacement effects may also lead to appearance of a solvent peak [173]. The solvent peak represents a local change of composition of the mobile phase. Under critical conditions small changes of the mobile phase composition (for example, 0.1 % wt.) have a large influence on polymer retention, thus the solvent peak could influence the elution of the macromolecules. If so, this could imply that a tabulated critical composition is not precisely that, which really correspond to the critical conditions. The real, acting critical composition of eluent may be, and likely is, the composition somewhere, in the middle, of the solvent peak. The presence of the solvent peak influences especially pronouncedly the elu-... 

For polymer-solvent systems, the critical composition occurs at low polymer concentrations. This results from the large volume of the polymer molecule compared with the solvent molecule. One consequence is that even the polymer-rich phase is relatively dilute. 

Simplified approaches could be taken [65] that neglected items 4 and 5 above and considered the system as a pseudobinary (diffusion of solvent into a water bath, and vice versa). Paul used these and the assumption that any phase change was instantaneous when a critical composition was reached to try to treat the... 

Numerous applications have been developed for a wide variety of compounds from different matrices, but surprisingly, only a few reviews dedicated to the extraction of medicinal plants have been pubhshed in the last few years [37-39]. ASE of cocaine and benzoylecgonine from coca leaves has been reported by Brachet et al. [40]. The influence of several extraction parameters such as the nature of the extracting solvent, the addition of alkaline substances, the pressure, the temperature, the extraction time, and the sample granulometry on cocaine recovery was systematically investigated. Methanol was fotmd to be the most suitable solvent. Critical parameters were found to be pressure, temperature, and extraction time. A central composite design has been used to optimize these 3 parameters and to assess the robustness of the extraction method. The optimal conditions for the quantitative extraction of cocaine from leaves were the following 20 MPa, 80 °C, 1 mL min , 10 min extraction time, with a particle size distribution between 90 and 150 pm. 

The vapour/liquid critical locus curve for Class B systems, instead of being continuous as in Figure 1.5, breaks into two parts as in Figure 1.9. The part originating at the solvent critical point (cs) terminates at the point U at which the composition of the solvent-rich liquid phase in the three phase mixture merges with that of the vapour. This part of the critical locus is marked branch I on Figure 1.9. It is very limited in extent for the CO2/H2O and C02/n-Ci6H32 systems shown, but is more extensive in some other systems. 

In Class B1 systems branch II of the critical locus spans the entire temperature range from the critical temperature of the heavy component (if this is accessible without decomposition) down to and below the critical temperature of the solvent, as in curves (a) and (b) in Figure 1.9. On raising the pressure at a constant temperature which is above the solvent critical temperature, complete miscibility between the liquid and supercritical fluid phases occurs at the pressure (the critical pressure) corresponding to this temperature on the locus curve. The dew- and bubble-point curves then merge giving a closed loop pressure/composition diagram. 

Class B1 systems show closed loop vapour/liquid pressure/composition diagrams in the vapour liquid region at all temperatures between the solvent critical temperature and the critical temperature of the heavy component. The system ethane/methanol shows this behaviour. Carbon dioxide/w-hexadecane is probably also of this type (Figure 1.10 and 1.11). 

As seen in chapter 1 the system carbon dioxide/water is a Class B2 system (Figure 1.6) and this is almost certainly true also of mixtures of carbon dioxide with the natural oils (Figure 1.8). Such systems show low mutual solubilities with the liquid solvent below its critical temperature and form open loop pressure/composition diagrams for a small range of temperatures above the solvent critical temperature. 

The critical behavior has been studied in details for polymer blends in a common good solvent. The composition fluctuations play an important role and the critical behavior is not of the mean field type except in the limit of... 

In this article we considered thermal composition fluctuations in binary polymer blends under various conditions. Samples of critical composition were studied in temperature and pressure fields, with small additions of a non-selective solvent, or in mixtures with a symmetric diblock copolymer with the same monomers as the homopolymers. Blends of critical composition were chosen in order to follow the fluctuations up to the critical point which represents the stabiUty limit of miscibility. The strength of thermal fluctuations is estimated by the Ginzburg criterion which in the incompressible limit follows the universal scaling law l/U and predicts that binary polymer blends... 

Pollution Prevention. Procedures haven been developed for recovery of composite ammonium perchlorate propellant from rocket motors, and the treatment of scrap and recovered propellant to reclaim ingredients. These include the use of high pressure water jets or compounds such as ammonia, which form fluids under pressure at elevated temperature, to remove the propellant from the motor, extraction of the ammonium perchlorate with solvents such as water or ammonia as a critical fluid, recrystalli2ation of the perchlorate and reuse in composite propellant or in slurry explosives or conversion to perchloric acid (166,167). 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

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