Solvents strength

Solvent Boiling point, °c Solvent strength parameter ... 

Solvent Boiling point, °C Solvent strength parameter Viscosity, mN s m (20°C) Refractive index (20°C) UV cutoff, nm... 

The process of changing the mobile phase s solvent strength to enhance the separation of both early and late eluting solutes. 

Organic Solvent/H20 Mobile-Phase Compositions Having Approximately Equal Solvent Strength... 

Solvent Strength of Pure Fluids. The density of a pure fluid is extremely sensitive to pressure and temperature near the critical point, where the reduced pressure, P, equals the reduced temperature, =1. This is shown for pure carbon dioxide in Figure 2. Consider the simple case of the solubihty of a soHd in this fluid. At ambient conditions, the density of the fluid is 0.002 g/cm. Thus the solubiUty of a soHd in the gas is low and is given by the vapor pressure over the total pressure. The solubiUties of Hquids are similar. At the critical point, the density of CO2 is 0.47 g/cm. This value is nearly comparable to that of organic Hquids. The solubiHty of a soHd can be 3—10 orders of magnitude higher in this more Hquid-like CO2. 

A paiticularly attiactive and useful feature of supeicritical fluids is that these materials can have properties somewhere between those of a gas and a hquid (Table 2). A supercritical fluid has more hquid-hke densities, and subsequent solvation strengths, while possessiag transport properties, ie, viscosities and diffusivities, that are more like gases. Thus, an SCF may diffuse iato a matrix more quickly than a Hquid solvent, yet still possess a Hquid-like solvent strength for extracting a component from the matrix. 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Adjustable solvent strength to tailor selectivities and yields. 

Extremely wide variation in solvent strength with temperature and pressure. Gollapse of structure due to capillary forces is prevented during solvent removal. 

Thermodynamic Properties The variation in solvent strength of a supercritical fluid From gaslike to hquidlike values may oe described qualitatively in terms of the density, p, or the solubihty parameter, 6 (square root of the cohesive energy density). It is shown For gaseous, hquid, and SCF CO9 as a function of pressure in Fig. 22-17 according to the rigorous thermodynamic definition ... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Analytical Supercritical Fluid Extraction and Chromatography Supercritical fluids, especially CO9, are used widely to extrac t a wide variety of solid and hquid matrices to obtain samples for analysis. Benefits compared with conventional Soxhlet extraction include minimization of solvent waste, faster extraction, tunabihty of solvent strength, and simple solvent removal with minimal solvent contamination in the sample. Compared with high-performance liquid chromatography, the number of theoretical stages is higher in... 

SCR chromatography due to the more favorable transport rates. A limitation in each of these apphcations is the low solvent strength of CO9 often cosolvents are required. 

Solvent systems for Neoprene - Predicting solvent strength. Du Pont Elastomers Bulletin. 

As we continue lowering the pressure, GC is the final limiting case when the mobile phase has zero solvent strength over the entire column length and where temperature is the only effective control parameter. Gas chromatography is shown in Figure 7.3. 

D development on the same monolayer stationary phase with mobile phases characterized by different total solvent strength (5t) and selectivity values (5y) ... 

These workers used binary solvent systems over a range of mole fractions to determine, for each solute, the constants a and b of equation (8.2). For methyl and phenacyl esters, TLC was used, while overpressured layer chromatography (OPLC) was used for dansyl amino acids. Nurok and co-workers (11) also evaluated how the quality of a simulated separation varies with changing solvent strength by using the inverse distance function (IDF) or planar response function (PRF), as follows ... 

When multiple development is performed on the same monolayer stationary phase, the development distance and the total solvent strength and selectivity values (16) of the mobile phase (17) can easily be changed at any stage of the development sequence to optimize the separation. These techniques are typically fully off-line modes, because the plates must be dried between consecutive development steps only after this can the next development, with the same or different development distances and/or mobile phases, be started. This method involves the following stages ... 

Clearly, the number of re-chromatography steps, the development distance, and the total solvent strength and/or selectivity value of the mobile phase can be freely varied, depending on the separation problems (19), as summarized in Table 8.1. 

The basis of automated multiple development (AMD) is the use of different modes of multiple development in whieh the mobile phase eomposition (5j and Sy values) is ehanged after eaeh, or several, of the development steps. Figure 8.11 illustrates the prineiple of AMD employing a negative solvent-strength gradient (deereasing 5-p values). 

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