Solvent strength table

PROCEDURE FOR ESTIMATING SOLVENT STRENGTHS Table III-3 (Continued)... 

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. 

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. 

Solvents grouped in the same region of the triangle will have similar selectivity, whereas solvents from other groups will have different selectivity even if their solvent strengths are similar. Acetone is a solvent characterized by a polarity value of 5.1 (group VI, e). Its X values given in Table 4.2 show its polar interaction properties that involve 35% proton-acceptor, 23% proton-donor, and 42% dipole interactions. Solvents near the comers have mainly one kind of selectivity. 

Thermodynamic Properties The variation in solvent strength of a SCF from gaslike to liquidlike values (see Table 20-12) may be described qualitatively in terms of the density p, as shown in Fig. 20-17, or the solubility parameter. 

The most important features of this equation are the emergence of a eluotropic series, i.e., an eluent-strength series based on e°. The key solvent-strength parameter, e°, which relates to the eluotropic series, was calculated by Snyder (14) for a wide variety of pure solvents. Table 2 lists some values. 

TABLE 9.6 Solvents of Chromatographic Interest (arranged in order of increasing solvent strength)... 

The mechanism of iodinolysis was suggested13 to be a simple bimolecular substitution of the tetraalkyltin by iodine, proceeding by mechanism SE2(open) through a transition state such as (VIII). Evidence for an open, polar transition state of this type is provided13 by the marked increase in the value of k2 with increase in the ionic strength of the solvent medium (Table 9). 

The following table contains the common solvents used in thin-layer chromatography, with a measure of their strengths on silica gel and alumina. The solvent strength parameter, s°, is defined as the relative energy of adsorption per unit area of standard adsorbent.13 It is defined as zero on alumina when pentane is used as the solvent. This series is what was called the eluotropic series in the older literature. For convenience, the solvent viscosity is also provided. Note that the viscosity is tabulated in cP for the convenience of most users. This is equivalent to mPasec in the SI convention. Additional data on these solvents may be found in the tables on high-performance liquid chromatography. 

The strength of the solvent is defined by the solvent strength parameter, e°, as listed in Table 2.2. A solvent with a low e° is chosen, and quantities of a second solvent with a greater s° are added until the desired separation is achieved. If the desired separation does not result from altering the concentration of the second solvent, either the nature of the second solvent can be changed or another additive can be introduced. Readers are directed to Refs. 1, 6, and 7 for in-depth discussions on the development of mobile phases for normal-phase chromatography. 

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