Alumina solvent strength data

Table 6.1 Solvent Strength Data on Alumina Adsorbent...

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. 

Fig. 8. Dependence of C-solvent strength ( on coverage of adsorbent surface 0. Mixtures A/C of nonpolar solvent A and polar localizing solvent C , isopropanol/hexane/silica , acetone/hexane/silica , ethyl ether/hexane/silica/ V, ethyl ether/pentane/silica O, isopropyl ether/pentane/alumina. Data taken from Snyder and Glujch (14) and Hara et al. (25). Curves through data are best fit to Eqs. (12) and (40).

The derivation of Eq. (8-3) assumes that the solvent Strength parameter e" can assume different values for different adsorbents. The possibility of gross changes in eluotropic series between different adsorbents is likewise not precluded. Table 8-2 presents some quantitative e data for polar adsorbents other than alumina. While it is apparent that these e values... 

Table 8-3 presents relative eluotropic series for separations on charcoal and polyamide. The series for charcoal is almost the reverse of that shown for alumina in Table 8-1. Solvent strength on charcoal is seen to increase with the size of the solvent molecule, and there is a good correlation between solvent strength and the calculated area (A,) required by an adsorbed solvent molecule. Jermyn (24) has reported additional data on the relative strengths of various solvents for adsorption on charcoal. The...

Ref. (2)]. While a dependence of e" on solvent dipole moment or dielectric constant has been refuted for numerous experimental adsorption systems [c.g.. Ref. (7)], the belief that dielectric constant defines both solvent strength and sample adsorption energy continues to find support by many workers. The dependence of e values on solvent dielectric constant is tested in Fig. 8-13 for adsorption on alumina. There is a tendency for solvent strength to follow solvent dielectric constant, but the exceptions to this relationship are both numerous and large (e.g., dioxane, e = 0.56 and dielectric constant = 2.2, versus methylene chloride, e = 0.42 and dielectric constant = 9.1). Consequently, the dielectric constant of the solvent is quite limited in its ability to predict solvent strength. The data of Fig. 8-13 do not support any fundamental relationship between solvent strength and dielectric constant. 

Solvent polarity parameters derived from Rohrshneider data. Solvent strength parameter for NPLC on alumina. 

E° (solvent elution strength as defined by Hildebrand on alumina). Data extracted from reference 2 and other sources. 

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