Silica, solvent strength parameters

Recently, Janjic et al. published some papers [33-36] on the influence of the stationary and mobile phase composition on the solvent strength parameter e° and SP, the system parameter (SP = log xjx, where and denote the mole fractions of the modiher in the stationary and the mobile phase, respectively) in normal phase and reversed-phase column chromatography. They established a linear dependence between SP and the Snyder s solvent strength parameters e° by performing experiments with binary solvent mixtures on silica and alumina layers. 

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 solvent-strength parameters for the common solvents used in normal phase chromatography on carbon are quite different from those of silica or alumina. Thus carbon offers quite different selectivities than alumina and silica for normal-phase chromatography. However, the lack of a reproducible commercial source for carbon was for many years a significant limitation to its widespread application. In addition the sensitivity of carbon to changes in solvent strength is much less than that of silica or alumina. 

Values of (the solvent strength parameter) for pure solvents on alumina can be obtained from Table 8-1. Similar values for adsorption on silica or other adsorbents are given in Tables 8-2 and 8-3, or can be estimated from e values for alumina through Eqs. (8-6a)-(8-6c). Values of e for a few binary solvents are listed in Appendix III. Other values can be calculated through Eq. (8-10) ... 

Note Solvent classification into groups based on solvent polarity selectivity parameters proton acceptor, proton donor, x dipole interactors) and solvent strength on alumina nd on silica gel 0. Physical constants viscosity (t)), surface tension (y), dielectric constant (8). Solvatochromic polarity parameters 7, j.(30) and Ej. ... 

According to eqn.(3.72) a higher value of ° will result in a lower value for the capacity factor. It can be concluded from table 3.4 that the solvent strength on silica and alumina stationary phases roughly increases with increasing polarity (6) of the solvent, but that there is no quantitative correlation between these two solvent properties. For example, ethers are much stronger solvents (especially on silica) than can be anticipated on the basis of their solubility parameters. 

Table A-11. Eluotropic solvent series for hydrophilic adsorbents such as alumina or silica, listed in order of increasing eluting power of the solvent , quantitatively measured by Snyder s empirical eluant strength parameter °. ...

The behavior of many low-polarity bonded phases and the colnmn packings based on crosslinked organic polymers (see section 11.5.2.3.2, Enthalpic Partition of Macromolecules) differs from that of polar column packings such as bare silica gel and the classical solvent strength concept shonld be re-evaluated. This is especially important for the alkyl bonded phases. In this case, both the snr-face and the interface adsorption of polymer species (see section 11.5.2.3.1, Adsorption of Macromolecules) play less important role and maeromolecules are mainly retained by the enthalpic partition (absorption) (see section 11.5.2.3.2). As explained, in order to ensure this kind of retention of maeromolecules, the mobile phase must be their poorer solvent than the solvated bonded phase. Only in that event, maeromolecules are pushed fiom the mobile phase into the station-aiy phase. Interactions of mobile phase with the bonded phase and (especially) with the sample maeromolecules largely control retention of polymers within the alkyl bonded phases. In other words, the decisive parameter that governs polymer retention in the reversed phases is the solvent quahty. 

---