Secondary solvent effects

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

Retention on these supports is adaquetely described by the adsorption displacement model. Nevertheless, the adsorption sites are delocalized due to the flexible moiety of the ligand, and secondary solvent effects play a significant role. The cyano phase behaves much like a deactivated silica toward nonpolar and moderately polar solutes and solvents. Cyano propyl columns appear to have basic tendencies in chloroform and acidic tendencies in methyl tertiobutyl ether (MTBE)... 

Xanthone, flavone and similar compounds. A difference in chloroform specificity from 2-methoxyethanol is again demonstrated In Figure 11. Xanthydrol is eluted before xanthone and flavone with chloroform as the modifier. Perhaps this shows the useful coupling of a proton donor solvent and proton acceptor solute (a large secondary solvent effect in an adsorption system (43)), the Interaction that was hoped for with the selection of chloroform as one of the selectivity triangle modifiers and apparent here because of a less strong adsorption of the xanthydrol on the fully active silica than some of the other basic solutes used in the preliminary studies. 

The chemical description of this interaction is still to be determined. It appears that there exists some threshold solvent power (defined either by the pure carbon dioxide density or the modifier identity and concentration in a modifier/carbon dioxide mixture) at which the solvent can begin to compete successfully with a particular stationary phase for a particular solute. Whether this involves a deactivation of active sites amenable to specific solute adsorption on the silica surface or a secondary solvent effect (43) where the mobile phase interacts with the solute as well as with the adsorption surface is unknown. 

Equation (8-3) can be corrected for secondary solvent effects by addition of a correction term A,... 

Some interesting secondary solvent effects have been noted for adsorption of the polycyclic aromatic hydrocarbons and certain of their derivatives on alumina 14). The more nearly linear isomers (e.g., anthracene relative to phenanthrene, 2-bromonaphthalene relative to 1-bromonaph-thalene) are preferentially adsorbed from most solvents, owing to the apparent weak localization of these compounds on linear site complexes see Section 11-2B, There is also a tendency for the preferential adsorption of strong solvent molecules on these same linear site complexes, with the result that strong solvents (or their solutions in weaker solvents) behave as selectively stronger solvents toward the preferentially adsorbed linear aromatics, relative to less linear isomers. As a consequence the ratio of values for two such isomers varies sharply with the solvent used, despite the fact that Eq. (8-3) predicts that this ratio should remain constant for all solvents i.e., A. is generally constant for two or more isomers. Jn extreme cases the ratio of K" values for two isomers of this type can be varied by a factor of 10 or more, depending upon the solvent used (see Table 11-4). 

D. Some Practical Aspects of Secondary Solvent Effects... 

Secondary Solvent Effects from Liquid Phase Hydrogen Bonding in Adsorption on Silica [data of Klouwen et al. (15)]... 

It is certain that some adsorbent-sample combinations resultjnxheihical interaction or bond formation between a reactive sample gfbtlp and a-corresponding adsorbent site. If a solvent molecule is also-capable of chemical interaction with the same adsorbent site, then that solvent should appear to be abnormally strong with respect to the given sample. Some workers have extended this principle to predict, for example, that alcohols are stronger solvents for alcohol samples, amines for amine. samples, etc. No verification of this hypothesis has yet been reported, particularly where other secondary elfects (as well as primary solvent effects) have been taken into account. However, some related secondary solvent effects, which arise from the preferential adsorption of certain samples and solvents onto particular adsorbent sites, have b n observed. 

A final corollary of the importance of the strong solvent component in determining secondary solvent effects is that solutions of some strong solvent G with various weaker solvents A, B, C, etc., are likely to give similar secondary effects in every case. That is, solutions A-G, B-G, C-G, etc., of equal strength are likely to be equivalent with respect to secondary solvent effects. Hence their use as equieluotropic series should be generally unprofitable. 

These aspects are largely discussed in Ref. If (Aeas)i and (Aeas)2 refer to the secondary solvent effects for a particular sample and solvents 1 and 2, respectively, then we can write ... 

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