Nonmodified silica gel

Chloroform has lower affinity toward silica gel-based adsorbents than THF. PMMAs were fully retained from this solvent on the nonmodified silica gel even in the presence of a 1% ethanol stabilizer (14). It has been shown that PMMAs were not retained even within the most interactive PS/DVB column using chloroform stabilized with 1% ethanol. Chloroform is considered a weak displacer. 

Nonmodified silica gel is used most commonly for the separation of substances of medical interest. The separation is based on the interactions (hydrogen bonding, van der Waals forces, and ionic bonding) between the molecules of drugs, lipids, bile acids, etc., and the silica gel. Alumina has similar properties but is rarely used. Successful separation of endogenous substances, drugs, or their metabolites can also be achieved using physically or chemically modified silica gel. 

As a medium strength liquid (Table 16.1), THF is commonly used also in the coupled methods of polymer HPLC. It promotes desorption of medium polar polymers such as poly(acrylate)s and poly(methacrylate)s including poly(methyl methacrylate) from the nonmodified silica gel. Other strong(er) solvents widely used in the coupled polymer HPLC methods are acetonitrile that exhibits high UV transparency, and dimethyl formamide. The latter solvent readily decomposes into amine and formic acid and its strength may differ from batch to batch. 

Thin-layer chromatography (TLC) and its refined version, high-performance thin-layer chromatography (HPTLC), are established separation techniques in synthetic organic chemistry and natural product chemistry, whereby lipids in particular are traditionally investigated by TLC [1-4]. Nonmodified silica gel is used in the majority of cases [2], so we will discuss here exclusively this type of stationary phase. 

In the case of nonmodified silica gels, adsorption of the test substances by the stationary phase is the decisive retention mechanism for chromatographic separation. Selective interactions of the sample molecules to be separated take place at the active surface centers of the silica gel. Forces that affect interactions include hydrogen bonding, dipole-dipole, and electrostatic interactions. The intensity of these forces depends on the following factors ... 

Supercritical alcohol drying typically induces hydrophobic properties even in nonmodified (hydrophobized) silica gels. At the high process temperatures and pressures, free silanol groups react with ethanol, leaving behind mostly alkoxy-decorated gel. This surfece chemistry, however, is not hydrolytically stable under normal ambient conditions. A TEOS-based HTSCD aerogel dried in supercritical ethanol loses its hydrophobicity within a few hours when stored in ambient air. 

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