Bonded Phases - Characteristics of Bonded Phases
Characteristics of Bonded Phases
To render the silica dispersive in character, the interactive surface must be chemically modified. For example, appropriate hydrocarbon moieties could be chemically linked to the surface hydroxyl groups. The first to attempt to bond an organic group to silica gel for chromatographic purposes was made by Halasz and Sebastian (1) who, incidentally, were the pioneers of bonded phases, The authors attached aliphatic hydrocarbon chains to the surface by means of the silicon-oxygen-carbon linkage. Coincidentally, Simpson (2) filed a patent for the synthesis of an analogous product and was granted a patent in 1975. Simpson, however, did not publish any chromatographic properties or other details, but did report some particulars of the material and examples of its use at an Informal Symposia held by the Chromatography Discussion Group in 1969. The procedure employed by Halasz was to reflux the silica gel with a high molecular weight aliphatic alcohol. This process esterified the hydroxyl groups of the silicic acid, and, as a consequence, aliphatic silyl ester groups were attached to the surface of the silica gel. One of the alcohols employed by Halasz was a low molecular weight polyethylene glycol.
The silicon-oxygen-carbon linkage is, however, very weak and the bonded phase of Halasz readily hydrolyzed from the surface, thus, regenerating the original hydroxyl groups of the silica gel. Nonetheless, the simple aliphatic moiety was sufficiently stable to give Halasz sufficient operating time to identify the desirable chromatographic properties of such phases and his discovery is, without doubt, one of the more important contributions to the evolution of LC.
Shortly after the work of Halasz, Kirkland (3) described an alternative bonding method that involved the use of chlorsilane reagents. When a chlorsilane reacts with a hydroxyl group of the silica gel surface, the hydrocarbon chain is attached by the much stronger and stable silicon-carbon link.
Treating silica, dried at 150oC with dimethyloctylchlorsilane that has been dissolved in a suitable solvent and reacted at elevated temperatures results in a dimethyloctylsilyl group being attached to the surface by the stable silicon-carbon bond. The product is far more stable than the carbon-oxygen-silicon bond, but even this bond is easily broken at extremes of pH.
Nevertheless, the silicon carbon bond has become the standard for the production of all classes of bonded phases. There are other methods of synthesis and there is now a wide range of silane reagents from which to choose. The alkoxyalkylsilanes reagents are of particular interest and react in a similar manner to the chlorsilanes but, as one would anticipate, under somewhat different reaction conditions. The reagents most commonly used are the ethoxy- and methoxysilanes as they are the most reactive of the alkoxysilanes and, consequently ideal for bonded phase synthesis.