Silanols, silica surfaces

The synthesis of MCM-41 structure in fluoride medium has been reported by Silva and Pastore [10], who used sodium silicate as silica source and carried out crystallization at 150 °C. They suggested the behaviour of Si02-CTMA+-F system was significantly different from that reported previously on the mesophase system. It is known that fluoride ions do influence the nature, activity and polymerizing capacity of silica precursors, and a fluorinated silica surface is much more hydrophobic and more resistant to the attack of water molecules than a silanol silica surface [11]. 

Fig. 1. Silanol groups of amorphous silica surface, where 0= Si Q — O and = H (a) isolated, (b) vicinal, and (c) geminal.

Silica gel, per se, is not so frequently used in LC as the reversed phases or the bonded phases, because silica separates substances largely by polar interactions with the silanol groups on the silica surface. In contrast, the reversed and bonded phases separate material largely by interactions with the dispersive components of the solute. As the dispersive character of substances, in general, vary more subtly than does their polar character, the reversed and bonded phases are usually preferred. In addition, silica has a significant solubility in many solvents, particularly aqueous solvents and, thus, silica columns can be less stable than those packed with bonded phases. The analytical procedure can be a little more complex and costly with silica gel columns as, in general, a wider variety of more expensive solvents are required. Reversed and bonded phases utilize blended solvents such as hexane/ethanol, methanol/water or acetonitrile/water mixtures as the mobile phase and, consequently, are considerably more economical. Nevertheless, silica gel has certain areas of application for which it is particularly useful and is very effective for separating polarizable substances such as the polynuclear aromatic hydrocarbons and substances... 

Porous silica packings do, however, sometimes suffer from adsorption between the sample and silanol groups on the silica surface. This interaction can interfere with the size exclusion experiment and yield erroneous information. In many cases, this problem is easily overcome by selecting mobile phases that eliminate these interactions. In addition, the surface of porous silica packings is routinely modified in order to reduce these undesirable interactions. Trimeth-ylsilane modified packing is typically used with synthetic polymers. Diol modified packing is typically used with proteins and peptides. 

Protein-Pak packings are designed for the size exclusion chromatography of proteins and related compounds. They are based on silica, which is deactivated with glycidylpropylsilane. The diol function prevents the interaction of the target analytes with the silica surface. However, because coverage of the silica surface is always incomplete, residual acidic silanols can interact with the analytes. For this reason, most applications are carried out with a salt concentration above 0.2 mol/liter, which eliminates the interaction of analytes with surface silanols. Protein-Pak packings are stable from pH 2 to pH 8. 

Problems with adsorption onto the packing material are more common in aqueous GPC than in organic solvents. Adsorption onto the stationary phase can occur even for materials that are well soluble in water if there are specific interactions between the analyte and the surface. A common example of such an interaction is the analysis of pEG on a silica-based column. Because of residual silanols on the silica surface, hydrogen bonding can occur and pEG cannot be chromatographed reliably on silica-based columns. Eikewise, difficulties are often encountered with polystyrenesulfonate on methacrylate-based columns. 

Figure 8 (a) Schematic diagram showing distribution of fillers in different parts of anionic elastomer [27]. (b) Proposed structural model showing interaction of silanol groups on silica surface with carboxylale groups [27]. 

An understanding of the surface chemistry of silica is required to interpret its chromatographic properties. The silica surface consists of a network of silanol groups, some of which may. be hydrogen bonded to water, and siloxane groups, as shown in Figure 4.2. A fully hydroxylat silica surface contains about 8... 

Exhaustive silanization with the most active of silanizing reagents cannot eliminate completely all silanol groups on the i silica surface. Encapsulating the deactivated surface by, ... 

Chemically bonded layers are prepared by reacting silica gel with various functionalized organosilane reagents forming siloxane Ssonds with some of the silanol groups originally present on the silica surface (10,23,71-77). Chemically bonded siloxane layers... 

Figure 2.6 Reagents used for the deactivation of silanol groups on glass surfaces. A - disilazanes, B > cyclic siloxanes, and C -silicon hydride polysiloxanes in which R is usually methyl, phenyl, 3,3,3-trifluoropropyl, 3-cyanopropyl, or some combination of these groups. The lover portion of the figure provides a view of the surface of fused silica with adsorbed water (D), fused silica surface after deactivation with a trimethylsilylating reagent (E), and fused silica surface after treatment with a silicon hydride polysiloxane (F).

Before reaction, the silica is treated with acid (eg refluxed for a few hours with 0.1 mol dm-3 HC1). This treatment produces a high concentration of reactive silanol groups at the silica surface, and also removes metal contamination and fines from the pores of the material. After drying, the silica is then refluxed with the dimethylchlorosi-lane in a suitable solvent, washed free of unreacted silane and dried. This reaction produces what is called a monomeric bonded phase, as each molecule of the silylating agent can react with only one silanol group. 

---