Modified silica gels

The reseai ch has been carried out by the liquid chromatograph Perkin-Elmer (Series 200), which has tandem detectors the diode array (X=210 nm) and the refractometer. The temperature of a column was 30 C, speed of a mobile phase is 1.5 ml/ min. As a mobile phase, mixtures of solvents methanol - water and acetonitrile - water with addition of sodium perchlorate. The columns with the modified silica gel C8 and Cl8 (4.6x220 mm, 5 pm) were used for sepai ation of the AIST and FAS components. In order to make the identification of AIST and FAS components more reliable the ratio of the values of the above-mentioned detectors signals of each substance analyzed. 

Fig. 1 Schematic view of the surface-modified silica gels at present commercially available.

In the case of substances whose structures are pH-dependent (e.g. phenols, carboxylic and sulfonic acids, amines etc.) it is possible to produce fluorescences or make them disappear by the deliberate manipulation of the pH [213] (Table 20). Shifts of the positions of the absorption and emission bands have also been reported. This is particularly to be observed in the case of modified silica gels, some of which are markedly acidic or basic in reaction (Table 25). 

With regard to the resolution of enantiomers, some applications can be found with modified silica gel supports. Thus, a Pirkle-type CSP was used for the separation of 200 mg of a racemic benzodiazepinone [75]. Also tris-(3,5-dimethylphenyl)carba-mate of cellulose coated on silica [91, 92] was applied successfully to the resolution of the enantiomers of 2-phenoxypropionic acid and to oxprenolol, alprenolol, propranolol among other basic drugs. However, the low efficiency of this technique and the relative high price of the CSPs limits its use to the resolution of milligram range of sample. 

The dried polyoxazoline-modified silica gel was immersed into distilled water. The adsorption property of the resulting gel was estimated by the water content. The water uptake was calculated from an expression of (W -W)jW, where Wis the weight of dried gel and W is the weight of water-absorbed gel. The modified gel showed a higher water-adsorption property than that of untreated silica gel, which absorbed 10.8 multiples of water. The water uptake of modified gel was up to 13.7 multiples of the weight of dried gel. Thus, silica gel has been made more hydrophilic by a polyoxazoline segment. 

Table 6. Preparation and water adsorption property of POZO modified silica gel...

These poly(2-alkyl-2-oxazoline) silane coupling agents were copolycondensed with tetraethoxysilane by acid-catalyst to produce poly(2-alkyl-2-oxazoline)-modified silica gel. The composite gel from 2-ethyl-2-oxazoline was also homogeneous and transparent glass. Poly(2-alkyl-2-oxazoline)-modified silica gels, especially gels based on poly(2-ethyl-2-oxazoline) absorbed water and also organic solvents such as DMF or alcohols as shown in Table 7. This result means that the obtained composite gel shows the amphiphilic adsorption property. 

The most frequently used sorbent type for straight phase chromatographic systems in planar chromatography is porous, nonsurface-modified silica gel. 

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. 

The original catalyst for this reaction is Pd(PPh3)4. Heterogenization of this catalyst was attempted by grafting it onto functionalized polystyrene or modified silica gels,185 which increased significantly the selectivity for the reaction of 3-acetoxy-5-carbomethoxy-l-cyclohexene with diphenylamine. Data concerning the stability of these solids are, however, not reported. 

Attachment of the Fe(n) complex of tetraphenylporphyrin to a modified silica gel support also resulted in an efficient dioxygen carrier (Basolo, Hoffman Ibers, 1975). In this system the silica gel was modified by direct attachment of imidazoyl-propyl groups to its surface. 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

The racemic ethyl ester of 1727 (R = Et) was resolved by HPLC with modified silica gels [89JAP(K)261380]. 

Surface-modified silica gels are used for a variety of separations, but organic polymer-based stationary phase materials are more useful for long-term operations, such as for an amino acid analyser and size-exclusion liquid chromatography. 

Galli, B. et al., Enantiomeric separation of DNS-amino acids and DBS-amino acids by ligand exchange chromatography with (5) - and (R)-phenylalaninamide modified silica gel, J. Chromatogr. A, 666,11, 1994. 

Srinivasan, G., Neumann-Singh, S., and Muller, K., Conformational order of n-alkyl modified silica gels as evaluated by Fourier transform infrared spectroscopy, J. Chromatogr. A, 1074, 31, 2005. 

Srinivasan, G, Sander, L.C., and Muller, K., Effect of surface coverage on the conformation and mobility of C-18-modified silica gels, AnaZ. Bioanal. Chem., 384, 514, 2006. 

Packed columns Porous and non-porous silica Lipophilic modified silica gels, e.g., C18 Mixed mode phases, e.g., C8/SCX ... 

Zimina, T. M., Smith, R. M., and Myers, P. (1997). Comparison of ODS-modified silica gels as stationary phases for electrochromatography in packed capillaries. /. Chromatogr. A 758, 191-197. 

Some successful attempts to immobilize catalysts for the oxidation of alcohols to carbonyl compounds involve the attachment of TEMPO-derivatives to a solid phase. Bolm et al. were the first to immobilize l-hydroxy-2,2,6,6-tetramethylpiperi-dine to modified silica gel (SG-TMP-OH) (11) and applied in the oxidation of multifunctional alcohols [68]. Other groups further investigated the use of polymer-supported TEMPO [69]. This system allowed the oxidation of alcohols to aldehydes and ketones, respectively, using bleach to regenerate the immobilized ni-troxyl radical (Scheme 4.6). 

When amine-modified silica-gel columns are eluted with a mobile phase of acetonitrile-water, carbohydrates are separate on the basis of normal, phase partitioning - increasing the water content of the mobile phase... 

---