Silica hydrogen bonding

The adsorption of water on a fully hydroxylated silica involves hydrogen bonding but is essentially physical in nature and is completely reversible in the low pressure range the isotherm is of Type II on a nonporous sample (Fig. 5.17(a)), and of Type IV, with no low-pressure hysteresis, on a porous sample (Fig. 5.18). 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

The amino group of the silane was also shown to hydrogen bond with silica surfaces. 

The reasons for the above phenomena are to be found in differing configurations of hydrogen bonds, the effect of pH, differences in the structures of fluorescence indicators and binders and differences in surface area. For example, silica gel 60 possesses a surface area of 500 m /g [211] while that of Si 50 000 lies below 5 m /g [212],... 

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. 

This relatively new class of CSPs incorporates glycopeptides attached covalently to silica gel. These CSPs can be used in the normal phase, reversed phase, and polar organic modes in LC [62]. Various functional groups on the macrocyclic antibiotic molecule provide opportunities for tt-tt complexation, hydrogen bonding, and steric interactions between the analyte and the chiral selector. Association of the analyte... 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

Likewise, synthetic 2//-azepines isomerize to 3//-azepines in refluxing chloroform (2-3 h) or in tert-butyl methyl ether at room temperature.291 The isomers can be readily separated by chromatography on silica gel, as the more basic 2//-azepines30 have lower Rf values. In contrast, 7-butyl-2//-azepin-2-acetic acid (11), obtained by heating the tert-butyl ester 10 with iodotrimethylsilane, is stabilized by intramolecular hydrogen bonding and shows no tendency to rearrange to the 3//-isomer.291... 

Kagel (29) found that whereas pyridine is hydrogen bonded to a silica gel surface 2-chloropyridine is not, the spectrum of 2-chloropyridine adsorbed on silica gel being identical with that of the liquid, and concluded that steric hindrance probably prevents hydrogen bond formation in this case. 

Separation by adsorption chromatography takes place preferentially as a result of hydrogen bonding or dipole-dipole interactions. Hence, separation of mixtures of substances on silica gel layers by lipophilic solvents primarily takes place according to polarity differences. Further separation within a polarity group can then be achieved either two-dimensionally or off-line by partition chromatography on anotho TLC plate (Fig. 4). 

The model in Figure 3.20a applies to ACM-silica, while Figure 3.20b suits the ENR-silica system. Kraus constant, C, determined from the slope of the plots in Figure 3.19, quantifies the mbber-silica interaction in these systems. CACM/siUca is 1-85 and CENR/siika is 2.30 and these values are significantly higher than the reinforcing black-filled mbber composites [65]. Hydrogen-bonded interaction between the SiOH and the vicinal diols in ENR is responsible for this, whereas dipolar interaction between ester and SiOH in ACM-silica only results in weaker adsorption of the mbber over the filler surfaces. 

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