Silicas adsorption behaviour

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

Takahashi, Y., Minai, Y, Ambe, S., Makide, Y. Ambe, F. 1999. Comparison of adsorption behaviour of multiple inorganic ions on kaolinite and silica in the presence of humic acids using the multiple sorption tracer technique. Geochimica et Cosmochimica Acta, 63, 815-836. 

C-155 and S-155 materials showed a sponge like structure as the microphotograph of figure 4-B, but when S-155 was submitted to methylene blue adsorption test it did not show adsorption properties C-155 material instead showed a high adsorption activity. The last observation demonstrates that carbon structure is the responsible of the adsorptive behaviour and that the silica structure in this case acts only as an inert skeleton, but at the same time, silica was the responsible of the expanded carbon network developed during the synthesis of the composite. 

In view of the complexity of the structure of most silica xerogels, it is to be expected that their adsorptive behaviour would be equally complex. The following... 

Figure 2 Adsorption behaviour of inhibitors on silica. (A) wfAFP (V) and (B) PVP (n) at 25 fjM adsorb to the Si02 surface, represented as D vs. f Different adsorption steps are indicated by the arrows.

Experiments with CPGs subjected to progressive thermal treatment leading to partial dehydroxylation of the surface and its enrichment in boron atoms showed specific adsorption properties of these materials differentiating them from silica gels [30,31,73]. It was found that depending on the type of adsorbate two trends in the adsorption behaviour of heated CPG can be observed [30,31]. The increase of thermal treatment time leads to the increase of CPG adsorption properties in relation to water, chloroform or hexane [30] (e.g. see Fig. 13). 

P. Somasundaran, E.D. Snell, Q. Xu, Adsorption behaviour of alkylarylethoxylated alcohols on silica,/. Colloid Interf. Sci. 

Stach, H., et al., Influence of pore diameter on adsorption behaviour of nonpolar molecules on silica adsorbents, Adsorpt. Sci. Technol., 3(4), 261-270 (1986). 

This molecular weight dependence of adsorption leads to fractionation, when the polymer is polydisperse, and exchange can take place freely. Hamori showed that there were significant differences in the adsorption behaviour of atactic and isotactic poly(methyl methacrylate) (PMMA) on silica from acetonitrile. Isotactic PMMA was adsorbed whereas atactic PMMA was not. Miyamoto et aV used this difference to separate isotactic and syndiotactic polymers. Botham and Thies found that isotactic poly(isopropyl acrylate) had a higher fraction of segments in direct contact with the surface than its syndiotactic or atactic equivalents. These experiments show the importance, even if second order, of the orientation and accessibility of the polymers adsorbing groups to the solid surface. 

GFP was immobilised into SBA-15 and on Aerosil OX-50 (Degussa) by physical adsorption. The latter is one of the commercially available products of Si02, and it consists entirely of a highly dispersed amorphous silica (SSA ca. 50 m2/g). Generally, siloxane and silanol groups are situated on the surface of Aerosil nanoparticles and the latter are responsible for its hydrophilic behaviour [4]. 

The next problem to be examined deals with the effect of water contained in the adsorbent on the chromatographic behaviour of macromolecules. The role of water in adsorption chromatography on polar adsorbents such as silica gel is extremely important63,64), because its varying content in the adsorbent often yields poorly reproducible results. 

Adsorption from solution behaviour can often be predicted qualitatively in terms of the polar/non-polar nature of the solid and of the solution components. This is illustrated by the isotherms shown in Figure 6.12 for the adsorption of fatty acids from toluene solution on to silica gel and from aqueous solution on to carbon. 

Due to their disperse character and small particle size, silicas are used as flow aids, i.e. they are used to improve the flow behaviour of other materials. The adsorption of the fine silica particles on other type powdered compounds reduces interparticle interactions. Particle adhesion, electrostatic adhesion, Van Der Waals forces and liquid bridge formation is reduced or avoided.33 This allows free-flowing behaviour of strongly interacting or irregularly shaped powdered materials. 

Summarizing we may state that, as adsorption isotherms revealed the formation of an equilibrium in the initial stage of the reaction, total loading data after two hours of reaction and filtration reveal that specific surface area and mean pore size are the controlling parameters in the loading step. Surface water causes hydrolysis and polymerization. On a dehydrated surface, a surface coverage irrespective of the number of hydroxyls is formed. For silica dehydroxylated at elevated temperature (1073 K) a different behaviour is observed, suggesting the participation of strained siloxanes. 

While limited changes in the features of the amide I band are observed after BSA adsorption on the two silicas the amide II band, present in the spectrum of the native BSA in solution (curve a) is absent in the spectrum of the protein adsorbed on A50 (curve b), whilst it is partially maintained in that of BSA on Qzm (curve c). This behaviour indicates that the interaction with the surface of the amorphous silicas resulted in an opening of the hydrophobic pockets, allowing the N-H groups therein contained to be converted by contact with D20 in N-D, producing an IR absorption at lower frequency (ca. 1450 cm, not shown). On Qzm the conformational changes are less pronounced. 

Solberg D, Wagberg L (2003) Adsorption and flocculation behaviour of cationic polyacrylamide and colloidal silica. Colloids Surf A Physicochem Eng Aspects 219 161... 

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

Several systems have been described which involve the use of silica with eluents of moderate to high polarity containing alcohols and/or water as major components. With such eluents, adsorption chromatography is most probably not the principal mechanism. The mechanisms are poorly understood, making the prediction of retention behaviour difficult nevertheless, many of these systems are very useful for drug analysis. 

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