Silica, isoelectric point

It should be emphasized that the acid- and base-synthesized silica mesophases have little in common other than sometimes having the same space-group symmetry. They do not have the same composition since mesophase samples synthesized below the silica isoelectric point require a counteranion, generally a halide anion, for each surfactant molecule that is present. Terminal Si—O groups are protonated so that the bulk compositions of M41S (e.g., MCM-41) and acid-prepared materials (e.g., SBA-3) made with the same surfactants are completely different in hydrogen and halide-ion content. 

Protein mixtures were well resolved on poly(aspartic acid)-silica columns using 0.05 mol/1 phosphate buffer, pH 6.0 and a gradient of sodium chloride from 0 to 0.6 mol/1. The columns displayed a high capacity and selectivity. Figure 3 shows the separation of several standard proteins with isoelectric points ranging from 4.7 to over 11. Peaks are sharp and show minimal tailing. The poly(aspartic acid) coating was quite stable the columns lasted for hundreds of hours of use without decrease in efficiency and capacity. 

The effect of pH on the protein adsorption on CMK-3 was also investigated [152], The monolayer adsorption capacities obtained under various pH conditions are plotted in Figure 4.12, where the maximum adsorption was observed in the pH region near the isoelectric point of lysozyme (pi of about 11). Near the isoelectric point, the net charges of the lysozyme molecule are minimized and would form the most compact assembly. A similar pH effect was also seen in the adsorption of cytochrome c on CM K-3. Although the nature of the surface of mesoporous silica and... 

To evaluate the interaction between protein and the mesoporous silica surface, the immobilisation of GFP was performed also on Aerosil amorphous silica nanoparticles. The isoelectric point (pi) of GFP is 5.78. At pH 7.4, above pi, protein is negatively charged whilst the silica surface is positively charged [5,6], Hence, the higher the protein negative charge, the stronger the electrostatic interaction between GFP and silica surface. 

Some like casein, salt free globulin and acid albumin are not heavily solvated in solution and are thus readily precipitated at the isoelectric point in a manner similar to the suspensions already considered. Others, e.g. glutin, gelatine and natural albumin, are solvated like silica which at the isoelectric point are not necessarily precipitated being maintained in the sol form by the solvent. On removal of the stabilising water however by the addition of alcohol or neutral salts precipitation will occur and this most readily at the isoelectric point. 

On the other hand, the data in Figure 5 show that for virtually the same Mo compositions, both activity and HCHO selectivity decrease with impregnation pH. This tendency can be explained in terms of equilibrium 2. At pH values close to isoelectric point of silica (ca. 2), molybdate appears to be uniformly adsorbed by electrostatic forces, however at medium and, especially, at higher pH, crystalline M0O3 is deposited during the impregnation step. 

Sodium Poly(4-styrene sulfonate). The sol—gel processing of TMOS in the presence of sodium poly-4-styrene sulfonate (NaPSS) has been used to synthesize inorganic—organic amorphous complexes (61). These sodium silicate materials were then isothermally crystallized. The processing pH, with respect to the isoelectric point of amorphous silica, was shown to influence the morphology of the initial gel structures. Using x-ray diffraction, the crystallization temperatures were monitored and were found to depend on these initial microstructures. This was explained in terms of the electrostatic interaction between the evolving silicate structures and the NaPSS prior to heat treatment at elevated temperatures. 

The surface of uncalcined aluminas consists of amphoteric hydroxyl groups that are even less acidic than those in silica gel. Electrophoretic measurements by Stigter et al. (90) show that the isoelectric point for alumina is attained at a pH of 9, much higher-than the value of 2 obtained in the case of silica gel. Thus, hydroxylated aluminas are relatively non-acidic, unless they are promoted with acid-producing impurities. 

The preparation can also take place under acidic conditions below the isoelectric point of the Si-OH-bearing inorganic species (pH 2) then the silica species are positively charged, that is, protonated silanol groups (Si-OHT) in order to produce an interaction with cationic surfactants, it is necessary to add a mediator ion X- (usually a halide), which gives rise to the S+X I+ pathway (Fig. 3.6b). 

The adsorption of ionic or polar surfactants on charged or polar surfaces involves coulombic (ion-surface charge interaction), ion-dipole, and/or dipole-dipole interaction. For example, a negatively charged silica surface (at a pH above the isoelectric point of the surface, i.e., pH >2-3)... 

Adsorption of ETES and VTES on Alumina. In the absence of acid or base catalysts it was not possible to obtain ETES films on a-alumina by retraction and the VTES films were unstable toward the organic test liquids as well as water. This failure of both the ETES and VTES to form strongly held films can be attributed to the weakly basic character of alumina surfaces. The isoelectric point of alumina is usually at a pH of 7.5 to 9.0 (11) which is a rough measure of the base strength of the —A1—OH surface groups. Silica, on the other hand, has an isoelectric point at a pH of 2, signifying a moderately strong acid character for the... 

Interactions between the ceria abrasives and the oxide surface have been investigated using both the chemical and the instrumental approaches. Suphantharida and Osseo-Asare [27] used zeta-potential measurements, silicate adsorption, and polishing experiment to investigate the role of ceria abrasives-Si02 surface interaction. T o determine the effect of pH on the surface charges, the zeta potentials of abrasive particles were measured (Fig. 13.21). The points of zero charge (pzc) or isoelectric point is at pH 6.0 for ceria and pH 1.5 for silica. These values are consistent with those reported by others [28,29]. 

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