Silica, isoelectric point measurement

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. 

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]. 

The zeta potential of slurry particles was measured as shown in Figure 1. Fumed silica showed a higher isoelectric point (lEP) at which the net charge and electrophoretic mobility is zero, than that of colloidal silica as shown in Figure 1 (a). Also zeta potentials of colloidal silica were around 20mV lower than that of fumed silica. Figure 1 (b) shows the zeta potentials of alumina, ceria and Mn02 particles. Due to their lower hardness than alumina, ceria has been... 

Surface Charge. For a number of applications the surface charge of silica is of eminent interest. The isoelectric point (point of zero charge) can be derived from electrophoretic mobility (zeta potential) measurements on... 

The point of zero charge (pzc) where the surface charge is zero and the isoelectric point where the electrical mobility of silica particles is zero have been measured by many methods. De Bassetti, Tschadek, and Helmy (47) measured the pzc for silica gel by a calorimetric method, from which they concluded the value must be between 2.5 and 3. However, the data may not preclude a value as low as pH 2 since the heat of neutralization becomes exceedingly small below pH 3. 

Electrostatic Forces— The AFM Tip. The most frequently used commercial AFM tip consists of microfabricated silicon nitride. Under normal laboratory conditions, this is, in reality, an ill-defined, hydroxylated melange of silicon oxynitride and silicon dioxide, the precise composition of which depends on a combination of manufacturing conditions and tip history. An isoelectric point of around pH=5 has been firequently observed in force-distance measurements under electrolyte in our laboratory (15), suggesting the oxidized oxynitride composition mentioned above. In order to achieve a better reproducibility, however, we have adopted an oxygen plasma-treatment procedure, which leaves the tip coated with hydroxylated silica, as evidenced by an isoelectric point of around pH=3 (16), as measured by force titration (75). 

---