Silanol dissociation

The mechanism proposed by Grubb and Osthoflf2 for the bulk polymerization of D4 catalysed by KOH is based on the assumption that the potassium silanolate dissociates into silanolate ions which are the active catalytic species in the polymerization. 

The pH is an important factor that can influence the ionization of the surface silica groups. As a result, C is directly dependent on the pH. Therefore, the relationship of /teof as a function of pH is governed by the behavior of the dissociation of the silanol groups. Different capillary materials result in different profiles of the electroosmotic mobility as a function of the pH (due to differences in Q. Typically a sigmoid curve behavior resembling the titration curve of the surface active groups is observed. ... 

Nevertheless, we recall that, in alkaline solutions, the silanol groups are partly dissociated into —Si—0 anionic sites owing to their acido-basic character. As a result, this oxide structure is loose and is permeable to water molecules and chemical reactants, so that the reaction can proceed by diffusion through the layer and lead to a uniform protecting layer. The acido-basic character is well demonstrated by the transformation operated by dipping the oxidized sample in an acidic HCl solution. The polarization resistance, which was equal to a few 10 f2 cm, suddenly is... 

The surface of a silica (or glass) particle contains a high density of silanol groups (about 1 per 25 A ) which dissociate to some extent in water to give a negatively charged surface. Figure 6.15. 

The magnitude of the electrostatic potential at the silica surface is, as expected from the law of mass action, pH-dependent. The variation in surface (or zeta) potential with pH must therefore be dependent on the dissociation constant of the surface silanol (Si-OH) group. 

The pK of a silanol group is not easy to determine because it is influenced by the surrounding environment however, at a pH of 2, the silanol is uncharged. Above this pH value it becomes increasingly dissociated and able to influence an extraction by virtue of its negative charge. Therefore, if a mixed-retention mechanism is present, the influence of residual silanols should be either reduced or enhanced, depending on the extraction mechanism desired. 

The eluent is rather acidic in most protein separations. With silica-based packings, the reason for the selection of this condition is related to the surface silanols which are weakly acidic groups. Above pH 3.5-4, these groups are dissociated and ionic interactions will occur with basic sample compounds. These interactions can be reduced simply using acidic mobile phases that prevent ionization of the silanols. Another reason is the stability of the silica which is higher in acidic media72). 

---