Silanol groups, acidic, dissociation

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 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). 

Electroosmosis occurs in fused silica capillaries because acidic silanol groups at the surface of the capillary dissociate when in contact with an electrolyte solution6,7 (buffer), according to... 

A. Effect of pH Acidic silanol groups at the surface of the capillary wall will dissociate when in contact with an electrolyte solution, as illustrated by Eq. (4.5). At high pH, the silanol groups are fully ionized, generating a dense compact layer and a high zeta potential. As a result, the magnitude of the EOF in untreated fused silica capillaries increases with increasing pH. 

Protein in untreated capillaries tend to stick to the inner capillary walls. Several approaches have been used to abolish or at least minimize this problem (McCormick, 1988 Bushey and Jorgenson, 1989 Green and Jorgenson, 1989 Emmer et al., 1991 Kajiwara, 1991). The easiest approach is to perform the separations at very high or very low pH values. In the former case, dissociation of the free amino groups is suppressed and, consequently, interactions between the dissociated silanol groups of the capillary surface and the free amino groups of the protein are minimized. At extremely acid pH, the opposite... 

Though negligible, the electrochemical route is nevertheless critical with respect to the surface topography. This route is responsible for the pitting on terraces, as Fig. 23 shows [122]. This is because the electrochemical hydrolysis of the Si-H bonds is a two-step process initiated by an acid-base dissociation of the Si-H bond which avoids steric problems by liberating a terrace site to form a silanol group which is then removed (Fig. 27, bottom). Additives may reduce the formation of etch pits (see Figs. 24 and 25). 

The dependence of these electrokinetic velocities on pH is shown in Fig. 1. In the case of capillary zone electrophoresis (CZE), with a bare fused-silica capillary, the pH greatly affects the EOF velocity (i.e., i eof significantly decreases with the decrease in pH from 8 to 3). In MEKC, however, the dependence of i eof on pH is different from that in CZE, especially under weakly acidic conditions (pH 7.0-5.5). In the range of pH between 7.0 and 5.5, qf slightly decreases with the decrease in pH, due to the adsorption of the SDS molecule or monomer on the inside wall of the capillary. On the other hand, of rapidly decreases with the decrease in the pH below 5.5. The decrease of qf is mainly caused by the decrease in the zeta-potential of the inside wall of the capillary, because the dissociation of silanol groups on the capillary wall is more suppressed as the solution becomes more acidic. 

The faster increase of the zeta potential above pH 6.5, from —70 mV to -110 mV at pH 7.2, may be attributed to the supplementary electrical negative charges resulting from the dissociation of the hydrogen bonded silanol groups. This result was additionally supported by the determination of the dissociation constant of the Al-OH group from the acid-base titration curve. 

The pK of dissociation of silanol groups is not known. But for the first dissociation of silicic acid ... 

By analogy with other oxyacids, such as phosphoric, one would expect the acidity of individual silanol groups to increase with the connectivity of the group to which they are attached the accepted values of the few known dissociation constants support this idea. (For orthosilicic acid, pKi 9.5, pK2 12.6 (7, 15) for silanol groups on silica gel, pK s7 (16).) However, this Ts more than compensated by the fact that in the course of polymerisation the total number of silanol groups is greatly reduced, so that despite the pK values just quoted the overall result is a reduction in acidity, and a consequent rise in pH. 

Figure 1.10. Surface hydroxyl groups (shaded) on kaolinite. Besides the OH groups on the basal plane, there are aluminol groups, associated with Lewis acid sites, and silanol groups protruding from the edge surface. The right side of the figure shows an outer-sphere surface complex between an ionized H2O and Na" ", as well as complexes between the silanol groups and OH (i.e., proton dissociation).

In case of high activities of hydroxyl ions (pH > 7) it must be taken into consideration that the protons from the acidic silanol groups also can react with hydroxyl ions, so that the dissociation equilibrium has to be formulated as... 

Selectivity Control by Mobile Phase pH and Column Temperature These two parameters are of major importance for selectivity control in HPLC, especially with acidic and/or basic analytes. The charge state of analyte molecules influences both their hydrophobic (dispersion type) interaction, as well as possible ionic secondary interactions (e.g., with dissociated residual silanol group on silica-based stationary phases). The relatively wide range of so-called mixed-mode phases make very effective use of ionic interactions, but specialty phases are outside the scope of this chapter (refer to Chapter 4 for more information). 

---