Hydrating zeta potential

For oxide CMP, the purpose of the solution is two fold. First, water weakens the Si—O bond in a silicon dioxide film and softens the surface as it becomes hydrated with Si—OH bonds [6,7]. Figure 10 shows the reaction mechanism. Second, the solution is to provide a basic environment (pH > 10), which accelerates the hydration rate. An environment with high pH values will allow the polishing-induced reaction to be further accelerated because the surface Si(OH) species will be partially dissolved into water. In the meantime, the zeta potential of silica increases with increasing pH values. At high zeta potentials silica particles will repel each other, whereby a better-suspended slurry is formed. 

Consider a simple interfacial region at a mercury/solution interface. The electrolyte is 0.01 M NaF and the charge on the electrode is 10 iC negative to the pzc. The zeta potential is -10 mV on the same scale. What is the capacitance of the Helmholtz layer and that of the diffuse layer Galculate the capacitance of the interfaces. Take the thickness of the double layer as the distance between the center of the mercury atoms and that of hydrated K+in contact with the electrode through its water layer. (Bockris)... 

Hydration, Electric Double Layer and Zeta Potential. 94... 

The zeta potential is the electric potential of a double layer at the slipping point (Fig. 9.8). The slipping point is known to be in front of the colloid s surface at a distance of at least a single hydrated cationic layer (Stern layer) (the exact location of the slipping point is not clear). Based on what was said above with respect to pH dependence and/or ionic strength dependence of the electric double layer, it is also known that as the ionic strength increases, or pH approaches the PZC, the zeta potential decreases (Fig. 9.9). 

In one and the same colloidal system, two opposite tendencies are embodied—a tendency toward a decrease in the total phase interface and enlargement of particles, and a tendency toward self-hardening due to adsorption of stabilizers—usually charged ions. Thus the stabilization of colloidal solutions is caused by the presence of electrolytes or hydration of ions, in particular of the counterions of the diffusional layer bound to the granule. All the factors that will raise the zeta-potential and increase the hydration of the micelles will enhance the stabihty of the sol. And con-... 

Viallis-Terrisse. H., Nonat, A., and Petit, J.C., Zeta-potential study of calcium sihcate hydrates interacting with alkaline cations, J. Colloid Interf. Sci., 244, 58. 2001. 

HYDRAQL (10) treats adsorption as surface complexation with bound hydroxide functional groups, SOH, and their ionization products, SO and SOH2. The calculations in this paper use HYDRAQL in its triple layer mode. Surface charge and countercharge accumulate in three layers (1) at the surface itself, i.e., in the plane of the SOH groups where the surface potential is T o (2) in the outer Helmholtz plane (OHP), where adsorbed ions retain their inner hydration sheaths (26) and the potential is and (3) in the diffuse layer. The triple layer model is ideal for our purposes because of its ability to compute an estimate of Pp. The computed T p can be compared with experimental measurements of the zeta potential, providing an additional means of constraining models. 

The treatment given above of the diffuse double layer is based on the assumption that the ions in the electrolyte are treated as point charges. The ions are, however, of finite size, and this limits the inner boundary of the diffuse part of the double layer, since the center of an ion can only approach the surface to within its hydrated radius without becoming specifically adsorbed (Fig. 6.4.2). To take this effect into account, we introduce an inner part of the double layer next to the surface, the outer boundary of which is approximately a hydrated ion radius from the surface. This inner layer is called the Stern layer, and the plane separating the inner layer and outer diffuse layer is called the Stern plane (Fig. 6.4.2). As indicated in Fig. 6.4.2, the potential at this plane is close to the electrokinetic potential or zeta ( ) potential, which is defined as the potential at the shear surface between the charge surface and the electrolyte solution. The shear surface itself is somewhat arbitrary but characterized as the plane at which the mobile portion of the diffuse layer can slip or flow past the charged surface. 

Unspecific adsorption refers to the electrostatic attraction of counter-ions and is, therefore, independent from ion size or chemical nature. Such an adsorption reduces the effective surface charge, but can never reverse its sign. That means that the isoelectric point (lEP), which refers to the zeta potential, agrees with the point of zero charge (PZC), which refers to the surface charge (Fig. 3.6). Since the attraction is purely Coulombic, the ions retain their hydration shells hence, they are located at the OHP. 

---