Negatively Charged Surfaces

The anionic nature of aluminum, when substituted for silica in the 4-coordinated state, was early recognized by Pauling in crystalline minerals (410) and soon thereafter in alumina-silica cracking catalysts (411). The formation of the alu-mino-silicate anion by reaction of aluminate ion with the surface of silica is 

Milliken, Mills, and Oblad (413) showed that the anion is stable only in the presence of a cation other than hydrogen, the free acid being unstable, and that excess silica must be present. The pH is thus an important factor only about 15% alumina in silica was stabilized in the adsorbed anionic form at pH 6 when the stabilizing cation was ammonium. Below pH 3, in the hydrogen form, the aluminum reverted to the 6-coordinate state. 

I silica so ver pure Pi but modified a sol gels 1 sodium the silica 20 silanol ited, they silica sol )resent in 415). The ises (416, to 25 nm to 10 1. U consist It topic. 

Sols of Silica Particles with Modified Surfaces 

Her (324) prepared a series of sols modified with various amounts of aluminate. One series was made from an unmodified sol of 22 nm particles (Ludox TM) and another of 14 nm particles (Ludox HS). The coverage by aluminate ions ranged from 1.8 to 25% of the total silanol groups on the surface (assuming 8 SiOH nm ). Stabilization as an aluminate ion in the particle surface requires that each aluminum atom be surrounded by three oxygens linked to silicon, which means that no more than 25% of the surface silicon sites can be replaced by aluminum atoms. This assumes, of course, that the underlying silica surface is nonporous and not accessible to reaction with Al(OH)4 ions. 

---

Inorganic Ions. Because of electrostatic attraction, positive ions are attracted to negatively charged surfaces and have a higher concentration near the surface than in the bulk. Negative ions are repeUed from the negative surface and have a lower concentration near that surface. Ions which are very strongly bound (// ds Stem layer, whereas those that can move into and out of the ionic atmosphere < kT) are in the Helmholtz... 

Unlike cations, the adsorption activity of CT, Br", and I at Pt electrodes is appreciable806 and increases in the given sequence of anions. At a 0, the <7, A curves for LiC104, NaCl,NaBr, and Nal coincide, which indicates that complete desorption of halide ions takes place at negatively charged surfaces. The values of Ea=0 for a renewed Pt electrode have been found to be -0.18, -0.24, and -0.33 V (SCE in H20) for NaCl, NaBr, and Nal in DMSO, respectively. 

Initiation of the fibrin clot in response to tissue injury is carried out by the extrinsic pathway. How the intrinsic pathway is activated in vivo is unclear, but it involves a negatively charged surface. The intrinsic and extrinsic pathways converge in a final common path-vray involving the activation of prothrombin to thrombin and the thrombin-catalyzed cleavage of fibrinogen to form the fibrin clot. The intrinsic, extrinsic, and final common pathways are complex and involve many different proteins (Figure 51-1 and Table 51-1). In... 

Binds to negatively charged surface at site of vessel wall injury activated by high-MW kininogen and kallikrein. 

All factors influencing the potentials of the inner or outer Helmholtz plane will also influence the zeta potential. For instance, when, owing to the adsorption of surface-active anions, a positively charged metal surface will, at constant value of electrode potential, be converted to a negatively charged surface (see Fig. 10.3, curve 2), the zeta potential will also become negative. The zeta potential is zero around the point of zero charge, where an ionic edl is absent. 

Negatively charged surface grains will attract a suspended particle with a positive charge. 

It is also possible for ions in the water, especially positively charged ions, or cations, to be attracted to the negatively charged surfaces. This leads to a zone of water and ions surrounding the clay particles, known as the diffuse double layer. 

The phenomena of cations being attracted to surfaces is commonly referred to as cation exchange because cations that are more attracted to the surface can displace those that are less attracted. For example, a +2 cation can displace a +1 cation from a surface because the +2 cation has a greater chemical drive to interact with a negatively charged surface than does the +1 ion. Thus, the +1 ion is exchanged for the +2 ion. 

---