Stabilization Against Particle Growth

Stabilization Against Particle Growth (Theory of Paul C Yates) 

The effect can most easily be understood by considering the changes in free energy that occur during the following hypothetical, reversible, isothermal steps in the three-component system SiOj-HjO-NaOH starting with an alkali-stabilized sol  

Desorb NaOH from the surface of the colloidal particles AFj. 

Desorb water from the surface of the colloid that is, the free energy of wetting the surface AF.  

---

From activity results (Table 2 Figure 3) it is evident that the extent of deactivation on high temperature treatments was more for Cu based catalysts.lt can be seen from Table 2 and Figure 4 that Cu catalysts experienced severe deactivation than Cu-Cr catalysts on hydrothermal treatment also. This observation indicates that addition of chromium, despite reducing the activity improves stability against particle growth which is likely to occur at severe operating conditions. 

E, R, Leite, A, P, Maciel, 1, T, Weber, P, N, Lisboa, E, Longo, C, O, Paiva-Santos, A, V, C, Andrade, C, A, Pakoscimas, Y, Maniette, and W, H, Schreiner, Development of metal oxide nanoparticles with high stability against particle growth using a metastable solid solution, Adv. Mater, 14 905-908, 2002... 

A confirmed the presence of unintercalated Na -montmorillonite. Nevertheless, surface area measurements and subsequent neutron scattering studies [5] of the silica sol - clay products were indicative of the formation of composite structures in which the sol particles bind to the basal surfaces of the clay aggregates and become stabilized against particle growth through Oswald ripening. 

Yates calculated the amount of alkali necessary to stabilize silica particles against growth as follows. 

The formation of hydroxyaluminosilicate species at near-neutral pH is unique. No interactions occur between silicic acid and Ca or Mg at less than pH 10, so that the transport and binding of these cations is unhindered. The interactions of Fe with silicic acid at near-neutral pH are very different from those of aluminum. Acidic solutions containing Fe (10 " M) with a threefold molar excess of silicic acid remain clear on neutralization no visible precipitate forms. Instead, Fe-0 polymers are formed as spherical hydrated ferric oxide particles 10-15 nm in diameter, stabilized against growth, aggregation, and precipitation by an adsorbed layer of silicic acid [23]. 

C. PRECIPITATED POLYMER STABILIZED AGAINST FLOCCULATION d. CHAIN GROWTH OCCURS VIA LIVING POLYMERIZATION BY BLOCK POLYMER DISPERSING AGENT BY MONOMER INCORPORATION AT THE PARTICLE... 

---