Temperature Effect

An increase in temperature increases the rate velocity of hydrolysis and condensation, and therefore the entire process of gel formation.

2.3) Gelation

The evolution of the condensation reaction causes growth of the polymer chains, and thereby increasing the viscosity of the colloidal solution. The gelation time can be considered as the time necessary for the transition from a viscous fluid to an elastic solid. That is, the change from sol to gel. The same variables which enhance the condensation rate velocity reduce the time of gelation. The characteristics of the gel keep evolving even after the gel point.

2.4) Non Hydrolytic Sol-Gel

When condensation occurs directly without hydrolysis, there’s non hydrolytic sol-gel. This reaction can be of various types.

a. Single component non hydrolytic sol-gel: This is when condensation is between precursors of the same type. For example, TEOS and SiCL₄

Fig. 2: A single component condensation

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b. Multiple-component non hydrolytic sol-gel: When there’s condensation between precursors of different metals, like SiCl₄ and Al[OCH(CH₃)₂]₃.

In this case, reaction has to be protected against the other possible reactions like;

2.5) Ageing And Hydro-Thermal Treatment

The working conditions used in this part of the process could make us identify two different processes

 Ageing: This is carried out at low temperatures, atmospheric pressures and consequently very long operating periods.

 Hydro-Thermal treatment: They are fast, and are carried out at higher temperatures and pressures.

Another difference is given by the fact that, the hydro-thermal treatment is a desired process while ageing is casual. Generally the structural and morphological characteristics that can be modified using the one or the other process are the following:

 A change in the crystal dimension

 A change in the dimension of the amorphous particles

 A transition of amorphous particles into crystalline particles

 Crystalline transitions (α→β→γ)

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 A change in the porosity.

These are transformations that follow the laws of thermodynamics, so they proceed in the direction where there’s a diminution of the free energy. Poly-condensation continues after gelation, increasing the connectivity of the network, by the formation of ulterior cross-links. A process favoured by the presence of hydroxyl groups.

Syneresis is the expulsion of small quantities of liquid from the pores as a consequence of further bonding due to the continuation of the condensation reaction. This continuation of the bonding causes contraction of the gel and the consequence is syneresis. The presence of organic solvents capable of forming hydrogen bonds with the hydroxyl groups of the network, causes a slow down of condensation and syneresis.

Coarsening is the non-reversible diminution of the surface area due to precipitation and re-dissolution of the surface particles. The superficial morphology makes it possible, for particles in different zones to have different solubilities. For example, convex surfaces are more soluble than concave surfaces. A gel in a liquid in which it is soluble, tends to re-precipitate the dissolved materials on the regions where the surface has a negative curvature (concave surfaces). This process causes an increase in the medium pore dimension and therefore a decrease in the specific surface area.

2.6) Drying

This is the evaporation of the solvent contained in the pores, a phenomenon governed by various parameters:

 Temperature

 Relative humidity

 Particle size

 Eventual air flow in the area.

Any rational treatment of drying takes into consideration capillarity. The expression which best expresses the vapour pressure inside a capillary tube is given by the Kelvin equation.

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P =Vapour pressure;

P° = Saturation vapour pressure;

γ = Surface tension of the liquid;

θ = Contact angle between liquid and solid;

V = Molar volume of the liquid;

R = Pore radius.

Pc = Capillary pressure

Fig 3: The various pressure forces involved in capillarity.

From the Kelvin equation we deduce a slow evaporation from tiny pores because of low vapour pressures. The exact contrary happens to large pores. Now when a solvent is trapped in a pore, the curved surface which separates the liquid from the vapour generates a capillary pressure. This capillary pressure developed in tiny pore can get so high that, they cause the break down the walls of the bigger pores, and a complete collapse of the structure of the gel.

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This could be avoided in the following ways:

 Increase particle dimension through ageing;

 Low oven temperatures during drying;

 Increase in the relative humidity;

 Minimum air flow currents.

From the above description, drying can be divided up into different phases. Initially the gel undergoes a volume decrease which is proportional to the quantity of evaporated liquid, and the liquid vapour interface is on the surface. Here the liquid/vapour meniscus’ radius is large with respect to the pore diameter, and the capillary pressure is low. At the arrival of a critical point where the radius of the meniscus is equal to the pore diameter, and the particles are so well packed that no further rearrangement is possible, the evaporating film continues inside the pore and this can bring about fractures on the pore walls. After this critical point, the liquid/vapour meniscus is no more on the surface, but is drawn back to a point where the liquid is isolated.

Here evaporation proceeds only by diffusion. There’s an increase in capillary pressure, which might provoke structural rearrangements, and make the gel compact. The stress that is generated from contraction is greater in gels prepared with acidic catalysis, because of the minor pore dimensions. Those prepared by the basic catalysis instead, are made of dense clusters separated by large empty spaces, and therefore generate very low superficial tension.

2.7) Thermal Treatment

To be able to give a final catalytically useful form to the gel, it is necessary to undergo specific thermal treatments. This is heating in the presence of various reactive gasses like in the air, oxygen or hydrogen. The heating eliminates all residual organic matter and oxidize or reduce the specimen. The exposure of a gel sample, to high temperatures for a certain period, causes a decrease in the surface area, and might also cause some re-crystallization. Normally the gel undergoes a more severe thermal treatment than the treatment it would encounter in the reactor

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