Vacuum preservation Preparation of gel

Vacuum preservation Preparation of gel

(1) Add a certain amount of tetrabutyl titanate, anhydrous ethanol, water and catalyst to a beaker, stir rapidly until a gel is formed, and then age it for several days. The amount of water and pH control can vary between groups. (2) Replace the residual water and reagents in the gel with absolute ethanol.

The surface tension of vacuum preservation no longer exists in supercritical drying

Adding surfactant to dry will introduce impurities. At present, the most effective method to eliminate the effect of liquid surface tension on gel damage is to use low-temperature freeze-drying or to drive out the liquid in the gel pores under the condition of supercritical fluid, that is, supercritical drying and supercritical drying. In the fluid in the supercritical state, the gas-liquid interface disappears, and the surface tension no longer exists in supercritical drying. At this time, there is no additional pressure generated by the surface tension in the capillary pores of the gel. Therefore, drying under supercritical fluid conditions , the original dispersion structure of the gel can be maintained, thereby avoiding the agglomeration and agglomeration of materials or nanoparticles caused by strong capillary contraction in the drying process of conventional drying techniques such as room temperature drying and baking drying, preventing material basic particles. Roughening, a sharp drop in specific surface and a large reduction in porosity are the consequences.

There is a close relationship between the surface properties of vacuum preservation.

D.4 Typical pressure diagram of pure substances D.4-1 Typical pressure-temperature diagram of pure substances D.4 Table D.4-1 Critical properties of some common substances

Thermophysics studies of fluids show that the surface tension of liquids is related to temperature as follows:

σ = σ 0 (1 T / TC )

In the formula: σ——surface tension of liquid; σ0——characteristic constant related to intermolecular attraction; TC is the critical temperature of matter. When the temperature of the system reaches the critical temperature (T = TC ), according to the above formula, the surface tension tends to zero, indicating that under critical conditions, the gas-liquid phase interface disappears and the surface tension ceases to exist. Before the gel is dried, the gel network structure is filled with liquid solvent. With the drying of the dried gel, after the solvent is partially volatilized, the liquid begins to form a meniscus in the capillary pores of the gel network, resulting in additional pressure. p = 2σ/r. The size of the gel capillary is generally 1~100nm. If the radius of the gel capillary is 20nm, when there is such a strong capillary contraction force formed by the surface tension of the ethanol liquid, the theoretical calculation will bear about the pressure. 2.3 Mpa. The capillary pore size is further reduced, and the additional pressure is further increased, so that the particles are further approached, aggregated and contracted, and the gel network structure collapses. Therefore, it is difficult to prevent the shrinkage and fragmentation of the gel by using the conventional drying process, and finally only a fragmented, hard and large-scale porous xerogel can be obtained.

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