Outgassing Materials and Solvents for Glove Boxes
Glove boxes are critical pieces of equipment for processing of air sensitive compounds for scientific research. When using a glove box, it is important to maintain a moisture- and oxygen-free atmosphere.
Sometimes the materials processed within these environments can directly impact the quality of the atmosphere and the ability of the system to maintain an inert atmosphere.
Minimizing moisture content within a glove box requires two things:
- Careful selection of material
- Removal of residual moisture in the materials
This will help you minimize your nitrogen or argon usage as the inert environment is easier to maintain. It will also ensure that sensitive samples are not affected by the presence of outgassed moisture within the box, improving the repeatability of your experiments.
Outgassing of Materials
All materials can absorb small amounts of moisture and oxygen. When materials are taken into a glove box, any absorbed moisture eventually evaporates from the surface and becomes part of the internal atmosphere. This process is called outgassing.
Moisture in Materials and Equipment
The scale of absorption in an ambient atmosphere depends on the exposed surface area of the material and the affinity between materials. While the scale of outgassing varies depending upon the material properties, physical shape and size, and prior exposure to atmospheric conditions.
A common item used in the laboratory, such as tissue paper, is a good example. At 50% relative humidity, paper is typically 6% moisture by weight. If a box of tissues weighs approximately 200 g, as much as 12 g could be water. This would outgas 12 g of moisture into the glove box, requiring nearly 10 m3 of nitrogen to flush it from the system.
Large pieces of equipment, such as spin coaters or hot plates, can also outgas in a glove box. The protective casings used are often made of polymer materials because they are low cost, durable, and have a high chemical resistance. However, they are more porous than metals and will absorb water in ambient conditions. Due to the large surface area of the equipment, even with small amounts of moisture, outgassing will significantly affect the inert environment.
Moisture in Solvents
Water absorption is an issue with liquid materials too, especially solvents used for the production of solutions.
The interaction between solvents and atmospheric moisture can result in a significant volume of water being present within the solvent. Any solvent exposed to air will absorb moisture until an equilibrium value is reached. For water soluble solvents, the percentage of moisture can be very high.
One example is the commonly used solvent, isopropyl alcohol (IPA). Once exposed to air, isopropyl alcohol rapidly absorbs water. The equilibrium point for isopropyl alcohol and moisture is 65% to 35%, meaning a 250 ml bottle of IPA could contain as much as 87.5 ml of water. This would require nearly 75 m3 of inert gas to remove the outgassed moisture from the internal environment.
Limiting Outgassing in a Glovebox
It is critical that any materials taken into the glove box are free from moisture as much as possible to reduce the risk of outgassing. There are several ways to limit outgassing and preserve the inert environment:
- Careful selection of materials used in a glove box is critical. Avoiding items that have high surface areas such as fibrous, porous, or powdered materials is ideal.
- Carry out several stages of degassing for porous, powdered, or hygroscopic materials before bringing them into the glove box.
In circumstances where materials with high surface areas or affinity to moisture must be used, there are steps that can be taken to limit the exposure of moisture.
For any solid and stable material, a short baking cycle before being taken into the glove box is a suitable method to remove moisture. The items should be heated to a high temperature to drive out any trapped moisture.
Similar to baking, vacuum annealing can be used to remove trapped moisture from the surface of the sample. In this process, materials are stored under vacuum for a prolonged period where the moisture will outgas before being extracted.
For solvent bottles, septum caps are a simple way of limiting moisture exposure. They enable the user to handle and extract solvents from the bottle with a needle, without removing the cap.
Anhydrous Grade Solvents
Solvents can be purchased as anhydrous grade solvents. These materials have processed to remove as much moisture as possible, and then sealed against the atmosphere to maintain purity.
Limiting the total volume of materials in the glove box can also limit outgassing. For example, if you can use a 100 ml bottle of solvent rather than a litre bottle, less outgassing will occur.
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