Air Sensitive Compounds
There are many reasons that a compound would be sensitive to air. The ideal storage conditions for every material can be found in their material safety data sheet, under "Handling and Storage".
The magnitude of an air-sensitive material's reaction to oxygen or moisture can vary dependent on the material properties.
- Some materials react violently with oxygen or moisture in air, making them very dangerous to work with.
- Some materials can simply degrade or decompose over time due to exposure to oxygen or moisture.
- These reactions can be accelerated by exposure to light or increased temperatures.
- Other materials can absorb moisture from the air causing them to aggregate.
Any of these reactions will impact the quality of your materials over time. It is important to store and handle air sensitive compounds in inert conditions, such as in a glove box.
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Pyrophoric materials react violently (often explosively) with oxygen. Examples of pyrophoric materials include:
- Metal alkyls and aryls
- Metal sulphides - such as barium sulphide, calcium sulphide, iron (II) sulphide, tin (ii) sulphide
- Some finely divided metals - for example, caesium, calcium, rubidium, sodium, cobalt, iron, lead, potassium.
You can handle small quantities of these materials in air free environments. Even then, users should take extreme caution when using these materials. You should not store these materials in large quantities. For these materials, you will need extremely secure inert processing conditions which can maintain extremely low levels of moisture and oxygen (such as <0.1 ppm glove box).
Thermal- and Photo-oxidation of Organic Materials
Lots of organic materials (both polymer and small molecules) will experience some kind of oxidation over time. The speed of this degradation will depend on the chemical composition of the compound, and the amount of external stress the compound is under. High stress (such as heat or light) may accelerate these reactions.
Accelerated polymer aging due to illumination is known as photo-oxidation and tends to happen at the surface of a polymer material. For example, polyvinyl chloride alone is vulnerable to degradation under illumination, but poly-sulfone is not. With thermal-oxidation, degradation of polymers is stimulated by raised temperatures, rather than light induced. The effects will permeate through the polymer as heat travels through it.
Lots of organic materials will experience some kind of oxidation over time. For example, rubrene is extremely photo-sensitive, and thin rubrene films will lose all optical characteristics after several hours in air. This limits the use of its impressive charge carrier mobility properties, as its stability is so bad. One way around this is to deposit and subsequently encapsulate thin films within an inert environment.
Hygroscopic materials can extract water from the air, and this will cause them to degrade. Examples of hygroscopic materials include many salts, such as organic halides or metal halides. For this reason, salts are often used to absorb moisture from the environment in the form of desiccants, used to create dry boxes and other moisture-free zones.
However, in a lot of cases, this can result in degradation of materials or aggregation of powders and salts making them harder to use or dissolve.
Examples of Air Sensitive Compounds
There are numerous reasons why a material might be best handled under inert conditions. A large number of materials that we sell at Ossila are best used under inert conditions. Examples of some of our air-sensitive materials include:
- DDP DTT
- Some perovskites (I301, I201)
We would recommend handling these materials using air-free processing methods suggested in this article. One main piece of advice would be to only open these materials within an inert environment, such as within a glove box.
How to Store Air Sensitive Materials
The long-term storage of air sensitive materials can be difficult, as it is important that they remain uncontaminated. One of the best ways to store air-sensitive compounds is within a glove box. This environment will provide a protective layer against oxygen, water vapor, and other reactive gases. When storing compounds in a glove box, it is important to use containers that are designed to prevent any air or moisture from entering. Materials are normally stored in containers made from glass or plastic, such as in a glass vial. It is also important to label the containers clearly, so that they can be easily identified when needed. When not in use, air-sensitive compounds should be stored in a dark place with minimal temperature fluctuations. It is also important to keep the compounds away from direct sunlight and other sources of heat or light.
We would recommend storing our air sensitive materials in a sealed amber vial within a glove box. Make sure it is tightly sealed so that no moisture can ingress through the lid. You should also avoid opening or disturbing the material unnecessarily. This will reduce the risk of contaminating materials with any substances or solvent vapours that may be trapped within the glove box. In an ideal situation, these materials should be stored in an isolated antechamber, or in a specific glove box, where no solvents are used.
Degassing and Drying
Air sensitive materials might be exposed to air at some point. Your air sensitive technique may be temporarily compromised, or this could happen when moving or using a sample. If this happens, you can use degassing or drying methods to remove moisture or trapped solvent vapours from the material. Additionally, if you are bringing porous materials (such as tissues or paper) into an air free environment, you should take some action to degas or dry them. Trapped moisture within pores can be released over time compromising the air free processing conditions you have created.
To degas a material, expose the material to a vacuum for a period of time. This will pull out any residual solvents. Alternatively, you can place the material or item in a drying oven for a prolong period to dry the materials. Vacuum ovens can do both these things at once. You can use the flow chart below to decide how to proceed when you are introducing materials or equipment into an air free environment.
Weighing Materials in Inert Conditions
If suitable, you can bring a weighing balance into a glove box to weigh out air sensitive materials. This will enable you to decant and weigh out materials without exposing them to oxygen or moisture. However, pressure variations within the glove box (for example when placing your hands into the gloves) can affect the accuracy of measurements. Therefore, there are other methods you can use to weigh materials accurately without risking exposure.
For example, say you are making up a solution using an air sensitive material. Start by weighing an empty bottle outside the glove box. You can bring this empty vial into the glove box, decant some air sensitive material into the vial and seal it. You then bring the sealed vial outside of the inert environment and weigh the filled bottle. From this, you can calculate the amount of material you have in the vial and therefore how much solvent you need to add.
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