Vacuum grease is low volatility lubricant which is used to provide a uniform and airtight seal between ground glass joints.
Silicone grease is the most commonly used grease for Schlenk line chemistry and vacuum applications. It is cheap and widely available, has a good operating temperature range (-40 to +200 °C) but has limited chemical resistance and is prone to ‘creep’ (i.e. undesirable spreading of the grease) and therefore readily contaminates reactions and resulting NMR spectra. It is not recommended to be used for more permanent joints such as the double oblique ground glass stopcocks on Schlenk lines but it is suitable for day-to-day operations using Schlenk flasks and vacuum traps.
Hydrocarbon based vacuum greases (such as Apiezon or Leybonol) are ‘creep’ resistant and are therefore recommended to be used for the double oblique ground glass stopcocks on Schlenk lines. Hydrocarbon greases can be used for day-to-day operations also, but it is considerably more expensive when compared to silicone grease, and due to its solubility in aromatic hydrocarbon solvents, can still easily contaminate reactions. Apiezon N is a fine grease which is best suited for fitted or moving joints on the Schlenk line such as double oblique ground glass stopcocks; it is also suitable for cryogenic temperature applications such as fitting liquid nitrogen traps. Apiezon M is often used for day-to-day operations such as Schlenk flask stopcocks or ground glass joints, whilst Apiezon H is employed for non-moving ground glass joints or for high temperature applications.
Polyfluorinated greases are made from PTFE (polytetrafluoroethane) or PFPE (perfluoropolyether) and therefore show excellent chemical resistance. They have a wide operating temperature range and are insoluble in most organic solvents making them ideal for performing distillations and to generally avoid grease contamination. Polyfluorinated greases can also be used for day-to-day operations, but they are significantly more expensive than silicone grease and are also prone to ‘creep’.
The amount of grease used is another important consideration; too much grease is likely to contaminate glassware and reactions, or be expensive when using hydrocarbon or polyfluorinated greases, whilst too little grease will lead to a poor seal and risk seizing of joints. A thin strip of grease applied along one length of a ground glass stopcock or stopper is typically sufficient and twisting the male component several times will ensure a uniform seal. Ensure that the joints are suitably greased and mobile before being evacuated glassware under vacuum to avoid seized joints or leaks.
Greasing and assembling a small Schlenk flask using a hydrocarbon-based grease.