Pressurizing the mixtures of two ionic liquids: Crystallization versus vetrification

A series of mixtures prepared with different content of the ionic liquids N-trimethyl-N-propylammonium bis(trifluoromethylsulfonyl)imide (TMPA-TFSI) and N-trimethyl-N-hexylammonium bis(trifluoromethylsulfonyl)imide (TMHA-TFSI), namely, (TMPA-TFSI)(100-x) (TMHA-TFSI)(x), is investigated by means of Raman spectroscopy as a function of pressure at room temperature. Both pure TMPA-TFSI and the x=2 mixture crystallize under an applied pressure lower than 1GPa. On the contrary, for x>7, none of the investigated mixtures crystallize, but they undergo a glass transition over the 1-2GPa pressure range. We suggest that the disorder induced by the high number of possible configurations for the longer alkyl chain of TMHA-TFSI in a compressed environment is a key factor in the suppression of the crystallization process.