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 1 GPa. On the contrary, for x > 7, none of the investigated mixtures crystallize, but they undergo a glass transition over the 1-2 GPa 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