Two-length scale description of hydrophobic room-temperature ionic liquid–alcohol systems

Abstract In pure hydrophobic 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C n C m im][TFSI] (2 ≤  n  ≤ 10, m  = 1), the crossover alkyl side-chain length of the C n C 1 im + cation ( n c ) is found to be equal to 6 by a simultaneous X-ray diffraction and differential scanning calorimetry. By adding butanol, a geometrical factor of butanol isomers became dominant in the liquid–liquid phase equilibrium (LLE) above n c . In the LLE of [C n C 1 im][TFSI]–butanol, upper critical solution temperatures ( T UCST ) and critical concentration ( x c ) at the top of the phase separation depended extensively on n and the butanol isomer effect. In primary alcohol (C l H 2l  +  1 OH), the alkyl chain length of alcohols, l , enables it to increase the T UCST at a fixed n . Density at T UCST and x c have the opposite tendency in the two-length scale ( n and l ). In Raman spectroscopy, the conformation ratio of the TFSI − anion as an instability factor of liquid mixing exhibits an anomaly in the vicinity of n c .