A sum-frequency generation vibrational spectroscopy studies on buried liquid/liquid interfaces of CCl4/[Cnmim][TFSA] (n = 4 and 8) hydrophobic ionic liquids

The liquid/liquid interfaces of room-temperature ionic liquids (RTILs) play a pivotal role in chemical reactions owing to their characteristic microscopic structure, yet the structure of hydrophobic liquid/RTIL interfaces remains unclear. We studied the structure at the liquid/liquid interfaces of carbon tetrachloride (CCl4) and 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Cnmim][TFSA]; n = 4 and 8) RTILs using infrared-visible sum frequency generation (SFG) vibrational spectroscopy. A comparison of the SFG spectra of the CCl4/RTIL and air/RTIL interfaces revealed that the solvation of the alkyl chains of the [Cnmim]+ cations by CCl4 reduces the number of gauche defects in the alkyl chain and the interface number density of the cation at the CCl4 interface. The orientational change of the [TFSA]- anion and concomitant increase in the area it occupies at the CCl4 interface was observed to be greater than that at the air interface. This is accompanied by the expansion of the space among the alkyl chains of the cations to be solvated by CCl4. The structural change of the CCl4 interface from the air interface can be attributed to the solvophilic effect of CCl4 on the alkyl chains of the cations at the CCl4/[Cnmim][TFSA] interface. This is in contrast with the solvophobic effect of CCl4 on the Langmuir film at the CCl4/water interface. This phenomenon is caused by the loosely packed alkyl chains of the cations at the RTIL surface and the flexible anion-cation arrangement owing to the weak basicity and acidity of the ions in the RTILs.