Ionic multilayers at the free surface of an ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, probed by x-ray reflectivity measurements

Abstract

The presence of ionic multilayers at the free surface of an ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide ([TOMA+][C4C4N−]), extending into the bulk from the surface to the depth of ∼60 Å has been probed by x-ray reflectivity measurements. The reflectivity versus momentum transfer (Q) plot shows a broad peak at Q∼0.4 Å−1, implying the presence of ionic layers at the [TOMA+][C4C4N−] surface. The analysis using model fittings revealed that at least four layers are formed with the interlayer distance of 16 Å. TOMA+ and C4C4N− are suggested not to be segregated as alternating cationic and anionic layers at the [TOMA+][C4C4N−] surface. It is likely that the detection of the ionic multilayers with x-ray reflectivity has been realized by virtue of the greater size of TOMA+ and C4C4N− and the high critical temperature of [TOMA+][C4C4N−].