Surfactant-Mediated Ion Exchange and Charge Reversal at Ionic Liquid Interfaces

Abstract

Room-temperature ionic liquids (ILs) exhibit a unique set of properties due to their charged character, presenting opportunities for numerous applications. Here, we show that the combination of charged surfactants with ILs leads to rich interfacial behavior due to the interplay between electrostatic and surface forces. Using traditional measures of surface activity and X-ray photoelectron spectroscopy (XPS), we find that sodium alkyl sulfates and alkyl trimethylammonium bromides are, indeed, surface-active at the air-IL interfaces of both [EMIM][EtSO(4)] and [BHEDMA][MeSO(3)]. XPS also reveals that surfactant counterions readily dissociate into the bulk, which when combined with the surfactant surface activity has striking consequences. We find that ion exchange occurs between surfactants and like-charged IL ions, with the greatest exchange for short surfactant alkyl chains. The initial negative surface charge of neat [EMIM][EtSO(4)] can be switched to positive by the addition of alkyl trimethylammonium bromides, with the effect most pronounced at short chain lengths. By contrast, the surface charge of [BHEDMA][MeSO(3)] is largely unaffected by the added surfactants, suggesting a key role for the strength of ion-pairing within the IL. The results here illustrate a simple but effective means of manipulating IL interfacial properties.

Structure and Dynamics of Nonionic Surfactants Adsorbed at Vacuum/Ionic Liquid Interfaces

“…At this point, we would like to bring into consideration an interesting observation that derives from recent surface tension and X-ray photoelectron spectroscopy experiments performed by Chen et al 48 on RTIL/air interfaces hosting ionic surfactants. The authors report an ionic exchange between surfactant counterions and RTIL ions of similar charge.…”

Mixtures of ionic liquids

“…, with the degree of surface enrichment increasing with the alkyl chain length of the surfactant. 130 At the same time, the level of the surfactants' counterions, Na + or Br À , at the surfaces were below detection limits. Again this data indicates that the surface layers contain both cations and anions and that [N 1,1,1,n ] + will displace ionic liquid cations at the surface, while [C n OSO 3 ] À will displace ionic liquid anions at the surface.…”

The Influence of the Interlayer Distance on the Performance of Thermally Reduced Graphene Oxide Supercapacitors

“…In addition, a VersaSTAT 4 potentiostat was employed to characterize the electrical performance of these two-electrode EDLC cells. It was found that the vibration mode of the CTAB head group was shifted due to the interaction with EMI-TFSI, implying the Coulombic or ion exchange interactions between the CTAB and EMI-TFSI [ 17 ]. Furthermore, the X-ray results revealed an interlayer distance of 0.41 nm as a result of CTAB intercalation in between the TRGC sheets.…”