Sum Frequency Generation Spectroscopy of Imidazolium-based Ionic Liquids with Cyano-functionalized Anions at Gas-Liquid and Solid-Liquid Interfaces

As essential media currently used in several technological applications, the accurate molecular level description of ionic liquids (ILs) at the gas-liquid interface is of utmost importance. The inherently surface-sensitive technique, sum frequency generation (SFG) spectroscopy, in combination with bulk-sensitive vibrational spectroscopic techniques such as FTIR and Raman, have been used in this report to characterize the surface of cyano-containing ILs, such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM], and [EMIM][TCB] at the gas-liquid interface. By structural variation of the anion while keeping the cation constant, emphasis on the molecular arrangement of the anion at the gas-liquid interface was reported. Vibrational modes seen in the C-H stretching region revealed the presence of the cation with an orientation independent of the type of anion. A similar arrangement at the surface as reported in previous studies was found. All three anions of varying symmetry, namely, [DCA]- (C2v), [TCM]-(D3h), and [TCB]- (Td) in ILs [BMIM]DCA], [BMIM][TCM] and [EMIM][TCB] were significantly tilted from the surface plane, while the linear [SCN]- in [BMIM][SCN] exhibited poor ordering, seen in the absence of its C-N stretching mode in the SFG vibrational spectra. Likewise, all four ILs in contact with two different solid salt surfaces, BaF2(111) single crystal and solid NaCl{100}, were discussed in this report. These studies described the nature of an ionic liquid-(solid) salt interface, contributing a new understanding to the molecular-level interactions involved in salts. Results showed that [BMIM]+ cations adhered closely via Coulombic interactions to the negatively-charged NaCl{100} surface, while [SCN]-, [TCM]-, and [DCA]- anions revealed a strong electrostatic affinity to the positively-charged BaF2(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer. At the BaF2(111)-[EMIM][TCB] interface, however, a strongly-bound layer of anions populating the first layer resulted to a much larger counter-ion charge delivered near the crystal salt surface than required to effectively neutralize the initial surface charge from the crystal. As a result, strong resonances from the cation were observed at the BaF2(111) surface, suggesting a more complicated structure of the double layer at the interface, than a simple Helmholtz-type model.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Houston

Year dc:date.issued

2012

Author dc:creator

• Penalber, Chariz 1984-

Advisor dc:contributor.advisor

• Baldelli, Steven

Committee members dc:contributor.committeemember

• Czernuszewicz, Roman S.
• Guloy, Arnold M.
• Jacobson, Allan J.
• Stein, Gila E.

• Ionic liquids
• Interfaces
• Sum-frequency generation

Statement dc:rights

• The author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10657/549

OAI identifier oai:identifier

oai:uh-ir.tdl.org:10657/549