Sum frequency generation spectroscopy of tetraalkylphosphonium ionic liquids at the air-liquid interface

Chariz Peñalber-Johnstone, Gabriela Adamová, Natalia V. Plechkova, Maryam Bahrami, Tahereh Ghaed-Sharaf, Mohammad Hadi Ghatee, Kenneth R. Seddon, Steven Baldelli*

*Corresponding author for this work

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Abstract

Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C - H stretching region (2750-3100 cm-1) for [P8 8 8 n][Cl], where n = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for n = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of n = 12 or n = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with n = 8, 10, or 12) confirmed gauche defects, with enhancement for the long alkyl chain and an abrupt increase of gauche occurrence from n = 8 to n = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented.

Original languageEnglish
Article number193841
Number of pages13
JournalJournal of Chemical Physics
Volume148
Issue number19
Early online date05 Apr 2018
DOIs
Publication statusPublished - 21 May 2018

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Peñalber-Johnstone, C., Adamová, G., Plechkova, N. V., Bahrami, M., Ghaed-Sharaf, T., Ghatee, M. H., ... Baldelli, S. (2018). Sum frequency generation spectroscopy of tetraalkylphosphonium ionic liquids at the air-liquid interface. Journal of Chemical Physics, 148(19), [193841]. https://doi.org/10.1063/1.5009674