Surface Nanocrystallization of an Ionic Liquid

Yoonnam Jeon, David Vaknin, Wei Bu, Jaeho Sung, Yukio Ouchi, Woongmo Sung, and Doseok Kim

Phys. Rev. Lett. 108, 055502 – Published 30 January 2012

Abstract

Surface crystallization at the vapor-liquid interface of the ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) is observed in synchrotron x-ray diffraction studies. Sharp Bragg reflections emerge in grazing-angle x-ray diffraction patterns $37 ° C$ above the bulk melting temperature, indicating the presence of a long-range ordered phase at the surface in coexistence with the bulk parent liquid. The unique structure of the vapor-liquid interface where butyl chains attached to the cations are expelled to the vapor side facilitates interionic electrostatic interactions that lead to the crystallization. Our results demonstrate the complexity of ionic-liquid structure with their tendency to spontaneously phase separate into nanodomains with finite correlation lengths in coexistence with the liquid phase. By virtue of interfacial boundary conditions, these nanodomains grow laterally to form quasi-two-dimensional crystals.

Received 8 September 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.055502

Authors & Affiliations

Yoonnam Jeon¹, David Vaknin², Wei Bu², Jaeho Sung¹, Yukio Ouchi³, Woongmo Sung¹, and Doseok Kim^1,*

¹Department of Physics, Sogang University, Seoul 121-742, Korea
²Ames Laboratory and Department of Physics, Iowa State University, Ames, Iowa 50011, USA
³Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan