Deposition Of Ionic Liquid Thin Films Onto Self-Assembled Monolayers

Abstract

To address the problem of friction in microelectromechanical systems (MEMS), a replenishable lubrication scheme is required. Recent research indicates that a combination of a mobile phase plus a bound phase offers synergistic lubrication performance that surpasses that for a single layer (mobile or bound). In this project, the ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim+][triflate-]) was synthesized and drop cast onto self-assembled monolayers (SAMs) to provide a combined mobile (IL)/bound (SAM) coating for potential applications in the lubrication of MEMS. Low-energy hydrophobic monolayers on Au and Si substrates were prepared from n-alkanethiols and from a variety of alkyltrichlorosilanes, respectively. The thicknesses of these monolayers increased as the chain length of the adsorbate molecules increased. Specifically, for the n-alkyltrichlorosilanes, each carbon in the adsorbate chain contributed ~1.3 Å to the thickness of the monolayer film. The IL would not coat these low-energy monolayer surfaces; thus, monolayers of varying surface energy were prepared on Au and Si substrates to determine the critical surface energy above which the IL would form a film. On both Au and Si substrates, the critical surface energy required for the formation of an IL film was characterized by an IL contact angle of ~27 (+/- 5), which corresponds to an advancing water contact angle of 70 (+/- 4). Transmission FTIR studies of the IL and RAIRS studies of IL thin films indicate orientation of the triflate anion occurs in thin films (~50 nm) of the IL but not in thicker films (~1 um).