Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 249

Ta b l e 1 .
Ionic liquids used in this study

Abbreviation Cation Anion

emim.BF 4 1-ethyl-3-methylimidazolium tetrafluoroborate

bmim.BF 4 1-butyl-3-methylimidazolium tetrafluoroborate

hmim.BF 4 1-hexyl-3-methylimidazolium tetrafluoroborate

omim.BF 4 1-octyl-3-methylimidazolium tetrafluoroborate

dmim.BF 4 1-decyl-3-methylimidazolium tetrafluoroborate

bmim.PF 6 1-butyl-3-methylimidazolium hexafluorophosphate

bmim.I 1-butyl-3-methylimidazolium iodide

bmim.TFA 1-butyl-3-methylimidazolium trifluoroacetate

bmim.NTf 2 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide

-CF 3 and -CF 2 - groups and is unaffected by the oxygen from the PDD monomers.
This apparent contradiction reflects the different sampling depths achieved with
XPS and SIMS (6-8 nm and 1-2 nm, respectively [66]) as compared with contact
angle measurements (

∼ 0.5 nm [64, 67]).

B.2. Ionic Liquids

The imidazolium-based ionic liquids used in this study are listed in Table 1.
Two series of pure ionic liquids were considered: (i) constant anion (1-alkyl-3-
methylimidazolum tetrafluoroborates, Rmim.BF 4 ) and (ii) constant cation (1-butyl-
3-methylimidazolium with various anions, bmim.X). Mixtures of bmim.BF 4 and
water with various concentrations were also included. All ionic liquids were pur-
chased from Merck and purified by extraction with activated charcoal, filtering
through a 0.2 µm Teflon filter, extraction with ethyl acetate, and evacuation under
medium vacuum (0.1 mbar) for 24 h [68].

B.3. Electrode

Glass slides, coated with a 30 nm Indium Tin Oxide (ITO) layer, were obtained
from Delta Technologies (Stillwater, MN). They were cleaned with isopropanol,
dried with nitrogen, and dip-coated with Teflon AF1600 (6% AF1600 dissolved in
Fluorinert FC-75, Derbyshire, UK). The AF1600 coating was dried in air (30 min
in a laminar flow cabinet and 30 h in an oven at 100 ◦ C). The thickness of the
AF1600 was determined by capacitance and stylus (Zeiss HandySurf profilometer)
measurements. The average thickness of the insulating layer was (3 . 8 ± 0 . 2) µm.

B.4. Electrowetting

The insulated electrode was immersed in a cell (Fig. 5) filled with hexadecane
(Sigma-Aldrich 99%, additionally purified by passing it through an aluminium ox-
ide column). A small droplet of ionic liquid (typical volume 10 µl) was deposited
on the electrode with an automated syringe. The droplet was then contacted with