|University of Pittsburgh
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Simultaneous hydrophilic/oleophobic materials are desired for various applications where anti-fogging is required. The ideal coating would be one that can be applied physically, such as an ionic liquid (IL) thin film. This thesis project explores the simultaneous hydrophilic/oleophobic wetting behavior of of a thin film of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIIm) on silica substrate. First, atomic force microscopy revealed that EMIIm forms a complete film on silica substrate with film dewetting occurring at 0.84 nm thickness. Next the molecular arrangement was analyzed with angle-resolved x-ray spectroscopy, by calculating the atomic ratio of nitrogen from the anion to nitrogen from the cation (N-/N+). For the three samples tested the ratio was above 0.5, indicating that the anion was organized over the cation even when the film was dewetted. This was determined to be due to the planar structure of the EMI cation. Further analysis done by changing the takeoff angle revealed a change in N-/N+ for 0.43 nm thin film; N-/N+ for the film bulk and the film surface was 0.8 and 0.9 respectively. This meant that for 0.43 nm film the ions are in a layered arrangement with the fluorinated anion layer at the surface. Finally, it was found that while the water contact angle (WCA) was low for all film thickness values tested (avg. 5.33° ± 3.91) the hexadecane contact angle (HCA) plateaued to 41.15° for samples greater than 0.55 nm thickness. Time-dependent HCA trials were conducted, for 24 hours and 3 hours. The trials confirmed that film penetration was occurring, as the HCA reached a final equilibrium value after 3 hours. The contact liquid is drawn to the high energy silica, so the liquid will pass through the intermolecular holes in the film to get to the substrate. Given that the WCA is low and constant, and the size difference between water and hexadecane, the hole size must be between 0.27 nm and 0.78 nm. In addition, the final HCA had a positive correlation to the film thickness.