# Superhydrophobic Polypyrrole-Coated Cigarette Filters for Effective Oil/Water Separation
## 3 Results and Discussion
### Surface Wettability  
Figure 4a shows photographs of water and oil droplets on the surface of an ordinary cigarette filter and a Ppy cigarette filter modified by dodecanethiol. A significant improvement in the wettability of the cigarette filter was observed. When the water droplet or oil droplet was dribbled on the original cigarette filter surface, the original cigarette filter absorbed both water droplets and oil droplets. Water droplets and oil droplets were completely spread on the original cigarette filter surface because of the inherently superhydrophilicity and superoleophilicity. Different from those on the unmodified cigarette filter, water droplets remained in a spherical shape on the Ppy cigarette filter surface modified by dodecanethiol, and oil droplets quickly penetrated into the interspace between the fibers of the Ppy cigarette filter modified by dodecanethiol. The results illustrated that the Ppy cigarette filter modified by dodecanethiol has superhydrophobicity and superlipophilicity. Figure 4b shows contact angle images of the modified filters with different pyrrole contents. When the pyrrole content was 0.1 mol/L, the water droplet remained hemi-ellipsoidal, indicating that the modified filter surface was hydrophobic. When the pyrrole content reached 1 mol/L, the water droplet had a spherical shape, which means that the filter surface was superhydrophobic.  
In order to further investigate the relationship between pyrrole content and surface wettability, the wettability of the modified filter was tested for different pyrrole contents, as shown in Figure 5. The contact angle increased with increasing pyrrole content, while the sliding angle decreased. When the pyrrole content was 0.5 mol/L, the contact angle was 150\({}^{\circ}\), and the sliding angle was about 10\({}^{\circ}\), fulfilling the requirements of superhydrophobicity. According to our SEM images, the pyrrole concentration can change the surface morphology of the filter. To our knowledge, the transition of the wettability of the cigarette filter from superhydrophilic to superhydrophobic can be realized by changes in the surface morphology and surface energy [39]. When the pyrrole content was lower, the filter surface was covered with unevenly distributed tiny Ppy nanoparticles. The water could wet the nanostructures but not spread on the filter surface owing to the hydrophobicity of dodecanethiol, which could be  
Figure 4: **(a)** Photographs of the wettability of an ordinary cigarette filter and a dodecanethiol-modified Ppy cigarette filter; **(b)** Contact angle images of dodecanethiol-modified Ppy cigarette filters with different Py concentrations.  
explained by the Wenzel mode [40]. With increasing pyrrole concentration, a large amount of Ppy particles aggregated and formed micro-nano hierarchical structures on the filter surface. With the help of low-surface-energy material, there was solid-liquid-vapor composite contact between the micro-nano hierarchical structures and water, which was consistent with the Cassie model [41]. Thus, a water droplet could easily roll off the modified cigarette filter surface owing to the existence of the air layer.