Hydrophobic-Oleophilic Property

Kapok fiber contains the pectin and wax substances that contribute to its hydro­phobic-oleophilic characteristic. On the glass slide coated with kapok extract, the diesel drop and water drop will show a different spreading radius and contact angle. The diesel drop can spread out rapidly, and in contrast, the water drop cannot spread out on the glass slide. As a result, a large spreading radius and small contact angle are observed for diesel drop, whereas a large contact angle is visualized for water drop, demonstrating that the oil is a wetting liquid for kapok fiber and the water is a non-wetting liquid for kapok fiber (Lim and Huang 2007). The static and dynamic contact angle of kapok fibers with different kinds of liquids such as vegetable oil, used oil, and engine oil is also investigated. It is found that kapok fiber is an excel­lent oleophilic and hydrophobic fiber with the contact angle of kapok fiber to water of 139.55o, but is less than 60o to various kinds of oil. The contact angle of kapok to water is constant as time flies. All the oil liquids on the kapok fibers have the quick spread rates, and the spread curves are similar though the spread rates varied with viscosity and surface tension of the liquids (Sun et al. 2011). This hydrophobic — oleophilic characteristic can be tuned by solvent treatments. Our study reveals that for untreated and NaClO2-treated kapok fiber, different wetting phenomenon can be observed using water drops, with a large contact angle of 116o and a large spreading

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Fig. 6.2 Pictures of water droplet (dyed with methylene blue) on (a) raw, (b) treated, and (c) superhydrophobic kapok fiber surface; oil droplet (dyed with oil red O) on (a1) raw, (b1) treated, and (cl) superhydrophobic kapok fiber surface (Wang et al. 2012b) (Copyright 2012, reproduced with permission from Elsevier)

radius for untreated and NaClO2-treated kapok fiber, respectively (Fig. 6.2) (Wang et al. 2012b). Here, another observation should also be mentioned. Before and after collecting the oils from water, the kapok fiber may float steadily on the water sur­face due to its light density and hydrophobic-oleophilic properties, a useful charac­teristic for oil spills cleanup. In addition to the thin hydrophobic plant wax layer covered on the surface of kapok fiber, the hydrophobic-oleophilic characteristic is also related to its micro-nano-binary structure (Zhang et al. 2013).

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