Contact Angle, Surface Free Energy, and Polar Component

13 Contact Angle, Surface Free Energy, and Polar Component 

The contact angle (0), another criterion for hydrophilicity and lipophilicity, is employed to evaluate the wettability of liquid to solid. Generally, the solid surface is considered to be wetted by liquid when 0 90° the solid surface is not considered to be wetted by liquid when 0 90° and the solid surface is wetted completely by liquid when 0 = 0°. As for inorganic whiskers, the bigger the contact angle between distilled water and whiskers, the better is the hydrophobicity of whiskers. 

Liu et al. 5 treated zinc oxide whiskers using several different coupling agents. Table 3.14 shows the contact angles using different coupling agents with a concentration of 4 wt%. 

As shown in Table 3.16, the whiskers modified with OTS have lower surface energy, lower polar component, and better hydrophobic and lipophilic properties than untreated whiskers. The dispersive component ratio of potassium titanate whiskers modified with OTS to OTS molecules is much closer to the dispersion value of the -CH2- group rather than the -CH3-group, which indicates that the whisker surface modified with 

OTS has mainly -CH2- groups and inclined alkyl chains of OTS. Furthermore, the polar component of whiskers modified with OTS decreases almost to zero, indicating that the surface of modified whiskers has stronger hydrophobicity and exhibits complete lipophilicity. 

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