Measurement of hydrophilicity

Measurement of Hydrophilicity
Contact angle measurement is a major parameter to quantify the hydrophilicity of a
substance, which is further indicative of wettability. Hydrophilic substances possess
good wettability. Wettability is the ability of the liquid to remain in contact with the
solid surface. The degree of wettability is measured using a contact angle. The
contact angle (θ) is the angle between the surface and the edge of the droplet. A
hydrophilic surface has a contact angle (θ) 90°, shown in Figure 1 (below). A higher contact angle
indicates the stronger liquid-liquid interaction rather than liquid-surface interaction
thus making the material hydrophobic.
Figure 1:
Representation of contact angle (Reproduced from Tavana H., Lam C., Grundke K., Friedel
P., Kwok D., Hair M., Neumann A. (2004). Contact angle measurements with liquids
consisting of bulky molecules. Journal of Colloid and Interface Science 279:493-502.)
If the liquid spreads out on a surface, wetting a large area of the surface, then the
contact angle is less than 90° and is considered hydrophilic, or water-loving (Figure
2). While, if a liquid forms droplet, the contact angle is more than 90° and is
considered to be hydrophobic or water-repelling (Figure 2). Wettability is an
important parameter for plants and animals. Lotus flower leaves and Rice leaves
exhibit a non-wetting surface, wherein leaves remain dry and water droplets roll out
from the surface of the leaves keeping them clean all the time. Certain animals like
Namib desert beetles, manage to survive in the dry region due to their ability to
absorb moisture from the environment via hydrophilic structures on their body
surface.
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