Hydrophobic / Hydrophilic

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Definition

Hydrophobic describes materials, molecules, or even parts of molecules which do not readily make contact with water. In contrast, Hydrophilic materials have an affinity for water.

A simple way to tell whether a certain solid is hydrophobic or hydrophilic is to place a drop of water on a flat piece of the solid. If the water beads up and has a contact angle <math>>90 ^\circ </math>, the solid is hydrophobic. If the water drop spreads out and touches the solid with a contact angle <math><90 ^\circ </math>, the solid is hydrophilic [1].

Applications

  • The lotus leaf is a naturally occurring hydrophobic surface. In the image below [ 2 ], the surfaces of the drops of water curve back in underneath the drops forming very high contact angles with the lotus leaf.

LotusLeaf2.jpg

  • Surfactant molecules found in soap have hydrophilic heads and hydrophobic tails. The combination of hydrophilic and hydrophobic parts makes surfactants useful for cleaning [1].
  • Innovative materials make use of hydrophilicity and hydrophobicity for applications such as water-repellent coatings and self-cleaning textiles and glass. This National Geographic article describes the development of one such textile [4].

References

[1] de Gennes, P., Brochard-Wyart, F. & Quere, D. "Capillarity and Wetting Phenomena," Springer Science+Business Media, LLC, New York (2004).

[2] Garg, J.M. http://commons.wikimedia.org/wiki/File:Indian_Lotus_%28Nelumbo_nucifera%29_leaf_I2_IMG_6266.jpg.

[3] Bowden, N., Choi, I.S., Grzybowski, B.A., & Whitesides, G.M. J. Am Chem. Soc. 121, 5373-5391 (1999).

[4] Roach, J. "New Water-Repellent Material Mimics Lotus Leaves," National Geographic (23 Feb 2003). http://news.nationalgeographic.com/news/2003/02/0227_030227_lotusmaterial.html