Hydrophobicity

Hydrophobicity is a vital phenomenon for life, as it plays an important role in the formation of the cell membrane. Probably at some point in our lives, we have heard the word hydrophobic and have been given as a definition: "material that repels water" or something similar. However, this is not what actually happens; there is simply no attractive force with water.


The term hydrophobic comes from the ancient Greek ὑδρόφόβος (hýdrophóbos) meaning "to be afraid of water", which, for that time, seemed to accurately describe what was happening. Nowadays, the hydrophobicity effect can be explained as the aggregation of non-polar molecules in an aqueous or polar medium (a fairly common example is when we try to mix water with oil and see that they end up separating).


But how does this happen? As we have mentioned, molecules can be polar or non-polar. Water is a polar molecule, where one part will have a positive charge and the other a negative charge. On the other hand, lipids are an example of non-polar molecules. Whether it is polar or non-polar can be attributed mainly to the electronegativity of the atoms and the molecular geometry. However, when we talk about hydrophobic materials, the surface of the material must also be taken into account. Therefore, in materials, the hydrophobic effect depends not only on the chemical composition of the material, but also on the microstructure of its surface. This is where the term contact angle becomes important.


The contact angle refers to the angle formed between the surface of a liquid in contact with the surface of a solid. Depending on the value of the angle, materials can be classified as hydrophilic (angles less than 90°), hydrophobic (angles between 90° and 150°) and superhydrophobic (angles greater than 150°). An example of the latter is the so-called lotus effect where, if you drop a few drops of water on lotus leaves, you can see how they bounce off the leaves. This happens because on the surface of the plant, there are papillae about 10 to 20 microns high and 10 to 15 microns wide, which are covered by hydrophobic waxes.



In recent years, thanks to its reach on microstructures, nanotechnology has played a very important role in the development of new superhydrophobic materials, since with the discovery of the lotus effect and its operation, it opened a wide field of study to manufacture materials that can "self-clean".