Have you ever heard the phrase 'oil and water don't mix'? Though this phrase is hardly ever used in a scientific setting, the basis behind the old saying is due to chemistry. Indeed, oil does not mix with water and, because of this, it is considered hydrophobic. Let's discuss what it means for a molecule to be hydrophobic, and why this is an important characteristic in chemistry.
The word hydrophobic comes from the Greek roots hydro- (meaning water) and -phobia (meaning fearing or hating). The word hydrophobic describes the fact that nonpolar substances don't combine with water molecules. Let's take a closer look at that definition. Water is a polar molecule, which means that it carries a partial charge between its atoms. Oxygen, as an electronegative atom, draws the electrons of each bond closer to its core, thus creating a more negative charge. Therefore, any materials with a charge, be it negative or positive, will be able to interact with water molecules to dissolve. (Think of how salt dissolves in water. This is due to the charges of the ions sodium and chlorine.)
So essentially, hydrophobic molecules are molecules that do not have a charge, meaning they are non-polar. By lacking a charge, these molecules do not have any charge-to-charge interactions that will allow them to interact with water. Hydrophobic materials often do not dissolve in water or in any solution that contains a largely aqueous (watery) environment. This characteristic of being hydrophobic - or non-polar - is important for many of the molecules found in nature, in other organisms, and even within our own bodies.
Waxes are practical examples of hydrophobic molecules that are used commercially and biologically because of their abilities to resist interacting with water. For example, if you have ever had your car waxed, you probably noticed that the water would bead up and roll off of the paint afterwards. This is because the applied wax is hydrophobic and will not interact with water. Waxes, in this case, help to keep water away from the surface of the car.
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On the other hand, in many biological systems, waxes may be used for other means. Some plants, for example, utilize waxes to prevent water from escaping through evaporation. Many of the succulent plants, such as aloe, produce waxes that allow their tissues to retain water for biological use. Still, the reason these plants are able to do so is the hydrophobicity of the waxes.
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Many of the key vitamins and hormones in the body are steroids, and these molecules are non-polar. These include testosterone, progesterone, and other hormones that are derived from cholesterol. Additionally, these include vitamins A, D, E, and K, which do not dissolve in water. (Coincidentally, most steroid molecules are soluble in fats and lipids.) This allows steroid hormones and vitamins to last longer within the system, and preserves them for future usage within the body. In addition, the hydrophobic nature of these molecules allow for these molecules to cross the plasma membrane and enter body cells easily.
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The word 'hydrophobic' comes from the Greek roots 'hydro' meaning 'water' and 'phobia' meaning 'fearing or hating.' To put it another way, hydrophic describes the fact that nonpolar substances don't combine with water molecules. Water is a polar molecule which means that it carries a partial charge between its atoms. Hydrophobic molecules are molecules that do not have a charge, meaning they're nonpolar. Hydrophobic materials often do not dissolve in water or in any solution that contains a largely aqueous (watery) environment. Oil, waxes, and steroids are all examples of hydrophobic materials and molecules.
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| Vocabulary | Definitions |
|---|---|
| Hydrophobic | hydro (water) and phobia (fearing or hating): nonpolar substances don't combine with water molecules |
| Polar molecule | water carries a partial charge between its atoms |
| Hydrophobic molecules | molecules that are nonpolar or do not have a charge |
| Aqueous | watery environment |
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