## Hydrophobic definition

Molecules that repel water because they are nonpolar molecules are described with the adjective hydrophobic. Hydrophobic molecules such as alkanes, oils, fats, and greases do not dissolve in water. This characteristic is called hydrophobicity. In chemistry, we call hydrophobic molecules hydrophobes. Their opposites, hydrophilic molecules, dissolve in water.

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## Hydrophobic molecules

Water, ${H}_{2}O$, is a polar molecule, that is, it has polarity, which is an uneven distribution of electron density among its atoms. The oxygen side of any water molecule is slightly negative, while the hydrogen side is slightly positive. Polar water does not bond with nonpolar or hydrophobic molecules.

The adjective hydrophobic has its roots in a Greek word, hydrophobos, which means “dreading or fearing water.” Hydrophobic molecules have the quality of hydrophobicity, meaning they repel water, do not dissolve in water, or are themselves repelled by water molecules.

### Characteristics of hydrophobic molecules

Because hydrophobic molecules occur across a wide range of named substances, we cannot lump all water-repelling substances under a single characteristic. Some overlapping characteristics emerge, however. Hydrophobic molecules tend to:

• Be nonpolar
• Prefer other neutral molecules and nonpolar solvents
• Dissolve poorly in water, water-repellent
• Cluster together in water, forming micelles (molecular clusters in colloids)
• Force water into high contact angles on contact with surfaces coated with hydrophobic substances (high contact angles cause water to bead up)
• Encourage molecular folding, making them useful for amino acids, proteins, phospholipids, and other biological building blocks

Substances with extremely high contact angles (greater than $150°$) are said to be superhydrophobic and have superhydrophobic surfaces. These substances, such as nanopin film and self-assembled monolayers (SAMs), may be useful in biomedicine, nanotechnology, energy efficiency, and improving thermodynamics in power plants.

## Examples of hydrophobic substances

Water forms hydrogen bonds, notable for their weakness and short lives. These weak bonds account for water’s ability to readily bond to other polar molecules, making water the “universal solvent.” But water is not a universal solvent; it cannot bond with a long list of nonpolar, hydrophobic substances:

• Waxes – paraffin, carnauba, beeswax
• Steroids – progesterone, testosterone, and other hormones derived from naturally hydrophobic cholesterol
• Greases – lithium-calcium grease, sulphonates, lithium complexes, and perfluorinated oils
• Oils – mineral oil, vegetable oil, and tung oil
• Fats – Molecules joining fatty acids with glycerol
• Alkanes – hydrocarbons such as methane, ethane, propane, and butane

All of these hydrophobic substances do not dissolve in water. Most of them repel water, as seen with car wax causing rainwater to bead on your car’s metal surfaces. You can use waxes to create hydrophobic surfaces.

The opposite of a hydrophobic substance is a hydrophilic substance or water-loving. An example is glucose.

## Lipophilic vs hydrophobic

Lipophilic substances will dissolve in lipids or fats, and they can absorb water. Hydrophobic substances cannot absorb water or be wetted by water.

## Hydrophobic substances quiz

1. Would you prefer to protect your car’s paint finish with a hydrophobic or hydrophilic substance?
2. What makes a molecule hydrophobic?
3. What is polarity in a molecule?
4. Please give three examples of categories of hydrophobic materials in everyday life
5. Please name one type of superhydrophobic material.

1. A car’s paint finish is better protected by a hydrophobic substance such as car wax.
2. A molecule is hydrophobic if it does not dissolve in water, repels water molecules, or is repelled by water molecules.
3. Polarity in a molecule means chemical polarity, the tendency of the molecule to have an electrically negative end and an electrically positive end.
4. Three examples of categories of hydrophobic materials in everyday life are waxes, steroids, and fats. You could also have mentioned greases, alkanes, or oils.
5. One type of superhydrophobic material is nanopin film. A water droplet on nanopin film forms a contact angle of 178°, making it an almost perfect sphere.

## What you learned:

After working your way through this lesson and video, you have learned:

• Hydrophobic materials in biology are substances that do not dissolve in water, repel water, or are themselves repelled by water molecules.
• Examples include greases, waxes, steroids, alkanes, and fats. Hydrophobic materials exhibit characteristics of nonpolarity, formation of micelles, and an affinity to bond to other nonpolar substances.
• Hydrophobic substances are useful in molecular folding, nanotechnology, biomedicine, and gains in energy efficiency.
Instructor: Malcolm M.
Malcolm has a Master's Degree in education and holds four teaching certificates. He has been a public school teacher for 27 years, including 15 years as a mathematics teacher.

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