The term hydrophobic derived from Ancient Greek ( ὕδωρ hydor " water " and φόβος Phobos "fear" ) and literally means "water -avoiding ". The measure of the expression level of the hydrophobicity of materials is the hydrophobicity.
Non-polar substances such as fats, waxes, alcohols having long alkyl groups - that is, with the exception of methanol, ethanol and propanol - alkanes, alkenes, etc. are hydrophobic. Upon release of hydrophobic substances in water generally occurs a so-called hydrophobic effect, and in some small, hydrophobic species, such as methane or xenon, even unfavorable entropic form clathrate structures. Therefore, the solubility of these substances in water is generally low.
Hydrophobic substances are almost always lipophilic, meaning they dissolve readily into fat and oil. An exception to this rule are fluorocarbons and some ionic liquids, such as BMIIm which are neither waterproof nor soluble in fat usually.
Surfaces with a contact angle greater than 90 ° with respect to water are also referred to as hydrophobic. Hydrophobic surfaces are typically composed of hydrophobic substances or of these are covered. Examples include the coating of surfaces with PTFE (Teflon) or the impregnation of insulating materials, and fabrics with hydrophobic materials such as wax or paraffin.
An extreme example of a hydrophobic surface is the surface of leaves and flowers of the Lotus. This is rough and additionally covered with hydrophobic substances. Due to this peculiarity it has contact angle of about 160 °, so that drops are on her almost round. From such surfaces water runs off very well. Superficial dirt particles are easily washed away. This effect is called the lotus effect.
The strongest repellency against water and oil is called Superamphiphob.
The opposite of hydrophobicity hydrophilicity. Molecules which possess both lipophilic and hydrophilic structural parts are called amphiphilic. This effect is exploited wash-active substances such as surfactants, or alkali salts of fatty acids (Na salt = core soap and K salt = soft soap). For you to solve hydrophobic soil in water.
Clarification of the term
Contrary to popular belief, there are no repulsive interactions between uncharged molecules. Even among the very hydrophobic surface of Teflon and water exists an attractive effect. Without such water droplets could adhere to the underside of hydrophobic surfaces, but would fall down. The reason for high contact angle with water is that this by itself stronger interactions ( hydrogen bonds ) may go down as with the hydrophobic surface with which only van der Waals interactions are possible. Therefore, an approximately spherical shape of water droplets is energetically most favorable.