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Hydrophobic and hydrophilic are opposites. The word stem ‘hydr’ comes from the greek ‘hydor‘ meaning water, therefore hydrophobic materials are ‘water-fearing‘, and do not mix with water, whereas hydrophilic materials are ‘water-loving‘ and have a tendency to be wetted by water. If you drip water onto a surface, sometimes it will spread out evenly when it hits the surface, for instance on a paper towel, and sometimes it will bead into droplets, like on a new waterproof jacket.

photographs by N. J. Rogers Simpson

Materials and molecules that have an affinity for water are hydrophilic; a hydrophilic surface will cause water to spread, and molecules that are hydrophilic tend to dissolve well in water. Many metal surfaces are hydrophilic, for example aluminium foil. Hydrophobic surfaces tend to repel water, causing it to form droplets; lotus leaves are incredibly hydrophobic and cause water to bead, due to their surface structure. The effect is so extreme it is often called ‘ultrahydrophobic’, and referred to as the ‘lotus effect‘. You can observe this yourselves with dried lotus leaves that are found in Chinese supermarkets.

The hydrophobicity/hydrophilicity of materials/surfaces can be measured by determining the contact angle:

This is the angle (θ) measured through the liquid, where the liquid/air boundary meets the solid surface. The bigger the contact angle is, the more hydrophobic the surface is and the more the water beads.

As well as describing solid surfaces, ‘hydrophobic’ and ‘hydrophilic’ can be used as adjectives to describe molecules. Hydrophilic molecules have an affinity for water and tend to dissolve well in polar solvents. Molecules that charge-polarised, form hydrogen bonds and/or exist as ions in solution tend to dissolve well in water because the charges can be stabilised by surrounding the molecules with polar water molecules. In contrast, nonpolar molecules (such as oil) do not have an affinity for water; it is more desirable for the oil molecules to stabilise each other via van der Waals forces, and the water molecules to stabilise each other via hydrogen bonding, than for the molecules to mix and interact with each other, and hence oil and water form layers.

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