Hydrophilicity
Definition and meaning of Hydrophilicity in chemistry.
Hydrophilicity is the tendency of a molecule, ion, or surface to interact favorably with water, driven by hydrogen bonding, dipole-dipole, or ion-dipole attractions. Hydrophilic ("water-loving") substances are typically polar or charged, making them soluble in or readily wetted by water.
In more detail
A substance is hydrophilic when its polar or ionic groups (such as hydroxyl, carbonyl, amino, or carboxylate groups) can form energetically favorable interactions with water's own polar molecules, offsetting the entropic cost of ordering water around a solute. This underlies the classic solubility rule "like dissolves like." Hydrophilicity also governs surface wetting: a droplet spreads on a hydrophilic surface, giving a water contact angle below 90 degrees. In biology, hydrophilic head groups of phospholipids face the aqueous environment on both sides of a cell membrane, while amphiphilic molecules like soaps use hydrophilic and hydrophobic regions together to solubilize grease in water.
Key facts
| Field | Physical Chemistry |
|---|---|
| Driving forces | Hydrogen bonding, ion-dipole and dipole-dipole interactions |
| Contact angle indicator | < 90° on a hydrophilic surface |
| Opposite property | Hydrophobicity |
Glucose (C6H12O6) is strongly hydrophilic because its five hydroxyl groups each form hydrogen bonds with surrounding water molecules, giving it high water solubility (about 900 g/L at 25°C).
Frequently asked questions
What structural features make a molecule hydrophilic?
Polar or ionizable functional groups, such as -OH, -NH2, -COOH, -COO-, or charged sites, that can hydrogen bond or electrostatically interact with water.
How is hydrophilicity measured experimentally?
For surfaces, via water contact angle (values below 90° indicate hydrophilic behavior); for molecules, via water solubility or the octanol-water partition coefficient (log P), where a low or negative log P indicates greater hydrophilicity.