hydrophilicity

Hydrophilicity refers to the affinity of a substance for water. It describes the tendency of a material, molecule, or surface to interact favorably with water, usually through hydrogen bonding, dipole–dipole interactions, and polar functional groups. Hydrophilic substances tend to attract and retain water, leading to high wettability and, in many cases, solubility in aqueous environments. By contrast, hydrophobic substances repel water and display low wettability.

Determinants include polar functional groups such as hydroxyl, carboxyl, amine, or sulfonate groups, which increase hydrophilicity; surface roughness and microstructures that can enhance apparent hydrophilicity via the Wenzel effect. Temperature, pH, and ionic strength can modulate the interaction by altering water structure and ion adsorption.

Measuring hydrophilicity commonly uses wettability quantified by the contact angle formed by a water drop on a surface. Contact angles less than 90 degrees indicate hydrophilicity, with smaller angles marking stronger wettability, and angles approaching 0° for highly hydrophilic surfaces. Surface energy and models such as Wenzel and Cassie–Baxter describe the influence of roughness and porosity on apparent contact angle.

Applications and relevance include the influence of hydrophilicity on coating adhesion, textile wetting, membrane filtration, and catalytic surfaces. In biology, many cell and protein surfaces are hydrophilic and interact with aqueous environments; hydrophilicity also affects protein solubility, folding, and aggregation. In materials science, hydrophilic surfaces are used to improve biocompatibility and to promote lubricity or anti-fouling strategies depending on design.

Overall, hydrophilicity is a relative property defined by favorable interactions with water, and it can be engineered through chemical functionalization, surface treatments, and morphology.